Time Farms Stay Naive for Porcine Reproductive and Respiratory Syndrome

Simple Summary Porcine Reproductive and Respiratory Syndrome (PRRS) is one of the main infectious diseases affecting swine herds in the U.S. Control and elimination are facing challenges, including the absence of a vaccine that confers sterilizing immunity. Elimination through herd closure can also be a long, uncertain, and costly process, with herds often taking around 41 weeks to start consistently weaning virus-free piglets after an outbreak is detected. Thus, producers and practitioners may hesitate in pursuing elimination if there is a perception that the herd will soon face another PRRS outbreak, opting instead to maintain some level of immunity in the herd indefinitely. Of all the breeding herds monitored over 12 years, only about 1/6 eliminated PRRS from their herds and remained, on average, PRRS-free for two years. After eliminating PRRS, the average percentage of new outbreaks per year was 23%, similar to the national average of 20-25% (regardless of the previous PRRS status). Additional factors might contribute to the decision to pursue elimination, and further studies are warranted. Background: Hesitation on eliminating Porcine Reproductive and Respiratory Syndrome virus (PRRSV) from breeding herds exists since it is difficult to predict how long the herd will remain virus-free. We aimed to estimate the time that breeding herds remained virus-free (naive) after PRRSV elimination was achieved. Methods: Production systems voluntarily shared their breeding herds' health status weekly between July 2009 and October 2021. PRRSV incidence rate and the total number of days a breeding herd remained virus-free were estimated. Results: A total of 221 (17%) herds reached the naive status 273 times. The median time sites remained in this status was approximately two years. The overall PRRS incidence rate after sites achieved a naive status was 23.43 PRRS outbreaks per 100 farm years. Conclusion: Estimates obtained here provide insights on how frequently and for how long sites remain naive, which contribute to informing management practices for PRRS control.

Automated summary

Porcine Reproductive and Respiratory Syndrome (PRRS) is a major infectious disease in U.S. swine herds, and eliminating the virus poses challenges. Only about 1/6 of breeding herds successfully eliminated PRRS and remained virus-free for an average of two years. After elimination, the average annual outbreak rate was 23%, similar to the national average of 20-25%. Further studies are needed to understand the factors influencing the decision to pursue elimination.