Background Past estimates have put the prevalence of schistosomiasis between 40% and 50% in the Malawi population overall based on studies undertaken ten years or more ago. More recent surveys in known high risk areas find similar levels. However control measures, changing ecology and migration may have led to changes in the prevalence of schistosomiasis in different parts of Malawi. A national schistosomiasis and soil-transmitted helminth (STH) survey was undertaken to measure the distribution, prevalence and intensity of infection in November 2002.
Background
Past estimates have put the prevalence of schistosomiasis between 40% and 50% in the Malawi population overall based on studies undertaken ten years or more ago. More recent surveys in known high risk areas find similar levels. However control measures, changing ecology and migration may have led to changes in the prevalence of schistosomiasis in different parts of Malawi. A national schistosomiasis and soil-transmitted helminth (STH) survey was undertaken to measure the distribution, prevalence and intensity of infection in November 2002.Methods A school was selected randomly from a random sample of 30 Traditional Authorities stratified by six distinct ecological zones, and 1,664 year 3 pupils (9–10 year olds) were questioned about recent illnesses and "red urine". Samples of urine and faeces were examined for the presence of eggs using the standard Kato-Katz technique for soil-transmitted helminths and intestinal schistosomiasis and urine samples using the filtration technique for  Schistosoma haematobium .
Methods
A school was selected randomly from a random sample of 30 Traditional Authorities stratified by six distinct ecological zones, and 1,664 year 3 pupils (9–10 year olds) were questioned about recent illnesses and "red urine". Samples of urine and faeces were examined for the presence of eggs using the standard Kato-Katz technique for soil-transmitted helminths and intestinal schistosomiasis and urine samples using the filtration technique for  Schistosoma haematobium .Schistosoma haematobiumResults The prevalence of  Schistosoma mansoni  is 0.4% (95% CI 0–1.3%),  S. haematobium  6.9% (95% CI 1.9 – 11.9%), hookworm 1.3% (95% CI 0.4–2.3%), Ascariasis 0.5% (95% CI 0.1–1.0%) and trichuriasis 0% in year 3 pupils (modal age 10 years of age). Intensity of infection is low for all infections except for 2.5% who have high intensity  S. haematobium  infection. The "red urine" question is 67% sensitive and 80% specific for positive  S. haematobium  microscopy.
Results
The prevalence of  Schistosoma mansoni  is 0.4% (95% CI 0–1.3%),  S. haematobium  6.9% (95% CI 1.9 – 11.9%), hookworm 1.3% (95% CI 0.4–2.3%), Ascariasis 0.5% (95% CI 0.1–1.0%) and trichuriasis 0% in year 3 pupils (modal age 10 years of age). Intensity of infection is low for all infections except for 2.5% who have high intensity  S. haematobium  infection. The "red urine" question is 67% sensitive and 80% specific for positive  S. haematobium  microscopy.Schistosoma mansoniS. haematobiumS. haematobiumS. haematobiumConclusions The reduction in prevalences may be real as a result of recent control measures, or false if historical results were based on surveys of high risk populations. Another explanation is that this survey used an unrepresentative sample of schools. Detailed analysis suggests this is unlikely. Recommendations include the use of a 30% positive threshold for the "red urine" screening question to be used in schoolchildren in high prevalence areas. This survey, based on a national probability sample excluding the northern region lakeside area, finds much lower overall prevalence and intensity of schistosomiasis and STHs than previous estimates based on selected surveys. Disease control featuring chemotherapy may be having a profound effect. The localised nature of the distribution of the infections means that control programmes may work best if undertaken at district level or below. "Red urine" questionnaire surveys may help identify hot spots.
Conclusions
The reduction in prevalences may be real as a result of recent control measures, or false if historical results were based on surveys of high risk populations. Another explanation is that this survey used an unrepresentative sample of schools. Detailed analysis suggests this is unlikely.Recommendations include the use of a 30% positive threshold for the "red urine" screening question to be used in schoolchildren in high prevalence areas.This survey, based on a national probability sample excluding the northern region lakeside area, finds much lower overall prevalence and intensity of schistosomiasis and STHs than previous estimates based on selected surveys. Disease control featuring chemotherapy may be having a profound effect. The localised nature of the distribution of the infections means that control programmes may work best if undertaken at district level or below. "Red urine" questionnaire surveys may help identify hot spots.

Background Chronic infections with soil-transmitted helminths and schistosomiasis are common in Malaŵi and cause considerable morbidity [ 1 ]. The Ministry of Health wishes to establish the distribution of these infections prior to a reassessment of national policy regarding their control. All water bodies in Malawi are considered to be potential transmission sites for schistosomiasis [ 2 ]. The most recent national estimate puts the prevalence of schistosomiasis between 40% and 50% of the population [ 3 ]. This is based on surveys carried out prior to 1991. Numerous local surveys have been conducted more recently in different parts of the country [ 4 - 7 ]. In a survey carried out by Randall et al in 1996 in northern Malawi,  S. mansoni  and  S. haematobium  were detected in 27% and 20% of schoolchildren respectively [ 8 ]. A survey performed by the Bilharzia Control Programme in Mangochi reported in 1999 showed between 60–80% of school children living along the lakeshore has  S. haematobium  [ 9 ]. This level of infection was confirmed in a later survey performed by the Lakeshore Schistosomiasis Control Project, which found the prevalence amongst school aged children (5–15 years old) to be as high as 80% along the lake shore, with some areas having 100% infection rates [ 2 ]. In Malawi there is little contemporary information available on helminth infection. Furthermore, fragmented control measures, changing ecology [ 10 ] (such as considerable water resource development and deforestation in the last 10 years) and migration may have led to marked changes in the prevalence of schistosomiasis within regions in Malawi since previous studies were done. A national schistosomiasis and soil-transmitted helminth (STH) survey was undertaken to measure the distribution, prevalence and intensity of infection in November 2002. Results from the survey are reported in this paper.
Background
Chronic infections with soil-transmitted helminths and schistosomiasis are common in Malaŵi and cause considerable morbidity [ 1 ]. The Ministry of Health wishes to establish the distribution of these infections prior to a reassessment of national policy regarding their control.All water bodies in Malawi are considered to be potential transmission sites for schistosomiasis [ 2 ]. The most recent national estimate puts the prevalence of schistosomiasis between 40% and 50% of the population [ 3 ]. This is based on surveys carried out prior to 1991. Numerous local surveys have been conducted more recently in different parts of the country [ 4 - 7 ]. In a survey carried out by Randall et al in 1996 in northern Malawi,  S. mansoni  and  S. haematobium  were detected in 27% and 20% of schoolchildren respectively [ 8 ]. A survey performed by the Bilharzia Control Programme in Mangochi reported in 1999 showed between 60–80% of school children living along the lakeshore has  S. haematobium  [ 9 ]. This level of infection was confirmed in a later survey performed by the Lakeshore Schistosomiasis Control Project, which found the prevalence amongst school aged children (5–15 years old) to be as high as 80% along the lake shore, with some areas having 100% infection rates [ 2 ].S. mansoniS. haematobiumS. haematobiumIn Malawi there is little contemporary information available on helminth infection. Furthermore, fragmented control measures, changing ecology [ 10 ] (such as considerable water resource development and deforestation in the last 10 years) and migration may have led to marked changes in the prevalence of schistosomiasis within regions in Malawi since previous studies were done.A national schistosomiasis and soil-transmitted helminth (STH) survey was undertaken to measure the distribution, prevalence and intensity of infection in November 2002. Results from the survey are reported in this paper.Methods Methods follow recent WHO advice to allow comparison with other international studies [ 11 ]. This includes a symptom and illness questionnaire and urine and faecal sampling. The use of questionnaires as a rapid assessment tool for urinary schistosomiasis has been evaluated in multi-country studies [ 4 ]. Seven areas (with the number of Traditional Authorities in each) represent the seven distinct ecology and soil conditions in Malaŵi (figure  1 ):- Northern Region highland (23) Northern Region lakeside Central Region highland (62) Central Region lakeside (17) Southern Region highland (51) Southern Region lakeside (31) Urban (3) The northern region lakeside had recently been surveyed by the Karonga Prevention Study [ 8 ]. To save expense this ecological area was not resurveyed. Hence only six areas were included in this survey. Study population Primary-school children were chosen as the target population. There are several reasons for conducting the survey in this age group. Children consistently have the highest prevalence and transmission of schistosomiasis and soil-transmitted nematodes (except hookworm); treatment via schools on a national scale is feasible [ 12 ] (and primary school enrolment is reasonably high in Malawi) and extremely cost-effective [ 13 ]. Third-year school classes (9–10 year olds) are commonly surveyed and this was the age-group for this survey. Sample frame and selection The sample was selected using a stratified multi-stage cluster design [ 14 ]. The six ecological zones constituted the strata. School children were clustered within schools which were clustered within Traditional Authorities (TA). Five Traditional Authorities (TA) were randomly selected from the total in each ecological zone using a computer generated random numbers technique. A list of all primary schools in these TAs was compiled by contacting the relevant District Education Officer and one school was selected at random. Private schools were included though the vast majority were government schools. In the three urban districts of Blantyre, Lilongwe and Mzuzu five schools were selected. A total of 30 schools were surveyed between 25 September and 14th November 2002. A census of all children attending standard 3 who were present on the day of the survey was taken and all were enrolled. Thus between 36 and 71 third year schoolchildren (aged 8–10 years) in 5 schools selected at random from each of six ecologically distinct regions of the country were sampled (Figure  2 ). A total of 1664 children were enrolled. Questionnaire All children were administered a standard symptom questionnaire by a class teacher in their local language [see  Additional file 1 ]. The questionnaire was completed before specimen collection and was designed to assess general health without biasing responses towards helminthiasis in particular. Children were asked about symptoms during the past month [e.g. cough, itch, headache, fever] and whether they had particular illnesses during the past month [e.g. malaria, diarrhoea, skin, eye disease, bilharzia]. Specimen collection methods School children were asked to provide a stool sample and at least 10 ml of urine (collected mid-morning). Stool samples were stored in a cool box during transfer to the laboratory, and then kept at 4°C until processed within 24 hrs of collection. Urine and faecal analysis Stool samples were tested for the presence of eggs using the standard Kato-Katz technique [ 15 ] for soil-transmitted helminths and intestinal schistosomiasis. Samples with ascaris more than 50,000 eggs per gram (epg), hookworms more than 4,000 epg, trichuria more than 10,000 epg and  S. mansoni  more than 400 epg were defined as high intensity. Urine samples were tested using the filtration technique for  S. haematobium . Infection with  S. haematobium  was classified as high intensity if at least 50 eggs per 10 ml of urine were detected or there was visible haematuria [ 16 ]. Data analysis Data were double entered into EpiInfo and then exported into STATA v7 for initial analysis. SPSS was used for chi squared, paired-t and McNemar tests. EpiInfo 2002 was used to calculate adjusted prevalence rates for ecological zones and national prevalence. The prevalences and 95% confidence intervals were computed incorporating the stratification by ecological zones. Sampling weights were computed as the product of the number of TA's in an ecological zone times the number of schools in the selected TA. For the urban zone, weights were adjusted for the fact that there were only three TA's and two schools were selected from two of them (Complex sample frequency analysis, Epi Info 2002, CDC Atlanta). The relationship between the prevalence and intensity of  S. haematobium  infection in schools was investigated using Pearson product moment correlation coefficient (SPSS).
