Spatial Analysis of the Spread of PCVAD in Ontario

A group of researchers from the University of Guelph has made an exploratory spatial analysis of the spread of porcine circovirus associated-disease (PCVAD) in pig herds in Ontario, Canada.

15 December 2010

4 minute read

The systemic form of porcine circovirus associated disease (PCVAD), also known as postweaning multisystemic wasting syndrome (PMWS) was initially detected in the early 1990s, explain Zvonimir Poljak and colleagues at the University of Guelph in Canada in a paper in BMC Veterinary Research. Starting in 2004, the Canadian swine industry experienced considerable losses due to PCVAD, concurrent with a shift in genotype of porcine circovirus type 2 (PCV2).

The objectives of their 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 swine herds from a large disease-monitoring project that included porcine reproductive and respiratory syndrome (PRRS) virus-positive herds identified by the diagnostic laboratory, and PRRS virus-negative herds directly from the target population. Herds were included if they had growing pigs 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 generalised 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 PRRS 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. Poljak and co-authors 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.

Reference

Poljak Z., C.E. Dewey, T. Rosendal, R.M. Friendship, B. Young and O. Berke. 2010. Spread of porcine circovirus associated disease (PCVAD) in Ontario (Canada) swine herds: Part I. Exploratory spatial analysis. BMC Veterinary Research, 6:59. doi:10.1186/1746-6148-6-59.