Project title: Assessment of Vertical Transmission from Parity One Sows Infected with a Low Dose and Mild Pathogenic PRRSV Isolate.

Institution: College of Veterinary Medicine, University of Minnesota.

Outcome/Results: Under conditions of this study, 4 days of age was the best age to test piglets. Stillbirths, mummies, and “very sick” pigs gave the best chance of finding positive pigs.

A very low dose of PRRS virus intramuscularly injected to naïve parity one sows at 90 days of gestation was able to effectively infect the sows and their litters generating clinical signs. Under the conditions of this study, 4 days of age seems to be the best age to test piglets compared to birth (prior to nursing) or weaning. In the present experiment, where completely susceptible sows were inoculated, litters born from “early farrowings” did not have a higher proportion of viremic piglets or higher viral load than sows that farrowed “on time”; however in a chronically infected farm, those sows that have not been exposed to the virus are more likely to farrow early compared to previously infected sows. After analyzing the amount of PRRS virus detected in serum, it could be suggested that stillbirths, mummies and “very sick” pigs tend to be a better sample than all other pigs to catch potential positive pigs. Under the conditions of this study there is no reason to sample lighter litters or piglets at birth but as expected affected litters will have a lower growing performance during lactation. Real time PCR on blood swab could be used as a tool for herd monitoring when frequent testing is required and when a veterinarian can not be present; however, clinical evaluation in order to select the best samples is important to increase the chances to catch those potentially positive pigs. The very high viral load detected in serum samples in the present experiment made pooling (5/1) a good alternative to increase the probability of detecting viremic piglets in the population without reducing individual sensitivity; however, in chronically infected farms, where a larger percentage of sows have been exposed, the viral load of viremic pigs might be lower.