Methods
Methods follow recent WHO advice to allow comparison with other international studies [ 11 ]. This includes a symptom and illness questionnaire and urine and faecal sampling. The use of questionnaires as a rapid assessment tool for urinary schistosomiasis has been evaluated in multi-country studies [ 4 ].Seven areas (with the number of Traditional Authorities in each) represent the seven distinct ecology and soil conditions in Malaŵi (figure  1 ):-Northern Region highland (23)Northern Region lakesideCentral Region highland (62)Central Region lakeside (17)Southern Region highland (51)Southern Region lakeside (31)Urban (3)The northern region lakeside had recently been surveyed by the Karonga Prevention Study [ 8 ]. To save expense this ecological area was not resurveyed. Hence only six areas were included in this survey.Study population Primary-school children were chosen as the target population. There are several reasons for conducting the survey in this age group. Children consistently have the highest prevalence and transmission of schistosomiasis and soil-transmitted nematodes (except hookworm); treatment via schools on a national scale is feasible [ 12 ] (and primary school enrolment is reasonably high in Malawi) and extremely cost-effective [ 13 ]. Third-year school classes (9–10 year olds) are commonly surveyed and this was the age-group for this survey.
Study population
Primary-school children were chosen as the target population. There are several reasons for conducting the survey in this age group. Children consistently have the highest prevalence and transmission of schistosomiasis and soil-transmitted nematodes (except hookworm); treatment via schools on a national scale is feasible [ 12 ] (and primary school enrolment is reasonably high in Malawi) and extremely cost-effective [ 13 ]. Third-year school classes (9–10 year olds) are commonly surveyed and this was the age-group for this survey.Sample frame and selection The sample was selected using a stratified multi-stage cluster design [ 14 ]. The six ecological zones constituted the strata. School children were clustered within schools which were clustered within Traditional Authorities (TA). Five Traditional Authorities (TA) were randomly selected from the total in each ecological zone using a computer generated random numbers technique. A list of all primary schools in these TAs was compiled by contacting the relevant District Education Officer and one school was selected at random. Private schools were included though the vast majority were government schools. In the three urban districts of Blantyre, Lilongwe and Mzuzu five schools were selected. A total of 30 schools were surveyed between 25 September and 14th November 2002. A census of all children attending standard 3 who were present on the day of the survey was taken and all were enrolled. Thus between 36 and 71 third year schoolchildren (aged 8–10 years) in 5 schools selected at random from each of six ecologically distinct regions of the country were sampled (Figure  2 ). A total of 1664 children were enrolled.
Sample frame and selection
The sample was selected using a stratified multi-stage cluster design [ 14 ]. The six ecological zones constituted the strata. School children were clustered within schools which were clustered within Traditional Authorities (TA). Five Traditional Authorities (TA) were randomly selected from the total in each ecological zone using a computer generated random numbers technique. A list of all primary schools in these TAs was compiled by contacting the relevant District Education Officer and one school was selected at random. Private schools were included though the vast majority were government schools. In the three urban districts of Blantyre, Lilongwe and Mzuzu five schools were selected. A total of 30 schools were surveyed between 25 September and 14th November 2002. A census of all children attending standard 3 who were present on the day of the survey was taken and all were enrolled. Thus between 36 and 71 third year schoolchildren (aged 8–10 years) in 5 schools selected at random from each of six ecologically distinct regions of the country were sampled (Figure  2 ). A total of 1664 children were enrolled.Questionnaire All children were administered a standard symptom questionnaire by a class teacher in their local language [see  Additional file 1 ]. The questionnaire was completed before specimen collection and was designed to assess general health without biasing responses towards helminthiasis in particular. Children were asked about symptoms during the past month [e.g. cough, itch, headache, fever] and whether they had particular illnesses during the past month [e.g. malaria, diarrhoea, skin, eye disease, bilharzia].
Questionnaire
All children were administered a standard symptom questionnaire by a class teacher in their local language [see  Additional file 1 ]. The questionnaire was completed before specimen collection and was designed to assess general health without biasing responses towards helminthiasis in particular. Children were asked about symptoms during the past month [e.g. cough, itch, headache, fever] and whether they had particular illnesses during the past month [e.g. malaria, diarrhoea, skin, eye disease, bilharzia].Specimen collection methods School children were asked to provide a stool sample and at least 10 ml of urine (collected mid-morning). Stool samples were stored in a cool box during transfer to the laboratory, and then kept at 4°C until processed within 24 hrs of collection.
Specimen collection methods
School children were asked to provide a stool sample and at least 10 ml of urine (collected mid-morning). Stool samples were stored in a cool box during transfer to the laboratory, and then kept at 4°C until processed within 24 hrs of collection.Urine and faecal analysis Stool samples were tested for the presence of eggs using the standard Kato-Katz technique [ 15 ] for soil-transmitted helminths and intestinal schistosomiasis. Samples with ascaris more than 50,000 eggs per gram (epg), hookworms more than 4,000 epg, trichuria more than 10,000 epg and  S. mansoni  more than 400 epg were defined as high intensity. Urine samples were tested using the filtration technique for  S. haematobium . Infection with  S. haematobium  was classified as high intensity if at least 50 eggs per 10 ml of urine were detected or there was visible haematuria [ 16 ].
Urine and faecal analysis
Stool samples were tested for the presence of eggs using the standard Kato-Katz technique [ 15 ] for soil-transmitted helminths and intestinal schistosomiasis. Samples with ascaris more than 50,000 eggs per gram (epg), hookworms more than 4,000 epg, trichuria more than 10,000 epg and  S. mansoni  more than 400 epg were defined as high intensity. Urine samples were tested using the filtration technique for  S. haematobium . Infection with  S. haematobium  was classified as high intensity if at least 50 eggs per 10 ml of urine were detected or there was visible haematuria [ 16 ].S. mansoniS. haematobiumS. haematobiumData analysis Data were double entered into EpiInfo and then exported into STATA v7 for initial analysis. SPSS was used for chi squared, paired-t and McNemar tests. EpiInfo 2002 was used to calculate adjusted prevalence rates for ecological zones and national prevalence. The prevalences and 95% confidence intervals were computed incorporating the stratification by ecological zones. Sampling weights were computed as the product of the number of TA's in an ecological zone times the number of schools in the selected TA. For the urban zone, weights were adjusted for the fact that there were only three TA's and two schools were selected from two of them (Complex sample frequency analysis, Epi Info 2002, CDC Atlanta). The relationship between the prevalence and intensity of  S. haematobium  infection in schools was investigated using Pearson product moment correlation coefficient (SPSS).
