Characterizing dynamics of PRRSV RNA positivity over time in sows after live virus exposure and associated risk...

As the sows constitute a major source of porcine reproductive and respiratory syndrome virus (PRRSV) infection in piglets, undetected PRRSV in sows poses a significant challenge to the success of PRRSV control and elimination programs. The dynamics of PRRSV persistence in individual sows remains unanswered majorly because of the lack of practical individual sampling tools. The objective of this study was to characterize the dynamics of PRRSV RNA positivity in sows and associated risk factors using a practical Tonsil-Oral-Scrubbing (TOSc) collection method on sows over a complete reproductive cycle. This field study was conducted in a commercial breed-to-wean farm intending to eliminate PRRSV by herd closure and using live virus inoculation (LVI) for whole herd exposure after an outbreak. Pre- and post-LVI, periodic TOSc samplings at four-week intervals were performed on 275 conveniently selected sows with evenly distributed parity categories (Parity 0, parity 1-2, and parity 3 and above) and production phases categories (wean-breed, early gestation, middle gestation, late gestation, and farrowing) until 24 weeks post-LVI.

Overall, PRRSV RNA positivity declined over time from the first sampling point- 2 days before LVI until the end of the trial. At 16 weeks post-LVI, the PRRSV RNA positivity reached 4.43%, below the defined threshold of a low prevalence scenario of 10% positivity, and remained low prevalence until the end of the study. In the low prevalence scenario, there was a rebound of positivity to 8.00% at 24 weeks post-LVI. Parity demonstrated a significant effect on PRRSV RNA positivity in sows. At a high prevalence scenario (>10% positivity), both P0 and P1-P2 groups constantly showed significantly higher PRRSV RNA positivity than the P3 + group (p<0.05 for all pairwise comparisons). The P0 positivity was consistently numerically higher than P1-P2. The production phase effect was evident when the herd reached low prevalence (<10% positivity); sows in farrowing had consistently higher PRRSV RNA positivity than sows in other production phases. The mean positivity increased to 28.56% and 34.49% in the farrowing group at 20 weeks post-LVI and 24 weeks post-LVI, respectively.

PRRSV RNA positivity in sows based on TOSc samples decreased over time and reached low prevalence at 16 weeks post-LVI. PRRSV RNA positivity decreased significantly with the increase in parity. And the production phase effect on PRRSV RNA positivity was more evident at low prevalence compared to high prevalence scenarios.