The present study evaluates the efficacy of serum inoculation to immunize gilts in prevention of reproductive PRRS following late-gestation challenge with the “homologous” virus. Here, the immunizing virus was 184-type virus and was > 98% similar to the challenge virus. Gilts were 113 days after inoculation, and challenged at 90 days of gestation, 209 days after the serum inoculation. The challenge isolate was a highly similar 184-type PRRSV isolated from a diseased sow in an outbreak in the same herd inoculated with virus A. This virus, referred to as “virus B,” was 98.2% identical to virus A in ORF5 sequence, and 98.3% identical across the entire genome.
Gilts were all viremic at 3 days after inoculation, and 11% were viremic at 25 days. All gilts were seronegative at day 3 and 100% were seropositive at 25 days and at 70 days. Following challenge at 90 days of gestation, all 15 sows showed fever, 2 aborted litters on day 5 and day 6, respectively and a third sow farrowed 15 live, but weak, piglets, all of which died shortly after birth. Thirteen of 15 challenged pigs farrowed, producing 12.8 pigs per litter. Ninety percent of farrowed pigs were born alive, and 71% of pigs farrowed also survived to weaning. Nineteen percent of piglets were viremic at one day of age, and 31% were viremic at weaning. Virus levels were low and did not increase over time. There was no change in antibody levels in piglets one day of age to weaning, and there was no difference in antibody levels between viremic and non-viremic piglets.
The findings show that immunity to a highly virulent PRRSV provides substantial protection against reproductive PRRS but failed to prevent transmission to piglets. Surprisingly, viremic piglets displayed no signs of disease. These findings highlight the difficulty in protecting sows against highly virulent PRRSV strains, and indicate the need to understand the consequences of healthy but PRRSV-viremic pigs weaned into negative nurseries.
M.P. Murtaugh, M. Wagner. Protecting the pregnant sow from PRRS: Research findings. 2010 AASV Annual Meeting: Implementing Knowledge 471.