Spread of porcine circovirus associated disease (PCVAD) in Ontario (Canada) XXXX herds: Part I. Exploratory spatial analysis.

@entity18723 @entity1707 @entity22131 @entity23006
BACKGROUND: The systemic form of porcine circovirus associated disease (PCVAD), also known as @entity22131 ( @entity22131 ) was initially detected in the early 1990s. Starting in 2004, the Canadian @entity1707 industry experienced considerable losses due to PCVAD, concurrent with a shift in genotype of @entity18723 ( @entity18723 ). Objectives of the current study were to explore spatial characteristics of self-reported PCVAD distribution in Ontario between 2004 and 2008, and to investigate the existence and nature of local spread. RESULTS: The study included 278 @entity1707 herds from a large disease-monitoring project that included @entity23006 ( @entity23006 ) virus-positive herds identified by the diagnostic laboratory, and @entity23006 virus-negative herds directly from the target population. Herds were included if they had growing @entity1707 present on-site and available geographical coordinates for the sampling site. Furthermore, herds were defined as PCVAD-positive if a producer reported an outbreak of circovirus associated disease, or as PCVAD-negative if no outbreak was noted. Spatial trend was investigated using generalized additive models and time to PCVAD outbreak in a herd using Cox's proportional hazard model; spatial and spatio-temporal clustering was explored using K-functions; and location of most likely spatial and spatio-temporal clusters was investigated using scan statistics. Over the study period, the risk of reporting a PCVAD-positive herd tended to be higher in the eastern part of the province after adjustment for herd @entity23006 status (P = 0.05). This was partly confirmed for spread (Partial P < 0.01). Local spread also appeared to exist, as suggested by the tentative (P = 0.06) existence of spatio-temporal clustering of PCVAD and detection of a spatio-temporal cluster (P = 0.04). CONCLUSIONS: In Ontario, PCVAD has shown a general trend, spreading from east-to-west. We interpret the existence of spatio-temporal clustering as evidence of spatio-temporal aggregation of PCVAD-positive cases above expectations and, together with the existence of spatio-temporal and spatial clusters, as suggestive of apparent local spread of PCVAD. Clustering was detected at small spatial and temporal scales. Other patterns of spread could not be detected; however, survival rates in discrete Ontario zones, as well as a lack of a clear spatial pattern in the most likely spatio-temporal clusters, suggest other between-herd transmission mechanisms.
