Production of PRRSV-negative pigs commingled from multiple, vaccinated, serologically stable, PRRSV-positive breeding herds

Objectives: To determine whether PRRSV-negative pigs could be produced from multiple PRRSV-positive herds and to investigate the serological stability of these herds and the impact of sow vaccination. Methods: Piglets originating from non-vaccinated and vaccinated herds were weaned at 8 to 14 days and housed separately in an off-site nursery for 90 days. Pigs were tested by PRRS IDEXX ELISA at weaning and at 30, 60, and 90 days of age. A subset of pigs was tested using PRRS reverse transcription-polymerase chain reaction (RT-PCR) and restriction fragment length polymorphism (RFLP) assays. Serological stability of breeding herds was assessed by PRRS IDEXX ELISA. Results: The five vaccinated herds were serologically stable. Piglets from these herds became seronegative after weaning and remained PRRSV negative up to 90 days of age. Pigs from two of the five non-vaccinated herds were PRRSV negative. One of these herds was serologically stable. Piglets from the other three non-vaccinated herds were PRRSV positive at weaning, and the number of seropositive piglets and their S:P values increased to 90 days of age. Two of these herds were not serologically stable. After vaccination of open sows and gilts, PRRSV-negative pigs were produced from one of these two herds. Implications: Piglets weaned at 8 to 14 days from multiple, serologically stable sow herds and commingled were PRRSV negative at 90 days of age. Vaccinating gilts twice prior to breeding and sows at weaning may increase the stability of breeding herds. Results must be validated in large commercial operations.