Risk Factors for the Spread of PCVAD in Ontario

From their results, Guelph researchers were unable to identify the mechanism of local spread through airborne transmission or indirectly through contaminated fomites or vectors of porcine circovirus-associated disease (PCVAD).

15 December 2010

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The emergence of PCVAD was associated with high mortality in swine populations worldwide, according to Zvonimir Poljak and colleagues at the University of Guelph, Canada, in a paper published in BMC Veterinary Research. Studies performed in different regions identified spatial, temporal and spatio-temporal trends as factors contributing to patterns of the disease spread. Patterns consistent with spatial trend and spatio-temporal clustering were already identified in this dataset. On the basis of these results, they further investigated the nature of local spread in this report.

The primary objective of the Guelph study was to evaluate risk factors for incidence cases of reported PCVAD.

Results

A time-matched case-control study was used as a study design approach, and conditional logistic regression as the analytical method. The main exposure of interest was local spread, which was defined as an unidentified mechanism of PCVAD spread between premises located within three kilometres of the Euclidean distance.

Various modifications of variables indicative of local spread were also evaluated.

The dataset contained 278 swine herds from Ontario originally sampled either from diagnostic laboratory submissions or directly from the target population. A PCVAD case was defined on the basis of the producer's recall. Existence of apparent local spread over the entire study period was confirmed (OR=2.26, 95% CI: 1.06, 4.83), and was further identified to be time-varying in nature – herds experiencing outbreaks in the later part of the epidemic were more likely than control herds to be exposed to neighbouring herds experiencing recent PCVAD outbreaks.

More importantly, the pattern of local spread was driven by concurrent occurrence of PCVAD on premises under the same ownership (OREXACTwithin ownership = 25.6, 95% CI: 3.4, +inf; OREXACToutside ownership = 1.3, 95% CI: 0.45, 3.3). Other significant factors included PRRSv status of a herd (OREXACT=1.9, 95% CI: 1.0, 3.9), after adjusting for geographical location by including the binary effect of the easting coordinate (Easting >600km=1; OREXACT=1.8, 95% CI: 0.5, 5.6).

Conclusions

These results preclude any conclusion regarding the existence of a mechanism of local spread through airborne transmission or indirectly through contaminated fomites or vectors, as simultaneous emergence of PCVAD could also be a result of concurrent change in contributing factors due to other mechanisms within ownerships, concluded Poljak and co-authors.

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 II. Matched case-control study. BMC Veterinary Research, 6:58. doi:10.1186/1746-6148-6-58