Swine mortality, particularly those caused by infectious pathogens, poses biosecurity risks and causes significant economic impact. Dead handling structures may act as sources of viral environmental contamination, but limited attention has been given to their role in biocontainment and bioexclusion of pathogens in and around farms. In this study, we tried to diminish this problem by assessing the environmental contamination around dead animal disposal structures in wean-to-finish farms and determining if the contamination levels differ according to how dead animals are managed.
Methods
We conducted a cross-sectional study evaluating 40 Midwestern U.S. wean-to-market farms that handle dead animals either through rendering (n=20) or composting (n=20). Farms had to be porcine reproductive and respiratory syndrome virus or porcine epidemic diarrhea virus positive to be included in this study. On each farm, 10 samples were collected from the vicinity of the dead handling structure, namely from the road, ground, leachate, wall of the structures and carcasses. Samples were submitted for diagnostic through RT-PCR for PRRSV-2, PEDV, porcine deltacoronavirus and transmissible gastroenteritis at the Veterinary Diagnostic Laboratory at the University of Minnesota. The three lowest Ct values obtained from samples of each farm were sent for virus isolation.
Results
Out of the 40 farms included in the study, 22 (55%) had at least one positive sample for any of the tested viruses. When stratified by mortality management method (Figure 1), more rendering sites (16/20, 80%) tested positive for at least one virus than composting sites (6/20, 30%) (p-value=0.001). PRRSV-2 and PDCoV were identified at both rendering and composting sites. PEDV was exclusively identified on four out of 20 (20%) rendering farms. The median cycle threshold obtained among all samples was 31.5, ranging from 20.8 to 37.9. The mean for PRRSV-2 positive tests was lower in rendering sites (Ct of 30.4) than in composting sites (Ct of 33.8, T-test p-value= 0.012). Out of 400 samples, 95 tested positive for at least one virus. More samples from rendering farms were positive for at least one virus (72/200; 36.0%) than from composting farms (23/200; 11.5%), (chi-square p-value = <0.001).
The lowest Ct value found for PRRSV was 20.8, for PEDV was 25.7 and for PDCoV was 24.1, collected from leachate, ground and carcass respectively. When stratified by sample type, the lowest Ct values for all pathogens were obtained from samples leachates or carcasses, suggesting that contact with carcasses or leachates originating from these structures, which sometimes drain to nearby roads, may pose a risk of unintended pathogen transmission. When stratified by dead disposal handling method (Figure 2), farms that perform rendering had lower Ct values among all diseases tested, suggesting that rendering sites may have a higher level of environmental contamination or greater potential for virus persistence compared to composting sites. None of the viral isolation attempts were positive.
Conclusions
Our results indicate that dead animal handling structures in wean-to-market farms are frequently contaminated and that this contamination is more frequent on rendering sites. The presence of viral genetic material in and around these structures supports the possibility of accidental transmission through contact with contaminated surfaces, even though no viral isolation was obtained. Lower Ct values (suggesting the present of more viral genetic material) were obtained in leachate and carcass samples, suggesting that practices minimizing contact with carcasses and fluids may improve biocontainment of pathogens. While biosecurity efforts are important, efforts in biocontainment such as diminishing contamination around dead handling structures may help not overload biosecurity plans farms have in place. We suggest that future efforts focus on identifying and implementing strategies to limit the environmental contamination at these sites.
This article was written by Rafael Medeiros de Ávila Melo, Kimberly VanderWaal, Cesar Corzo, Mariana Kikuti and Igor A. D. Paploski of the University of Minnesota for the National Hog Farmer.