Data analysis
Data were double entered into EpiInfo and then exported into STATA v7 for initial analysis. SPSS was used for chi squared, paired-t and McNemar tests. EpiInfo 2002 was used to calculate adjusted prevalence rates for ecological zones and national prevalence. The prevalences and 95% confidence intervals were computed incorporating the stratification by ecological zones. Sampling weights were computed as the product of the number of TA's in an ecological zone times the number of schools in the selected TA. For the urban zone, weights were adjusted for the fact that there were only three TA's and two schools were selected from two of them (Complex sample frequency analysis, Epi Info 2002, CDC Atlanta). The relationship between the prevalence and intensity of  S. haematobium  infection in schools was investigated using Pearson product moment correlation coefficient (SPSS).S. haematobiumResults The sample A total of 1,664 children were surveyed. Between 36 and 71 children were included from each school. There were 1,662 completed questionnaires, 1,546 stool samples and 1,638 urine specimens collected. Just over half (50.7%) of the sample was female. The mean age (for 1,658 children where age was recorded) was 10.7 years [range 6–17 years]. The mean age of boys was 10.9 years and of girls 10.5 years. Prevalence and intensity of infection Schistosomiasis A total of 140 children from 19 different schools had  S. haematobium  detected by urine microscopy, of which 45 had high intensity infection. The prevalence in schools ranged between 0 (11 schools) and 43.1% (a school in the southern lowlands). Twenty one children from eight different schools had visible haematuria. Only 8 children from 2 different schools had  S. mansoni  detected by stool microscopy and none were high intensity infections. Overall, the national prevalence of  S. mansoni  was found to be 0.4%. The national prevalence of  S. haematobium  was found to be 6.9% (95% CI 1.9–11.9%) (Table  1 ). 2.5% of these were high intensity infections [range 0–23.3%]. There was a clear association between prevalence and intensity (r = 0.9, n = 30, p = <0.01); schools with high prevalence of  S. haematobium  also had high intensity of infection. There was a wide range of ages in standard 3, with mode of 10 years of age. The observed prevalence was higher in older children (see figure  3 ) and boys (9.0%) compared with girls (8.2%). Soil transmitted helminths The number of children with detectable helminth infestation was low. Twenty-six children from 13 schools had hookworm detected from stool, none of which were high intensity. Eighteen children from nine different schools had ascaris eggs detected and no trichuris eggs were detected at all. There was no obvious pattern of co-infection with schistosomiasis and soil-transmitted helminths across the schools sampled; in 16 schools there was concordance with either both or neither type of infection detected; in 14 schools there was no concordance. The prevalence of STHs was much lower than that of schistosomiasis (Table  1 ). Only 1.8% [95% C I 0.6–3.1%] of children had evidence of infection with any of hookworm, ascaris or trichuriasis on stool microscopy. There were no heavy intensity STH infestations detected. Prevalence and intensity of infection by ecological region Schistosomiasis S. haematobium  infection was detected in all ecological areas (Table  2 ). The highest rates were in the Southern Lowland (23.2%) and the Central lowland (9.5%); the other four areas examined all showed a prevalence of between 2% and 7.4%. There was a wide range in the prevalence of  S. haematobium  infection in all areas.  S. mansoni  was rarely detected, and only found in the Southern Highlands (1.3%). To assess the sampling method used to select schools we classified the selected schools into high and low expected prevalence of  S. haematobium  based on ecology and known previous prevalence and compared our survey results with these expectations. Of the 12 expected high prevalence schools seven were found to be high and 5 unexpectedly low. Of the 18 expected low prevalence schools 12 were found to be low and 6 unexpectedly high. Unexpected results were found in the expected low prevalence as much as in the expected high prevalence schools using the McNemar test for symmetry actual (p = 0.29). We also compared the historical results for Malawi districts compiled by the London School of Hygiene and Tropical Medicine for the Schistosomiasis Control Initiative in 2001 (personal communication with Simon Brooker) with our results. Prevalence has fallen overall by 24% (95% confidence intervals of 16–32%). A formal paired t-test finds this reduction is true for low as well as high prevalence schools (t = 6.4 with 29 degrees of freedom and highly statistically significant). Soil transmitted helminths There was also a low prevalence of STHs in all areas, though urban areas had the highest rates for any STH (6.6%) and hookworm was most prevalent in the Northern Highland area (2.6%). There were no STHs detected in the Central lakeshore schools. Questionnaire responses Total number of respondents was 1,662 (Table  3 ). It is possible to calculate the sensitivity and specificity of the "red urine" question in respect of the microscopic presence of  S. haematobium  (Table  4 ). The sensitivity of the "red urine" question in respect of  S. haematobium  being seen by microscopy is 67% and the specificity 80%. At a national average prevalence of 8.5% the "red urine" question has a positive predictive value of 24% and a negative predictive value of 96%. At the higher prevalence (22.5%) found in the Southern Lowlands zone the positive predictive value is 49% and the negative predictive value is 89%.
Results
The sample A total of 1,664 children were surveyed. Between 36 and 71 children were included from each school. There were 1,662 completed questionnaires, 1,546 stool samples and 1,638 urine specimens collected. Just over half (50.7%) of the sample was female. The mean age (for 1,658 children where age was recorded) was 10.7 years [range 6–17 years]. The mean age of boys was 10.9 years and of girls 10.5 years.
The sample
A total of 1,664 children were surveyed. Between 36 and 71 children were included from each school. There were 1,662 completed questionnaires, 1,546 stool samples and 1,638 urine specimens collected. Just over half (50.7%) of the sample was female. The mean age (for 1,658 children where age was recorded) was 10.7 years [range 6–17 years]. The mean age of boys was 10.9 years and of girls 10.5 years.Prevalence and intensity of infection Schistosomiasis A total of 140 children from 19 different schools had  S. haematobium  detected by urine microscopy, of which 45 had high intensity infection. The prevalence in schools ranged between 0 (11 schools) and 43.1% (a school in the southern lowlands). Twenty one children from eight different schools had visible haematuria. Only 8 children from 2 different schools had  S. mansoni  detected by stool microscopy and none were high intensity infections. Overall, the national prevalence of  S. mansoni  was found to be 0.4%. The national prevalence of  S. haematobium  was found to be 6.9% (95% CI 1.9–11.9%) (Table  1 ). 2.5% of these were high intensity infections [range 0–23.3%]. There was a clear association between prevalence and intensity (r = 0.9, n = 30, p = <0.01); schools with high prevalence of  S. haematobium  also had high intensity of infection. There was a wide range of ages in standard 3, with mode of 10 years of age. The observed prevalence was higher in older children (see figure  3 ) and boys (9.0%) compared with girls (8.2%). Soil transmitted helminths The number of children with detectable helminth infestation was low. Twenty-six children from 13 schools had hookworm detected from stool, none of which were high intensity. Eighteen children from nine different schools had ascaris eggs detected and no trichuris eggs were detected at all. There was no obvious pattern of co-infection with schistosomiasis and soil-transmitted helminths across the schools sampled; in 16 schools there was concordance with either both or neither type of infection detected; in 14 schools there was no concordance. The prevalence of STHs was much lower than that of schistosomiasis (Table  1 ). Only 1.8% [95% C I 0.6–3.1%] of children had evidence of infection with any of hookworm, ascaris or trichuriasis on stool microscopy. There were no heavy intensity STH infestations detected.
Prevalence and intensity of infection
Schistosomiasis A total of 140 children from 19 different schools had  S. haematobium  detected by urine microscopy, of which 45 had high intensity infection. The prevalence in schools ranged between 0 (11 schools) and 43.1% (a school in the southern lowlands). Twenty one children from eight different schools had visible haematuria. Only 8 children from 2 different schools had  S. mansoni  detected by stool microscopy and none were high intensity infections. Overall, the national prevalence of  S. mansoni  was found to be 0.4%. The national prevalence of  S. haematobium  was found to be 6.9% (95% CI 1.9–11.9%) (Table  1 ). 2.5% of these were high intensity infections [range 0–23.3%]. There was a clear association between prevalence and intensity (r = 0.9, n = 30, p = <0.01); schools with high prevalence of  S. haematobium  also had high intensity of infection. There was a wide range of ages in standard 3, with mode of 10 years of age. The observed prevalence was higher in older children (see figure  3 ) and boys (9.0%) compared with girls (8.2%).
Schistosomiasis
A total of 140 children from 19 different schools had  S. haematobium  detected by urine microscopy, of which 45 had high intensity infection. The prevalence in schools ranged between 0 (11 schools) and 43.1% (a school in the southern lowlands). Twenty one children from eight different schools had visible haematuria. Only 8 children from 2 different schools had  S. mansoni  detected by stool microscopy and none were high intensity infections.S. haematobiumS. mansoniOverall, the national prevalence of  S. mansoni  was found to be 0.4%. The national prevalence of  S. haematobium  was found to be 6.9% (95% CI 1.9–11.9%) (Table  1 ). 2.5% of these were high intensity infections [range 0–23.3%]. There was a clear association between prevalence and intensity (r = 0.9, n = 30, p = <0.01); schools with high prevalence of  S. haematobium  also had high intensity of infection.S. mansoniS. haematobiumS. haematobiumThere was a wide range of ages in standard 3, with mode of 10 years of age. The observed prevalence was higher in older children (see figure  3 ) and boys (9.0%) compared with girls (8.2%).Soil transmitted helminths The number of children with detectable helminth infestation was low. Twenty-six children from 13 schools had hookworm detected from stool, none of which were high intensity. Eighteen children from nine different schools had ascaris eggs detected and no trichuris eggs were detected at all. There was no obvious pattern of co-infection with schistosomiasis and soil-transmitted helminths across the schools sampled; in 16 schools there was concordance with either both or neither type of infection detected; in 14 schools there was no concordance. The prevalence of STHs was much lower than that of schistosomiasis (Table  1 ). Only 1.8% [95% C I 0.6–3.1%] of children had evidence of infection with any of hookworm, ascaris or trichuriasis on stool microscopy. There were no heavy intensity STH infestations detected.
Soil transmitted helminths
The number of children with detectable helminth infestation was low. Twenty-six children from 13 schools had hookworm detected from stool, none of which were high intensity. Eighteen children from nine different schools had ascaris eggs detected and no trichuris eggs were detected at all. There was no obvious pattern of co-infection with schistosomiasis and soil-transmitted helminths across the schools sampled; in 16 schools there was concordance with either both or neither type of infection detected; in 14 schools there was no concordance.The prevalence of STHs was much lower than that of schistosomiasis (Table  1 ). Only 1.8% [95% C I 0.6–3.1%] of children had evidence of infection with any of hookworm, ascaris or trichuriasis on stool microscopy. There were no heavy intensity STH infestations detected.Prevalence and intensity of infection by ecological region Schistosomiasis S. haematobium  infection was detected in all ecological areas (Table  2 ). The highest rates were in the Southern Lowland (23.2%) and the Central lowland (9.5%); the other four areas examined all showed a prevalence of between 2% and 7.4%. There was a wide range in the prevalence of  S. haematobium  infection in all areas.  S. mansoni  was rarely detected, and only found in the Southern Highlands (1.3%). To assess the sampling method used to select schools we classified the selected schools into high and low expected prevalence of  S. haematobium  based on ecology and known previous prevalence and compared our survey results with these expectations. Of the 12 expected high prevalence schools seven were found to be high and 5 unexpectedly low. Of the 18 expected low prevalence schools 12 were found to be low and 6 unexpectedly high. Unexpected results were found in the expected low prevalence as much as in the expected high prevalence schools using the McNemar test for symmetry actual (p = 0.29). We also compared the historical results for Malawi districts compiled by the London School of Hygiene and Tropical Medicine for the Schistosomiasis Control Initiative in 2001 (personal communication with Simon Brooker) with our results. Prevalence has fallen overall by 24% (95% confidence intervals of 16–32%). A formal paired t-test finds this reduction is true for low as well as high prevalence schools (t = 6.4 with 29 degrees of freedom and highly statistically significant).
