Factors affecting Porcine Reproductive and Respiratory Syndrome virus time-to-stability in breeding herds in the Midwestern United States

The time needed to wean porcine reproductive and respiratory syndrome (PRRS) virus negative pigs consistently from a breeding herd after an outbreak is referred to as time-to-stability (TTS). TTS is an important measure to plan herd closure as well as to manage economic expectations. Weekly PRRS incidence data from 82 sow farms in six production systems located in the Midwestern United States were used for the analysis. The objective of this study was to evaluate the effect of recorded predictors on TTS in participant sow farms. The median TTS was 41.0 weeks (1st quartile 31.0 weeks–3rd quartile 55.0 weeks). In the final multivariable mixed-effects Cox model, farms that experienced winter (hazard ratio (HR) 2.18, 95% confidence interval (CI) 1.28–3.70) and autumn (HR 1.91, 95% CI 1.16–3.13) PRRS outbreaks achieved stability sooner than farms that experienced PRRS outbreaks during summer. No statistically significant difference (p = 0.76) was observed between the TTS of farms that had a PRRS outbreak during spring and summer (HR 1.09, 95% CI 0.62–1.91). Additionally, farms that had a PRRS outbreak associated with a 1-7-4 restriction fragment length polymorphism (RFLP) cut pattern took significantly longer to achieve stability (HR 0.44, 95% CI 0.27–0.72) compared to farms which had a non-1-7-4 PRRS outbreak. Finally, farms that had a previous PRRS outbreak within a year achieved stability sooner (HR 2.18, 95% CI 1.23–3.86) than farms that did not have a previous PRRS outbreak within a year. This study provides information that may result useful for planning herd closure and managing expectations about the time needed to wean PRRS virus negative pigs in breading herds according to the season of the year when the outbreak occurred and the RFLP cut pattern associated with the outbreak virus.