Leman: Gilt risk factors for PRRS

Porcine reproductive and respiratory syndrome (PRRS) continues to cause significant economic and pig production losses. Despite elimination efforts, PRRS outbreaks and elimination failures continue to occur. Linhares et al. (2022) showed that the median time-to-stability (TTS) for herds undergoing a herd-closure within the past decade was 36 weeks, approximately ten weeks longer than previously observed. Age at the time of PRRS exposure may influence how long pigs remain infected and their ability to transmit the virus. 

Previous research has shown that younger pigs exhibit longer periods of viremia and higher viral loads in pulmonary macrophages compared to older pigs. These findings have prompted questions in the field regarding the role of gilt age at introduction and exposure in the success of PRRS elimination efforts. 

This study evaluated the infection dynamics post-live virus inoculation (LVI) of different gilt ages at introduction, and the effect of gilt age on the probability of producing PRRSV-negative litters. An observational study was conducted on a commercial 2,500-head category 1A PRRS infected sow farm. The farm had an on-site gilt isolation area and allowed for the entry of various aged gilts at the start of closure. Two-to six-month-old gilt cohorts were longitudinally sampled throughout the closure, starting at the initial LVI event. 

Oral fluids were collected from each gilt age cohort at approximately 1, 3, 7, 11, and 16 weeks post-LVI. Tonsil-oral scrubbing samples were collected from 15 randomly selected gilts and longitudinally sampled at 30 and 90 days of gestation and prior to farrowing. Aggregated processing fluids were collected weekly from gilt and parity 2+ litters throughout the closure. Samples were tested for the presence of PRRS genetic material via RT-PCR. 

A significant difference in the time it took different age groups to test negative for PRRS post-LVI was observed. Notably, 4-month-old gilts were more likely to clear the virus sooner than 2-month-old gilts. Despite these differences in infection duration, no numerical differences were observed in the rate of PRRS-negative litters produced across gilt age groups.

These findings suggest that younger gilts may require a longer cooling-off period post-exposure but do not appear to negatively impact litter status. In conclusion, the age of gilts at the time of PRRS exposure influences how quickly they clear the infection, though it may not impact litter status at farrowing.