Prevalence and intensity of infection by ecological region
Schistosomiasis S. haematobium  infection was detected in all ecological areas (Table  2 ). The highest rates were in the Southern Lowland (23.2%) and the Central lowland (9.5%); the other four areas examined all showed a prevalence of between 2% and 7.4%. There was a wide range in the prevalence of  S. haematobium  infection in all areas.  S. mansoni  was rarely detected, and only found in the Southern Highlands (1.3%). To assess the sampling method used to select schools we classified the selected schools into high and low expected prevalence of  S. haematobium  based on ecology and known previous prevalence and compared our survey results with these expectations. Of the 12 expected high prevalence schools seven were found to be high and 5 unexpectedly low. Of the 18 expected low prevalence schools 12 were found to be low and 6 unexpectedly high. Unexpected results were found in the expected low prevalence as much as in the expected high prevalence schools using the McNemar test for symmetry actual (p = 0.29). We also compared the historical results for Malawi districts compiled by the London School of Hygiene and Tropical Medicine for the Schistosomiasis Control Initiative in 2001 (personal communication with Simon Brooker) with our results. Prevalence has fallen overall by 24% (95% confidence intervals of 16–32%). A formal paired t-test finds this reduction is true for low as well as high prevalence schools (t = 6.4 with 29 degrees of freedom and highly statistically significant).
Schistosomiasis
S. haematobium  infection was detected in all ecological areas (Table  2 ). The highest rates were in the Southern Lowland (23.2%) and the Central lowland (9.5%); the other four areas examined all showed a prevalence of between 2% and 7.4%. There was a wide range in the prevalence of  S. haematobium  infection in all areas.  S. mansoni  was rarely detected, and only found in the Southern Highlands (1.3%).S. haematobiumS. haematobiumS. mansoniTo assess the sampling method used to select schools we classified the selected schools into high and low expected prevalence of  S. haematobium  based on ecology and known previous prevalence and compared our survey results with these expectations. Of the 12 expected high prevalence schools seven were found to be high and 5 unexpectedly low. Of the 18 expected low prevalence schools 12 were found to be low and 6 unexpectedly high. Unexpected results were found in the expected low prevalence as much as in the expected high prevalence schools using the McNemar test for symmetry actual (p = 0.29).S. haematobiumWe also compared the historical results for Malawi districts compiled by the London School of Hygiene and Tropical Medicine for the Schistosomiasis Control Initiative in 2001 (personal communication with Simon Brooker) with our results. Prevalence has fallen overall by 24% (95% confidence intervals of 16–32%). A formal paired t-test finds this reduction is true for low as well as high prevalence schools (t = 6.4 with 29 degrees of freedom and highly statistically significant).Soil transmitted helminths There was also a low prevalence of STHs in all areas, though urban areas had the highest rates for any STH (6.6%) and hookworm was most prevalent in the Northern Highland area (2.6%). There were no STHs detected in the Central lakeshore schools.
Soil transmitted helminths
There was also a low prevalence of STHs in all areas, though urban areas had the highest rates for any STH (6.6%) and hookworm was most prevalent in the Northern Highland area (2.6%). There were no STHs detected in the Central lakeshore schools.Questionnaire responses Total number of respondents was 1,662 (Table  3 ). It is possible to calculate the sensitivity and specificity of the "red urine" question in respect of the microscopic presence of  S. haematobium  (Table  4 ). The sensitivity of the "red urine" question in respect of  S. haematobium  being seen by microscopy is 67% and the specificity 80%. At a national average prevalence of 8.5% the "red urine" question has a positive predictive value of 24% and a negative predictive value of 96%. At the higher prevalence (22.5%) found in the Southern Lowlands zone the positive predictive value is 49% and the negative predictive value is 89%.
Questionnaire responses
Total number of respondents was 1,662 (Table  3 ). It is possible to calculate the sensitivity and specificity of the "red urine" question in respect of the microscopic presence of  S. haematobium  (Table  4 ). The sensitivity of the "red urine" question in respect of  S. haematobium  being seen by microscopy is 67% and the specificity 80%. At a national average prevalence of 8.5% the "red urine" question has a positive predictive value of 24% and a negative predictive value of 96%. At the higher prevalence (22.5%) found in the Southern Lowlands zone the positive predictive value is 49% and the negative predictive value is 89%.S. haematobiumS. haematobiumDiscussion Prevalence and distribution The overall prevalence of schistosomiasis is well below expectations. Based on previous studies in Malawi the overall prevalence of schistosomiasis was thought to be between 40% and 50% in the population. Of course these were surveys of selected populations, perhaps undertaken in the season of high transmission. Certainly the disease is localised in Malawi. In the study conducted in the northern lakeshore area in 2000 by Randall et al [ 8 ], schoolchildren from four schools were screened and there was a wide range of prevalence: 5%–57% on  S. haematobium  and 6%–42% in the case of  S. mansoni . The school located closest to rice fields had a high prevalence of schistosomiasis. In another recent survey carried out at the Cape McClear peninsula in the Southern Lake zone results showed markedly different  S. haematobium  and  S. mansoni  prevalence amongst adjacent villages situated on the lake shore and inland (Paul Bloch, personal communication). Our survey, representative of all schoolchildren in the country, and undertaken just before the rainy season, suggests far lower levels of 7% for  S. haematobium  and 0.4% for  S. mansoni . The finding highlights two important features of schistosomiasis in Malawi. Firstly, these infections are highly localised and secondly they are not common in general. As expected higher prevalence rates are found in the Southern lake/lowlands zone.  S. mansoni  does not seem to be a generalised problem in any particular ecological area in the country. The overall prevalence of STHs is lower than expected. The survey conducted in Karonga District (Randall et al., 2000 [ 8 ]) finds a higher prevalence of STHs as well as of  S. mansoni  and  S. haematobium . The differences could be due to seasonal variation (the survey in Karonga district was conducted after the rainy season). This possibility is supported by a low prevalence found in Karonga district in samples collected in November 2001, prior to the rains (unpublished observation, MA Perez). Supporting evidence is seen in the prevalence rates found in urban areas where rains would not affect transmission as much as in rural areas. Similar results are found in previous studies in urban Blantyre:  S. mansoni  (0%), Ascaris (18%) and Hookworm (0.4%) [ 7 ]. Another explanation for the higher prevalence rates found in Karonga District could be the different methodology used to process the stools. The Kato-Katz method [ 16 ] was used during our survey in order to calculate intensity of infection. Due to time and personnel constraints, only one Kato-Katz slide was read from each sample, and this could have affected the sensitivity of our testing [ 17 , 18 ]. On the other hand the survey in Karonga District was done using a commercial kit (Parasep ® , Intercep Ltd) in order to concentrate the stools and achieve high diagnostic sensitivity. Because of the dissimilar laboratory methods we have been unable to include results for the Northern Lakeland Ecological zone in the estimate of overall prevalences for the whole country. The northern lakeshore zone has tended to have high prevalences. The impact of omitting this zone is likely to underestimate the national results. We intend to survey the zone using our own laboratory methods when funds are available to complete the national picture. Although this survey is designed to be representative of this age group in Malawi, it is limited in scale by the budget ($12,000). This is an important limitation when one considers that the distribution of schistosomiasis in populations is likely to be highly localized [ 19 ]. The population sampled is representative of the general population of school children in standard three with respect to age and sex distribution. However, this survey does not represent children in Malawi who do not attend school or who were absent on the day of the survey due to illness, or who live in the northern lakeland zone. Despite the introduction of free primary education, the net enrolment rate in Malawi is 78%. There is little difference between poor and non-poor households in regard to the proportion of primary school-aged children in school [ 20 ]. However, poor children are likely to drop out before reaching standard 5, especially girls in rural areas. (Malawi poverty reduction strategy p.7). Children enrolled but absent due to sickness may have higher rates of schistosomiasis. In future if localised surveys in areas of low school enrolment are contemplated surveyors should consider including children who are not attending school or are absent due to illness. Drug treatment has been widely if intermittently available. The National Schistosomiasis Control Programme does not have documented evidence of where and when universal drug treatment has been offered to communities. Nor does it have details of where and when Praziquantel has been available for prescription to symptomatic individuals at health centres and hospitals. Supplies and universal treatment programmes have tended to depend on intermittent interest and funding from NGOs. However Praziquantel seems to have been widely available (Kayuni SA. National Survey to find out difficulties people face in taking regular anti-schistosomiasis drugs in Malawi. Fourth Year Student Project. College of Medicine, Blantyre, 2003) and may have had an effect. In this survey of 264 people living in high risk villages in eleven randomly selected districts 71% reported receiving anti-schistosomiasis treatment. Several NGOs such as Save the Children (US) in Mangochi and World Vision in Chikwawa in recent years have studied helminth infections in schoolchildren. Save the Children (US) surveyed and treated schoolchildren in the entire Mangochi District after finding a prevalence of  Ascaris lumbricoides  (1.9%), Hookworm (24%), and  S. mansoni  (16%) in four schools. The last survey done in 2000 showed an overall prevalence of  A. lumbricoides  (0.4%) and Hookworm (1.5%) on the four schools after two consecutive courses of treatment (Mary Mukaka, Save the Children, personal communication). Similar interventions may have had an effect on the prevalence of infections in other parts of the country. Another possible explanation for the unexpectedly low prevalence results could be due to a sampling method that selected a low preponderance of schools in high risk areas. However the results of the analysis looking at expected and actual prevalence found in each school suggest this did not happen. Unexpected results were found in the expected low prevalence as much as in the expected high prevalence schools using the McNemar test for symmetry actual (p = 0.29). The reason for the comparative low prevalence rate of  S. haematobium  now is not due to asymmetric sampling of low risk schools; it is due to a marked trend of a reduced prevalence across the board. It is not just the high risk schools that have lower prevalence now. Low risk schools have lower rates as well. Compared to the historical results for Malawi districts compiled by the London School of Hygiene and Tropical Medicine for the Schistosomiasis Control Initiative in 2001, prevalence has fallen overall by 24%, and this reduction is true for low as well as high prevalence schools. Our interpretation is that the results reflect a widespread reduction in schistosomiasis prevalence and not a biased sample in the current survey which inadvertently missed schools in high risk areas. However bias may still be an explanation for the change. The widespread reduction in prevalence could be due to bias in the selection of high risk schools in the historic studies rather (than in the selection of low risk schools in our survey). Our sampling fraction of 30 of a possible 3,900 schools is sufficient to give statistical confidence in our overall prevalence result of 6.9% (95% CI 1.9 – 11.9%) for  S. haematobium  so long as our schools and Traditional Authorities were chosen randomly, which they were. Questionnaire results Of the questions asked, only the red urine question is sufficiently sensitive and specific to be used as a screening test. It is a cheap way of assessing schistosomiasis prevalence and as such may be useful to identify hot spots within districts. Implications for the control programme Past control programmes, despite their fragmentary application in Malawi, seem to have achieved much in recent years. High intensity infections are now rare, and it is of course these that are thought to cause morbidity. There remain some high prevalence locations but these are uncommon now. The survey findings suggest that future approaches to control can focus on the identification of hot spots through local surveys. These should be undertaken in districts in high prevalence ecological zones, probably through "red urine" questionnaire surveys of school children. Because of the low positive predictive value of the "red urine" question, a low threshold should be used to categorise each school. The categorisation as proposed in Table  5  takes this into account and provides a simple way to identify those schools that require a urine microscopy survey.
