Introduction/Background
Porcine reproductive and respiratory syndrome (PRRS) remains one of the most economically significant endemic diseases affecting the United States swine industry, with recent estimates exceeding $1.2 billion annually in losses.1 Despite widespread implementation of herd closure and live-virus exposure strategies, time-to-stability following PRRSV outbreaks has increased substantially in recent years.
Replacement gilts represent a critical population during PRRSV elimination due to their potential to sustain viral circulation and introduce actively shedding animals into breeding herds. Factors including age at exposure, immune maturity, acclimation strategy, population flow, and opportunities for re-exposure may influence PRRSV shedding duration within replacement populations.
Factors Influencing PRRSV Shedding Duration
Immune maturity at exposure likely plays an important role in viral clearance dynamics. Younger gilts may exhibit delayed or less consistent immune responses, potentially contributing to prolonged shedding compared to older, more immunologically mature animals. Consequently, acclimation at older ages, when feasible, may improve immune consistency and reduce shedding duration.3-5
Acclimation design also influences PRRSV persistence within replacement populations. Continuous exposure, repeated viral recirculation, and population commingling may prolong instability and delay viral clearance. Controlled acclimation protocols emphasizing rapid homologous exposure, minimized re-exposure, and strict population flow management are preferred to reduce ongoing viral circulation.
Importantly, PRRSV clearance should not be defined solely by negative serum PCR results. Previous literature has demonstrated prolonged viral persistence within sampling the tonsillar and oropharyngeal tissues despite resolution of detectable viremia, suggesting some animals may continue to pose shedding risk after serum negativity.6
Diagnostic Monitoring Approaches
Diagnostic surveillance remains critical during acclimation and elimination programs. Serum sampling has traditionally been used to evaluate PRRSV status, however, previous studies have demonstrated variation in positivity rates across different sample types, including serum, oral fluids, and tonsil-oral-scrubbing (TOSc), depending on stage of infection and viral persistence.
Selection of sample type should align with the surveillance objective. Serum sampling may be more useful for evaluating viremia and active viral circulation, whereas TOSc or tonsil scraping may better detect prolonged viral persistence within tonsillar tissues after viremia has waned. Oral fluids and TOSc samples may also provide practical population-level surveillance approaches for monitoring ongoing circulation within populations.
PRRSV sequencing additionally remains important to confirm circulation of the expected variant and reduce the risk of introducing heterologous strains into replacement populations or breeding herds.
Discussion
Although no single intervention is likely to eliminate prolonged shedding risk entirely, biologically sound acclimation strategies combined with strict population flow management, adequate post-exposure stabilization time, and structured diagnostic surveillance may improve elimination success and reduce downstream instability risk.
References
Batista, L., Dee, S.A., Rossow, K.D., Deen, J., Pijoan, C. (2002) Assessing the Duration of Persistence and Shedding of Porcine Reproductive and Respiratory Syndrome Virus in a Large Population of Breeding-age Gilts. Can J Vet Res. 66(3),196-200.
Betlach, A.M., Linhares, D., Bisek, C., Yeske, P. (2025) Assessing the Role of Gilt-related Risk Factors for PRRSV Time-to-low-prevalenc. Allen D. Leman Conference.
Holtkamp D, and Osemeke O. (2024) PRRS Now Costs Pork Producers $1.2 Billion per Year. National Hog Farmer, July 30, 2024. https://www.nationalhogfarmer.com/livestock-management/prrs-now-costs-pork-producers-1-2-billion-per-year.
Li, P., Poeta Silva, A., Carnevale de Almeida Moraes, D., Yeske, P., Osemeke, O.H., Magalhaes, E.S., Silva, G., Linhares, D. (2024) Comparison of a Novel Rapid Sampling Method to Serum and Tonsil Scraping to Detect PRRS in Acutely Infected Sows. Prev Vet Med: 106082, 223.
Linhares DCL. (2025) PRRS Outbreak Management Program (POMP) database. Accessed February 6, 2025.
Mainquist-Whigham, C.E., Mauch-Swinford, E.D., Stephenson, E.W., Madigan, J.A., Cross, A.J., Rathje, T., McNeil, B.M. (2025) Risk Factors Associated with Prolonged Infection of Porcine Reproductive and Respiratory Syndrome Virus Determined by Whole-herd Sampling Methods. J Swine Health Prod. 33(3), 90-97.