James D. McKean, Faculty, Veterinary Diagnostic and Production Animal Medicine
PRRS is recognized as a major swine disease that costs the USA industry over $560 million annually. This virus disease is widespread throughout the swine dense areas of the USA, but a true prevalence based on a statistical sampling of finishing operations has not been completed to date. Such a survey would provide a better understanding of the epidemiology of PRRS and assist in development of control/eradication strategies. The market swine surveillance program, developed for Pseudorabies virus detection, provided a resource for PRRS surveillance that was economical and timely. USDA-APHIS-VS, ISU Veterinary Diagnostic Laboratory and the participating packers cooperated on this survey.
The objectives of this survey are 1) to enumerate market swine lots presented at eight Iowa abattoirs that contain PRRS positive antibodies by utilizing meat juice technologies, 2) to benchmark within lot PRRS antibody levels, and 3) to generate a substantive sample population of further epidemiologic evaluations.
Two sampling periods (Winter and Summer) were utilized to generate ~ 110,000 test results each from Midwestern swine finishing sites. The market swine surveillance program collecting meat juice samples in eight Iowa abattoirs, representing 25+% of the national daily harvest, was utilized from Winter (November 2005 to March 2006) and Summer (June 2006 to September 2006) to generate this benchmark data. The population sample size is sufficiently robust to enable statistical comparisons to measure seasonal variability, changes in demographics or area prevalence and other factors related to the spread of PRRS virus within this swine population. Individual samples for the Winter period were found as 74.8% (81,728/109,260) positive. A total of 21,986 lots were identified and 85.8% (18,864/21,986) were judged positive by containing one or more positive samples. A total of 3,866 producers were classified with 89.7% (3,474/3,866) determined to be positive. Producer classifications provided a more varied distribution. The 1-1000 annual production grouping was significantly different from all others. The 1001-2000 and 2001-4000 groups were similar, but the 1001-2000 grouping was different from all others. The 4001-9999 group was not statistically different from the 2001-4000 group, nor from >10,000 groups. A sub-population, consisting of 3,073 samples (2.8%), 615 lots (2.8%) and 175 producers (4.5%) generated 50.2% positive samples, 63.6% positive lots and 72.6% positive producers, that supplied known negative weaned pigs was statistically different from the general population. The Summer data is currently under statistical review but cursory examination indicates that minimal seasonal effects were observed and no shift in population prevalence by herd size or geographic distribution could be identified. Further evaluations will be made and reported.
This project demonstrates the flexibility of an established market swine surveillance system to answer short-term disease-related epidemiologic questions as part of a routine disease monitoring program and on a rapid and cost-effective basis. In this survey the prevalence of PRRS antibodies has been determined with sufficient size to ensure benchmark values will be comparable in future studies for individual, lot and ownership prevalence values. Increasing herd size and location in hog-dense areas have been demonstrated as significant risk factors associated with this status. Evaluation of a known negative weaned pig population indicates that infections occurred regularly during the grow-finish stage within Iowa, but that pigs from such sources are less likely to become infected during finishing. This data supports a major role for unknown mechanisms of inter-herd infection, but does not elucidate causation.
150 Iowa Pork Industry Center
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October 24, 2006
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