Discussion
Prevalence and distribution The overall prevalence of schistosomiasis is well below expectations. Based on previous studies in Malawi the overall prevalence of schistosomiasis was thought to be between 40% and 50% in the population. Of course these were surveys of selected populations, perhaps undertaken in the season of high transmission. Certainly the disease is localised in Malawi. In the study conducted in the northern lakeshore area in 2000 by Randall et al [ 8 ], schoolchildren from four schools were screened and there was a wide range of prevalence: 5%–57% on  S. haematobium  and 6%–42% in the case of  S. mansoni . The school located closest to rice fields had a high prevalence of schistosomiasis. In another recent survey carried out at the Cape McClear peninsula in the Southern Lake zone results showed markedly different  S. haematobium  and  S. mansoni  prevalence amongst adjacent villages situated on the lake shore and inland (Paul Bloch, personal communication). Our survey, representative of all schoolchildren in the country, and undertaken just before the rainy season, suggests far lower levels of 7% for  S. haematobium  and 0.4% for  S. mansoni . The finding highlights two important features of schistosomiasis in Malawi. Firstly, these infections are highly localised and secondly they are not common in general. As expected higher prevalence rates are found in the Southern lake/lowlands zone.  S. mansoni  does not seem to be a generalised problem in any particular ecological area in the country. The overall prevalence of STHs is lower than expected. The survey conducted in Karonga District (Randall et al., 2000 [ 8 ]) finds a higher prevalence of STHs as well as of  S. mansoni  and  S. haematobium . The differences could be due to seasonal variation (the survey in Karonga district was conducted after the rainy season). This possibility is supported by a low prevalence found in Karonga district in samples collected in November 2001, prior to the rains (unpublished observation, MA Perez). Supporting evidence is seen in the prevalence rates found in urban areas where rains would not affect transmission as much as in rural areas. Similar results are found in previous studies in urban Blantyre:  S. mansoni  (0%), Ascaris (18%) and Hookworm (0.4%) [ 7 ]. Another explanation for the higher prevalence rates found in Karonga District could be the different methodology used to process the stools. The Kato-Katz method [ 16 ] was used during our survey in order to calculate intensity of infection. Due to time and personnel constraints, only one Kato-Katz slide was read from each sample, and this could have affected the sensitivity of our testing [ 17 , 18 ]. On the other hand the survey in Karonga District was done using a commercial kit (Parasep ® , Intercep Ltd) in order to concentrate the stools and achieve high diagnostic sensitivity. Because of the dissimilar laboratory methods we have been unable to include results for the Northern Lakeland Ecological zone in the estimate of overall prevalences for the whole country. The northern lakeshore zone has tended to have high prevalences. The impact of omitting this zone is likely to underestimate the national results. We intend to survey the zone using our own laboratory methods when funds are available to complete the national picture. Although this survey is designed to be representative of this age group in Malawi, it is limited in scale by the budget ($12,000). This is an important limitation when one considers that the distribution of schistosomiasis in populations is likely to be highly localized [ 19 ]. The population sampled is representative of the general population of school children in standard three with respect to age and sex distribution. However, this survey does not represent children in Malawi who do not attend school or who were absent on the day of the survey due to illness, or who live in the northern lakeland zone. Despite the introduction of free primary education, the net enrolment rate in Malawi is 78%. There is little difference between poor and non-poor households in regard to the proportion of primary school-aged children in school [ 20 ]. However, poor children are likely to drop out before reaching standard 5, especially girls in rural areas. (Malawi poverty reduction strategy p.7). Children enrolled but absent due to sickness may have higher rates of schistosomiasis. In future if localised surveys in areas of low school enrolment are contemplated surveyors should consider including children who are not attending school or are absent due to illness. Drug treatment has been widely if intermittently available. The National Schistosomiasis Control Programme does not have documented evidence of where and when universal drug treatment has been offered to communities. Nor does it have details of where and when Praziquantel has been available for prescription to symptomatic individuals at health centres and hospitals. Supplies and universal treatment programmes have tended to depend on intermittent interest and funding from NGOs. However Praziquantel seems to have been widely available (Kayuni SA. National Survey to find out difficulties people face in taking regular anti-schistosomiasis drugs in Malawi. Fourth Year Student Project. College of Medicine, Blantyre, 2003) and may have had an effect. In this survey of 264 people living in high risk villages in eleven randomly selected districts 71% reported receiving anti-schistosomiasis treatment. Several NGOs such as Save the Children (US) in Mangochi and World Vision in Chikwawa in recent years have studied helminth infections in schoolchildren. Save the Children (US) surveyed and treated schoolchildren in the entire Mangochi District after finding a prevalence of  Ascaris lumbricoides  (1.9%), Hookworm (24%), and  S. mansoni  (16%) in four schools. The last survey done in 2000 showed an overall prevalence of  A. lumbricoides  (0.4%) and Hookworm (1.5%) on the four schools after two consecutive courses of treatment (Mary Mukaka, Save the Children, personal communication). Similar interventions may have had an effect on the prevalence of infections in other parts of the country. Another possible explanation for the unexpectedly low prevalence results could be due to a sampling method that selected a low preponderance of schools in high risk areas. However the results of the analysis looking at expected and actual prevalence found in each school suggest this did not happen. Unexpected results were found in the expected low prevalence as much as in the expected high prevalence schools using the McNemar test for symmetry actual (p = 0.29). The reason for the comparative low prevalence rate of  S. haematobium  now is not due to asymmetric sampling of low risk schools; it is due to a marked trend of a reduced prevalence across the board. It is not just the high risk schools that have lower prevalence now. Low risk schools have lower rates as well. Compared to the historical results for Malawi districts compiled by the London School of Hygiene and Tropical Medicine for the Schistosomiasis Control Initiative in 2001, prevalence has fallen overall by 24%, and this reduction is true for low as well as high prevalence schools. Our interpretation is that the results reflect a widespread reduction in schistosomiasis prevalence and not a biased sample in the current survey which inadvertently missed schools in high risk areas. However bias may still be an explanation for the change. The widespread reduction in prevalence could be due to bias in the selection of high risk schools in the historic studies rather (than in the selection of low risk schools in our survey). Our sampling fraction of 30 of a possible 3,900 schools is sufficient to give statistical confidence in our overall prevalence result of 6.9% (95% CI 1.9 – 11.9%) for  S. haematobium  so long as our schools and Traditional Authorities were chosen randomly, which they were.
