Sampling Strategies in PRRS Elimination in Hungary: An Observational Study Involving Four Farrow-to-Finish Swine Herds

SIMPLE SUMMARY: Porcine reproductive and respiratory disease (PRRS), a viral cause of morbidity and mortality, seriously affects the productivity of the swine industry. Although the economic losses due to PRRS can be controlled by vaccination, disease eradication is challenging that requires concerted efforts. Hungary initiated a national PRRS eradication program in the mid-2010s and achieved significant countrywide success in all herd types by the early 2020s. Farrow-to-finish herds are particularly challenging from the perspective of disease control and prevention due to the simultaneous rearing of pigs of different ages. In this study, we present data about different sampling strategies of large farrow-to-finish swine farms. We demonstrate that our sampling strategy, where it was systematically applied, was successful, and these successes should guide future efforts of PRRS elimination programs in other regions. ABSTRACT: PRRS elimination strategies often rely on depopulation-repopulation. However, this approach is accompanied by a long-term loss of production. With adequate control measures, such as well-designed immunization programs and technological changes along with prevalence-based laboratory testing, the virus-free status of the most vulnerable age groups in swine herds can be achieved. The most common reason for acquiring PRRSV at large farrow-to-finish swine farm units is that the previously settled fattening pigs serve as a source of infection for the newly reared PRRS-free animals. Following such unwanted events, PRRSV may persist in an affected establishment for several years. In this observational study, we selected four farrow-to-finish type swine herds. We implemented different laboratory testing protocols to find the most optimal solution for a successful PRRS elimination program. To aid our objectives, we used a DIVA PCR technique. The PRRS DIVA PCR assay is a fast, reliable method to identify sows shedding farm-specific PRRSV strain(s). As a result of elimination efforts at the sentinel pig herds, we found that reliable detection of wild-type PRRSV shedding among sows requires sampling at least three weaned piglets per litter. The strict adherence to this sampling protocol, the systematic use of laboratory methods that quickly detect the presence of wild virulent virus in the herd during the rearing period and the culling of DIVA PCR positive litters and their sows decreased the presence of the resident virus markedly. These procedures at Hungarian farrow-to-finish type farms successfully inhibited the wild-type PRRSV infection of different age groups. The results of this study demonstrate that applying this methodology together with strict biosecurity measures enabled us to reach PRRS-vaccinated-free status in large, farrow-to-finish herds within two years.