G. W. Stevenson, W. G. Van Alstine, C. L. Kanitz, K. K. Keffaber.
After the initial outbreaks of PRRS 2 farms were chosen to investigate the evolution of 2 farms during the endemic course of the disease.
On December 1991 from each farm 8 “typical” acutely affected nursery pigs of 6-8 weeks old were selected to collect blood samples and to perform a complete necropsy (4 pigs).
Microscopic examination and laboratory tests for common agents (bacteria and virus) were completed for all piglets.
Farm 1 a 500-sow farrow-to-finish farm with all-in/all-out in the farrowing room and continuous flow in nursery, growing, and finishing rooms.
3 months after the initial outbreak the reproductive performance and mortality in suckling pigs went back to the levels before PRRS was present; nevertheless mortality in nursery pigs was still high. It was 3.1% in average for the 11 months before the PRRS infection, and it was 7.4% for the 34 months after the outbreak. Since PRRS appeared, the mortality in the nursery had 5-6 months of low mortality (3-5%) and then 5-6 months of high mortality (7-16%).
Piglets were lethargic at the beginning with anorexia and skin cyanosis. After 2 days, they showed dyspnoea and had a poor response to broad spectrum antibiotics.
Farm 2 a 1000-sow farrow-to-finish farm with all-in/all-out in the farrowing room and continuous flow in nursery, growing, and finishing rooms
Also 3 months after the initial outbreak the reproductive performance and mortality in suckling pigs went back to the levels before PRRS was present. And since then, recurring increases in nursery mortality had been reported.
Microscopic lesions reported were purulent bronchopneumonia in the cranial ventral lung lobes, diffuse interstitial pneumonia in the dorsal caudal lung lobes, hepatitis with multifocal necrosis. All piglets submitted showed lesions typical of septicemic salmonellosis, which supports the enhancement of bacterial diseases by PRRSv.
The most severe elevations in nursery mortality occurred in winter months confirming also the interaction of PRRS with environmental factors: low room temperature, increased thermal fluctuation, low ventilation rates, and high relative humidity.
Nursery mortality in farm 1 was 3.1% in average for the 11 months before the PRRS infection, and 7.4% for 34 months after the outbreak.