Prevalence and distribution
The overall prevalence of schistosomiasis is well below expectations. Based on previous studies in Malawi the overall prevalence of schistosomiasis was thought to be between 40% and 50% in the population. Of course these were surveys of selected populations, perhaps undertaken in the season of high transmission. Certainly the disease is localised in Malawi. In the study conducted in the northern lakeshore area in 2000 by Randall et al [ 8 ], schoolchildren from four schools were screened and there was a wide range of prevalence: 5%–57% on  S. haematobium  and 6%–42% in the case of  S. mansoni . The school located closest to rice fields had a high prevalence of schistosomiasis. In another recent survey carried out at the Cape McClear peninsula in the Southern Lake zone results showed markedly different  S. haematobium  and  S. mansoni  prevalence amongst adjacent villages situated on the lake shore and inland (Paul Bloch, personal communication).S. haematobiumS. mansoniS. haematobiumS. mansoniOur survey, representative of all schoolchildren in the country, and undertaken just before the rainy season, suggests far lower levels of 7% for  S. haematobium  and 0.4% for  S. mansoni . The finding highlights two important features of schistosomiasis in Malawi. Firstly, these infections are highly localised and secondly they are not common in general. As expected higher prevalence rates are found in the Southern lake/lowlands zone.  S. mansoni  does not seem to be a generalised problem in any particular ecological area in the country.S. haematobiumS. mansoniS. mansoniThe overall prevalence of STHs is lower than expected. The survey conducted in Karonga District (Randall et al., 2000 [ 8 ]) finds a higher prevalence of STHs as well as of  S. mansoni  and  S. haematobium . The differences could be due to seasonal variation (the survey in Karonga district was conducted after the rainy season). This possibility is supported by a low prevalence found in Karonga district in samples collected in November 2001, prior to the rains (unpublished observation, MA Perez). Supporting evidence is seen in the prevalence rates found in urban areas where rains would not affect transmission as much as in rural areas. Similar results are found in previous studies in urban Blantyre:  S. mansoni  (0%), Ascaris (18%) and Hookworm (0.4%) [ 7 ]. Another explanation for the higher prevalence rates found in Karonga District could be the different methodology used to process the stools. The Kato-Katz method [ 16 ] was used during our survey in order to calculate intensity of infection. Due to time and personnel constraints, only one Kato-Katz slide was read from each sample, and this could have affected the sensitivity of our testing [ 17 , 18 ]. On the other hand the survey in Karonga District was done using a commercial kit (Parasep ® , Intercep Ltd) in order to concentrate the stools and achieve high diagnostic sensitivity. Because of the dissimilar laboratory methods we have been unable to include results for the Northern Lakeland Ecological zone in the estimate of overall prevalences for the whole country. The northern lakeshore zone has tended to have high prevalences. The impact of omitting this zone is likely to underestimate the national results. We intend to survey the zone using our own laboratory methods when funds are available to complete the national picture.S. mansoniS. haematobiumS. mansoniAlthough this survey is designed to be representative of this age group in Malawi, it is limited in scale by the budget ($12,000). This is an important limitation when one considers that the distribution of schistosomiasis in populations is likely to be highly localized [ 19 ]. The population sampled is representative of the general population of school children in standard three with respect to age and sex distribution. However, this survey does not represent children in Malawi who do not attend school or who were absent on the day of the survey due to illness, or who live in the northern lakeland zone. Despite the introduction of free primary education, the net enrolment rate in Malawi is 78%. There is little difference between poor and non-poor households in regard to the proportion of primary school-aged children in school [ 20 ]. However, poor children are likely to drop out before reaching standard 5, especially girls in rural areas. (Malawi poverty reduction strategy p.7). Children enrolled but absent due to sickness may have higher rates of schistosomiasis. In future if localised surveys in areas of low school enrolment are contemplated surveyors should consider including children who are not attending school or are absent due to illness.Drug treatment has been widely if intermittently available. The National Schistosomiasis Control Programme does not have documented evidence of where and when universal drug treatment has been offered to communities. Nor does it have details of where and when Praziquantel has been available for prescription to symptomatic individuals at health centres and hospitals. Supplies and universal treatment programmes have tended to depend on intermittent interest and funding from NGOs. However Praziquantel seems to have been widely available (Kayuni SA. National Survey to find out difficulties people face in taking regular anti-schistosomiasis drugs in Malawi. Fourth Year Student Project. College of Medicine, Blantyre, 2003) and may have had an effect. In this survey of 264 people living in high risk villages in eleven randomly selected districts 71% reported receiving anti-schistosomiasis treatment.Several NGOs such as Save the Children (US) in Mangochi and World Vision in Chikwawa in recent years have studied helminth infections in schoolchildren. Save the Children (US) surveyed and treated schoolchildren in the entire Mangochi District after finding a prevalence of  Ascaris lumbricoides  (1.9%), Hookworm (24%), and  S. mansoni  (16%) in four schools. The last survey done in 2000 showed an overall prevalence of  A. lumbricoides  (0.4%) and Hookworm (1.5%) on the four schools after two consecutive courses of treatment (Mary Mukaka, Save the Children, personal communication). Similar interventions may have had an effect on the prevalence of infections in other parts of the country.Ascaris lumbricoidesS. mansoniA. lumbricoidesAnother possible explanation for the unexpectedly low prevalence results could be due to a sampling method that selected a low preponderance of schools in high risk areas. However the results of the analysis looking at expected and actual prevalence found in each school suggest this did not happen. Unexpected results were found in the expected low prevalence as much as in the expected high prevalence schools using the McNemar test for symmetry actual (p = 0.29).The reason for the comparative low prevalence rate of  S. haematobium  now is not due to asymmetric sampling of low risk schools; it is due to a marked trend of a reduced prevalence across the board. It is not just the high risk schools that have lower prevalence now. Low risk schools have lower rates as well. Compared to the historical results for Malawi districts compiled by the London School of Hygiene and Tropical Medicine for the Schistosomiasis Control Initiative in 2001, prevalence has fallen overall by 24%, and this reduction is true for low as well as high prevalence schools. Our interpretation is that the results reflect a widespread reduction in schistosomiasis prevalence and not a biased sample in the current survey which inadvertently missed schools in high risk areas. However bias may still be an explanation for the change. The widespread reduction in prevalence could be due to bias in the selection of high risk schools in the historic studies rather (than in the selection of low risk schools in our survey). Our sampling fraction of 30 of a possible 3,900 schools is sufficient to give statistical confidence in our overall prevalence result of 6.9% (95% CI 1.9 – 11.9%) for  S. haematobium  so long as our schools and Traditional Authorities were chosen randomly, which they were.S. haematobiumS. haematobiumQuestionnaire results Of the questions asked, only the red urine question is sufficiently sensitive and specific to be used as a screening test. It is a cheap way of assessing schistosomiasis prevalence and as such may be useful to identify hot spots within districts.
Questionnaire results
Of the questions asked, only the red urine question is sufficiently sensitive and specific to be used as a screening test. It is a cheap way of assessing schistosomiasis prevalence and as such may be useful to identify hot spots within districts.Implications for the control programme Past control programmes, despite their fragmentary application in Malawi, seem to have achieved much in recent years. High intensity infections are now rare, and it is of course these that are thought to cause morbidity. There remain some high prevalence locations but these are uncommon now. The survey findings suggest that future approaches to control can focus on the identification of hot spots through local surveys. These should be undertaken in districts in high prevalence ecological zones, probably through "red urine" questionnaire surveys of school children. Because of the low positive predictive value of the "red urine" question, a low threshold should be used to categorise each school. The categorisation as proposed in Table  5  takes this into account and provides a simple way to identify those schools that require a urine microscopy survey.
Implications for the control programme
Past control programmes, despite their fragmentary application in Malawi, seem to have achieved much in recent years. High intensity infections are now rare, and it is of course these that are thought to cause morbidity. There remain some high prevalence locations but these are uncommon now. The survey findings suggest that future approaches to control can focus on the identification of hot spots through local surveys. These should be undertaken in districts in high prevalence ecological zones, probably through "red urine" questionnaire surveys of school children. Because of the low positive predictive value of the "red urine" question, a low threshold should be used to categorise each school. The categorisation as proposed in Table  5  takes this into account and provides a simple way to identify those schools that require a urine microscopy survey.Conclusions The highly effective drugs now available and the control methods used in Malawi seem to have reduced the prevalence of both schistosomiasis and soil helminths to low levels. There remain some localised areas of high prevalence for which local control measures will be required. It seems that the epidemiology of these diseases makes local control at district level or below the preferred public health approach.
Conclusions
The highly effective drugs now available and the control methods used in Malawi seem to have reduced the prevalence of both schistosomiasis and soil helminths to low levels.There remain some localised areas of high prevalence for which local control measures will be required. It seems that the epidemiology of these diseases makes local control at district level or below the preferred public health approach.Competing interests The author(s) declare that they have no competing interests.
Competing interests
The author(s) declare that they have no competing interests.Authors' contributions BS conceived the survey and provided logistical advice. CB designed the survey and undertook the final analysis and first drafting. BP supervised the survey and undertook the initial analysis. PM undertook the randomization and led the survey team. MP supervised the quality control of the laboratory work. All contributed to the final report.
Authors' contributions
BS conceived the survey and provided logistical advice. CB designed the survey and undertook the final analysis and first drafting. BP supervised the survey and undertook the initial analysis. PM undertook the randomization and led the survey team. MP supervised the quality control of the laboratory work. All contributed to the final report.Pre-publication history The pre-publication history for this paper can be accessed here:
Pre-publication history
The pre-publication history for this paper can be accessed here:Supplementary Material Additional File 1 questionnaire; the questionnaire in Chichewa used in the schools to find out about the health of the pupils surveyed. Click here for file
Supplementary Material
Additional File 1
questionnaire; the questionnaire in Chichewa used in the schools to find out about the health of the pupils surveyed.Click here for file


Background The embryonic and larval peripheral nervous system of  Drosophila melanogaster  is extensively studied as a very powerful model of developmental biology. One main advantage of this system is the ability to study the origin and development of individual sensory cells. However, there remain several discrepancies regarding the organization of sensory organs in each abdominal segment A1-A7.
Background
The embryonic and larval peripheral nervous system of  Drosophila melanogaster  is extensively studied as a very powerful model of developmental biology. One main advantage of this system is the ability to study the origin and development of individual sensory cells. However, there remain several discrepancies regarding the organization of sensory organs in each abdominal segment A1-A7.Drosophila melanogasterDescription We have constructed a web site called FlyPNS (for Fly Peripheral Nervous System) that consolidates a wide range of published and unpublished information about the embryonic and larval sensory organs. It communicates (1) a PNS pattern that solves the discrepancies that have been found in the recent literature, (2) the correspondence between the different nomenclatures that have been used so far, (3) a comprehensive description of each sensory organ, and (4) a list of both published and unpublished markers to reliably identify each PNS cell.
Description
We have constructed a web site called FlyPNS (for Fly Peripheral Nervous System) that consolidates a wide range of published and unpublished information about the embryonic and larval sensory organs. It communicates (1) a PNS pattern that solves the discrepancies that have been found in the recent literature, (2) the correspondence between the different nomenclatures that have been used so far, (3) a comprehensive description of each sensory organ, and (4) a list of both published and unpublished markers to reliably identify each PNS cell.Conclusions The FlyPNS database integrates disparate data and nomenclature and thus helps understanding the conflicting observations that have been published recently. Furthermore, it is designed to provide assistance in the identification and study of individual sensory cells. We think it will be a useful resource for any researcher with interest in  Drosophila  sensory organs.
Conclusions
The FlyPNS database integrates disparate data and nomenclature and thus helps understanding the conflicting observations that have been published recently. Furthermore, it is designed to provide assistance in the identification and study of individual sensory cells. We think it will be a useful resource for any researcher with interest in  Drosophila  sensory organs.Drosophila

Background The  Drosophila  abdominal larval PNS is composed of a constant number of neurons and associated cells, whose characteristics and positions are reproducible between individuals [ 1 - 5 ]. Its stereotyped pattern has made it an ideal system to study many aspects of developmental biology such as cell determination, asymmetric cell divisions, cell lineages, development and remodeling of axons and dendrites, cell migration, and cell death. Despite the extraordinary utility of the PNS for understanding these diverse problems, a prominent current limitation is the lack of agreement in the literature of the exact number, position and nomenclature of sensory neurons. For example, many different names have been used to describe abdominal neurons, reflecting the number of cells within a cluster, their position [ 6 , 7 ], their dendritic arborization [ 8 , 19 ] or their cell lineage origin [ 9 ]; and the correspondence between names of cells has remained vague. Furthermore, there has been no consensus on the exact number of neurons located in the dorsal cluster, which is one of the most extensively studied regions of the PNS. First studies indicated that 10 [ 4 ], 11 [ 2 ], or 12 [ 4 , 6 ] neurons are present. Following these studies, this cluster was usually thought to contain 12 neurons. A single compilation of the different nomenclatures that accounts for all peripheral neurons would not only benefit those within the field, but should also make it easier for researchers outside of the immediate field to comprehend the literature.
Background
The  Drosophila  abdominal larval PNS is composed of a constant number of neurons and associated cells, whose characteristics and positions are reproducible between individuals [ 1 - 5 ]. Its stereotyped pattern has made it an ideal system to study many aspects of developmental biology such as cell determination, asymmetric cell divisions, cell lineages, development and remodeling of axons and dendrites, cell migration, and cell death.DrosophilaDespite the extraordinary utility of the PNS for understanding these diverse problems, a prominent current limitation is the lack of agreement in the literature of the exact number, position and nomenclature of sensory neurons. For example, many different names have been used to describe abdominal neurons, reflecting the number of cells within a cluster, their position [ 6 , 7 ], their dendritic arborization [ 8 , 19 ] or their cell lineage origin [ 9 ]; and the correspondence between names of cells has remained vague. Furthermore, there has been no consensus on the exact number of neurons located in the dorsal cluster, which is one of the most extensively studied regions of the PNS. First studies indicated that 10 [ 4 ], 11 [ 2 ], or 12 [ 4 , 6 ] neurons are present. Following these studies, this cluster was usually thought to contain 12 neurons. A single compilation of the different nomenclatures that accounts for all peripheral neurons would not only benefit those within the field, but should also make it easier for researchers outside of the immediate field to comprehend the literature.Construction and content We have constructed a website, named FlyPNS, that consolidates a wide range of published and unpublished information regarding the embryonic and larval sensory organs and their associated glial cells. Motoneurons and other glial cells have not been included. The FlyPNS web site is arranged in 6 sections: General references, Nomenclature, Abdominal PNS organization, Sensory organ description, Antibodies and enhancer-traps, Gal4 lines, and a Search option. The  General references  section provides a list of papers on various aspects of sensory organ morphology and development. Subcategories include general descriptions, descriptions of dendritic arborizations, axonal pathways, developmental changes, and functions of multidendritic neurons. Links are provided to PubMed entries for access to abstracts or full-texts of papers. In the  Abdominal PNS organization  section, we have represented the PNS pattern that we have observed (Fig.  1 ), with the notable presence of 13 neurons in the dorsal cluster (as also mentioned recently in [ 10 , 11 ]). In previous representations that indicated a smaller number of cells in the cluster, either the anterior-located Cut-negative neuron that we have named dmd1 (Fig.  1 ) or one of the Cut-positive multidendritic neurons may have been missed. Since we found that the precise position of sensory cells may vary with segment, embryo and developmental stage (unpublished data), the most reliable criteria for identification of  Drosophila  embryonic and larval sensory organs are marker expression and cell morphology, and these should be used whenever possible. Our study of the cell markers Cut, Collier, E7-2-36, E7-3-49, Elav, Engrailed and Nubbin/Pdm1 [ 11 - 13 ] and unpublished data), as well as neuron morphology [ 8 , 13 ], revealed that distinctive features can be attributed to each neuron, thereby allowing the unambiguous identification of every PNS neuron. The sections entitled Sensory organ description, Antibodies and enhancer-traps, and Gal4 lines relate these observations. It should also be noted that the precise geometry of the PNS pattern presented in the web site might vary somewhat depending on the stage examined and the techniques used to dissect and mount the preparation. Figure 1 Diagram of the sensory cells of an embryonic or larval abdominal hemisegment (A1-A7) . Three types of sensory organs are found: (1) external sensory organs, composed of three accessory cells (oval shape) and one or several neurons (circular shape), (2) chordotonal organs, composed of accessory cells (oval shape) and neurons (elongated triangular shape), (3) multidendritic neurons (diamond shape). The most conspicuous sensory dendrites are represented as a straight line. Note that the dmd1/dda1 neuron presents a unique dendritic arborization different from the other da neurons. Anterior is left and dorsal is up. The section on  Nomenclature , presented in table form, lists all nomenclatures that have been used in the literature, drawing explicit connections between the various names wherever possible (Table  1 ). This table is aimed to resolve inconsistencies in the literature, especially when particular cells have been omitted or subtle denominations interchanged. Many different names have been used for both the external sensory organs and the multidendritic neurons. The multidendritic neurons have been classified and named based on their gross morphology and the substrate upon which they extend their dendrites [ 14 ]. They were therefore called tracheal dendrite (md-td), bipolar dendrite (md-bd) and dendritic arborization (md-da) neurons. In recent papers, the "md" has often been dropped and they have been referred to as td, bd, or da neurons. Whereas the bd and td neurons usually reside singly and their identification is usually not problematic, the da neurons are found grouped as clusters of cells and it has been more difficult to assign individual identities. Early papers named these clusters according to the number of da neurons they contain. The dorsal cluster was therefore called dmd5 (for dorsal multidendritic cluster of 5) [ 15 ] or dmd6 [ 6 ]. Merritt and Whitington were the first to name nearly all da neurons as individuals [ 7 ] by providing each with an alphabetic designation reflecting their ventral-to-dorsal position. However, their dorsal cluster contained only 5 da neurons, and since we now know that this cluster typically contains an additional, sixth da neuron, it has not been possible to assign their specific names to the cluster. When Grueber et al. characterized the morphology of all da neurons [ 8 ], and Sweeney et al. the morphology of the dorsal cluster neurons [ 19 ], each group chose to assign names in the same vein as Merritt and Whitington. However, the typical relative positions of some neurons were not resolved because specific markers were lacking. Cut antibodies were subsequently used to discriminate the different neurons, and their typical relative positions were established [ 13 ]. Table 1 Correspondence between the nomenclatures previously used to name sensory organs or neurons. [5] [16] [3] (names es organs) [4] (names es and ch neurons) [6] (names es, ch and md neurons) [7] (names es, ch and md neurons) [1] (names sensory organs) [8] (names md da neurons) other (Only the first paper using this other nomenclature is indicated) external sensory (es) organs - - p1 vesA vesA vesA vp1 - vc1 [17] - - p2 vesB vesB vesB vp2 - vc2 [17] - - p3 vesC vesC vesC vp3 - vc3 [17] - - p4 v'esA v'esA v'esA vp4 - vc4 [17] - - p5 v'esB v'esB v'esB vp4a - vc4a [17] - - p6 v'es2 v'es2 v'es2 vp5 - vc5 [17] b sensory hair C h1 lesA lesA lesA lh2 - - - - p7 lesB lesB lesB lp2, lc1 - - H sensory hair C h2 lesC lesC lesC lh1 - - b+st sensory hair F h4 desB des2 desA2 dh2 - h3 desA desB desB dh1 - - s - p8 desC desC desC dp1, dc1 - - b - p9 desD desD desD dp2, dc2 - - chordotonal (ch) organs - - - vchA vchA vchA vch1 - - - - - vchB vchB vchB vch2 - - - - - lch5 lch5 lch5 lch5 - - - - - v'ch1 v'ch1 v'ch1 lch1 - - multidendritic (md) neurons - - - - vbd vbd - vmd3 [9] - - - - vdaC vdaC vmd2 [9] - - - - vdaA vdaD vmd1 [9] - - - - vmd5 vdaD vdaA-D vdaA vmd4 [9] - - - - vdaB vdaB vmd1a [9] - - - - vpda vpda vdap vpda - - - - - v'ada v'ada vdaa v'ada vmd4a [9] - - - - - v'td1 [this paper] - - - - v'td2 v'td2 vtd1/2 - v'td2 [this paper] - - - - v'pda v'pda v'dap v'pda - - - - - ldaA ldaA lda ldaA - - - - - lbd - isbp - ldb, lbd [18] - - - - ltd ltd istd - - - - - - ldaB ldaB ltd ldaB - - - - - dbd dbd dbp - - - - - - ddaA ddaA the ddaA-B-C-D-E cluster has also been called - - - - ddaB or ddaC? ddaA-E ddaB dmd5 [15] - - - - dmd6 ddaB? ddaC - - - - - ddaD ddaF [19] - - - - ddaD? ddaE - - - - ddaD? ddaF ddaD [19] - - - - - ddaE - - dda1 [18] dmd1 [13] Each nomenclature presented in Table  1  has its own advantages, and no single nomenclature is yet uniformly agreed upon by all researchers. The nomenclature presented on some FlyPNS pages, such as the "Sensory organ description" page, simply reflects the names that we use in our own work and that have also been found in several recent publications. We hope and think that with time researchers will agree on a common constructive nomenclature that also allows wide comprehension of the wealth of data already available on PNS development. The  Sensory organ description  section gives an extensive description for each individual sensory organ: position, name(s), markers, morphology, development and related references. A few unpublished observations relating to maker expression and morphology are also mentioned. A typical sensory organ description is shown in Fig.  2 . Links to other sensory organ descriptions, antibodies, enhancer-trap markers and references are available. Clicking on the various cells depicted in the adjacent diagram also allows navigating between sensory organ descriptions. The extensive cross-linkage of the various pages of the web site is designed to facilitate the comprehension of the available data. Figure 2 Description of the sensory organ v'ada . Links to other sensory organ descriptions, antibodies, enhancer-trap markers and references are shown. Clicking on a cell depicted in the diagram links to its sensory organ description. The  Antibodies, enhancer-traps and Gal4 lines  pages display a collection of published and unpublished data on expression patterns of antibodies, enhancer-traps and Gal4 lines, with links to the gene/insertion information contained in Flybase when available. Finally, the  Search  option provides a convenient link to the information about any of the organs/markers.
Construction and content
We have constructed a website, named FlyPNS, that consolidates a wide range of published and unpublished information regarding the embryonic and larval sensory organs and their associated glial cells. Motoneurons and other glial cells have not been included. The FlyPNS web site is arranged in 6 sections: General references, Nomenclature, Abdominal PNS organization, Sensory organ description, Antibodies and enhancer-traps, Gal4 lines, and a Search option.The  General references  section provides a list of papers on various aspects of sensory organ morphology and development. Subcategories include general descriptions, descriptions of dendritic arborizations, axonal pathways, developmental changes, and functions of multidendritic neurons. Links are provided to PubMed entries for access to abstracts or full-texts of papers.General referencesIn the  Abdominal PNS organization  section, we have represented the PNS pattern that we have observed (Fig.  1 ), with the notable presence of 13 neurons in the dorsal cluster (as also mentioned recently in [ 10 , 11 ]). In previous representations that indicated a smaller number of cells in the cluster, either the anterior-located Cut-negative neuron that we have named dmd1 (Fig.  1 ) or one of the Cut-positive multidendritic neurons may have been missed. Since we found that the precise position of sensory cells may vary with segment, embryo and developmental stage (unpublished data), the most reliable criteria for identification of  Drosophila  embryonic and larval sensory organs are marker expression and cell morphology, and these should be used whenever possible. Our study of the cell markers Cut, Collier, E7-2-36, E7-3-49, Elav, Engrailed and Nubbin/Pdm1 [ 11 - 13 ] and unpublished data), as well as neuron morphology [ 8 , 13 ], revealed that distinctive features can be attributed to each neuron, thereby allowing the unambiguous identification of every PNS neuron. The sections entitled Sensory organ description, Antibodies and enhancer-traps, and Gal4 lines relate these observations. It should also be noted that the precise geometry of the PNS pattern presented in the web site might vary somewhat depending on the stage examined and the techniques used to dissect and mount the preparation.Abdominal PNS organizationDrosophilaThe section on  Nomenclature , presented in table form, lists all nomenclatures that have been used in the literature, drawing explicit connections between the various names wherever possible (Table  1 ). This table is aimed to resolve inconsistencies in the literature, especially when particular cells have been omitted or subtle denominations interchanged. Many different names have been used for both the external sensory organs and the multidendritic neurons. The multidendritic neurons have been classified and named based on their gross morphology and the substrate upon which they extend their dendrites [ 14 ]. They were therefore called tracheal dendrite (md-td), bipolar dendrite (md-bd) and dendritic arborization (md-da) neurons. In recent papers, the "md" has often been dropped and they have been referred to as td, bd, or da neurons. Whereas the bd and td neurons usually reside singly and their identification is usually not problematic, the da neurons are found grouped as clusters of cells and it has been more difficult to assign individual identities. Early papers named these clusters according to the number of da neurons they contain. The dorsal cluster was therefore called dmd5 (for dorsal multidendritic cluster of 5) [ 15 ] or dmd6 [ 6 ]. Merritt and Whitington were the first to name nearly all da neurons as individuals [ 7 ] by providing each with an alphabetic designation reflecting their ventral-to-dorsal position. However, their dorsal cluster contained only 5 da neurons, and since we now know that this cluster typically contains an additional, sixth da neuron, it has not been possible to assign their specific names to the cluster. When Grueber et al. characterized the morphology of all da neurons [ 8 ], and Sweeney et al. the morphology of the dorsal cluster neurons [ 19 ], each group chose to assign names in the same vein as Merritt and Whitington. However, the typical relative positions of some neurons were not resolved because specific markers were lacking. Cut antibodies were subsequently used to discriminate the different neurons, and their typical relative positions were established [ 13 ].NomenclatureCorrespondence between the nomenclatures previously used to name sensory organs or neurons.[5][16][3] (names es organs)[4] (names es and ch neurons)[6] (names es, ch and md neurons)[7] (names es, ch and md neurons)[1] (names sensory organs)[8] (names md da neurons)other (Only the first paper using this other nomenclature is indicated)external sensory (es) organschordotonal (ch) organsmultidendritic (md) neuronsEach nomenclature presented in Table  1  has its own advantages, and no single nomenclature is yet uniformly agreed upon by all researchers. The nomenclature presented on some FlyPNS pages, such as the "Sensory organ description" page, simply reflects the names that we use in our own work and that have also been found in several recent publications. We hope and think that with time researchers will agree on a common constructive nomenclature that also allows wide comprehension of the wealth of data already available on PNS development.The  Sensory organ description  section gives an extensive description for each individual sensory organ: position, name(s), markers, morphology, development and related references. A few unpublished observations relating to maker expression and morphology are also mentioned. A typical sensory organ description is shown in Fig.  2 . Links to other sensory organ descriptions, antibodies, enhancer-trap markers and references are available. Clicking on the various cells depicted in the adjacent diagram also allows navigating between sensory organ descriptions. The extensive cross-linkage of the various pages of the web site is designed to facilitate the comprehension of the available data.Sensory organ descriptionThe  Antibodies, enhancer-traps and Gal4 lines  pages display a collection of published and unpublished data on expression patterns of antibodies, enhancer-traps and Gal4 lines, with links to the gene/insertion information contained in Flybase when available.Antibodies, enhancer-traps and Gal4 linesFinally, the  Search  option provides a convenient link to the information about any of the organs/markers.SearchUtility, discussion and conclusions Much of the discordance in the literature regarding the larval and embryonic PNS pattern seems to be rooted in variation in the precise position of sensory cells with segment, embryo, and time, and a lack of specific markers for some organs. These factors might explain why, for example, some dorsal cluster neurons were often missed in previous publications. We have attempted to resolve remaining ambiguities by (1) identifying the total number of sense organs and neurons in each abdominal segment A1-A7, (2) resolving different nomenclatures used for each organ, and (3) providing a collection of molecular markers and descriptions that provide an unambiguous guide for cell identification. This data compilation will hopefully also increase the accessibility of the fly PNS literature to non- Drosophila  investigators.
Utility, discussion and conclusions
Much of the discordance in the literature regarding the larval and embryonic PNS pattern seems to be rooted in variation in the precise position of sensory cells with segment, embryo, and time, and a lack of specific markers for some organs. These factors might explain why, for example, some dorsal cluster neurons were often missed in previous publications. We have attempted to resolve remaining ambiguities by (1) identifying the total number of sense organs and neurons in each abdominal segment A1-A7, (2) resolving different nomenclatures used for each organ, and (3) providing a collection of molecular markers and descriptions that provide an unambiguous guide for cell identification. This data compilation will hopefully also increase the accessibility of the fly PNS literature to non- Drosophila  investigators.DrosophilaAvailability The FlyPNS web site is available from the link under Miscellaneous in the FlyBase Allied Data section  . Comments and additions are welcomed by e-mail to VO.
Availability
The FlyPNS web site is available from the link under Miscellaneous in the FlyBase Allied Data section  . Comments and additions are welcomed by e-mail to VO.List of abbreviations PNS: peripheral nervous system da neuron: neuron displaying a dendritic arborization
List of abbreviations
PNS: peripheral nervous systemda neuron: neuron displaying a dendritic arborizationAuthors' contributions The PNS pattern was established based on the observations of WBG and VO. VO created the FlyPNS web site and WBG made significant improvements and corrections to the web site. Both VO and WBG wrote the manuscript.
Authors' contributions
The PNS pattern was established based on the observations of WBG and VO. VO created the FlyPNS web site and WBG made significant improvements and corrections to the web site. Both VO and WBG wrote the manuscript.