Key Messages

• Two farrow-to-finish farms were studied during the endemic phase of the disease.
• Three months after the initial outbreak the reproductive performance went back to the previous levels (before PRRS).
• During the next 2.5 years, cyclical increases in nursery mortality were reported with highest levels in winter months.
• On average, nursery mortality was twice much higher after PRRS had settled down in sows than before it appeared.
• Piglets were lethargic at the beginning with anorexia and skin cyanosis. They had dyspnoea and had a poor response to broad spectrum antibiotics.
• The unusual features of septicemic salmonellosis supported the viral enhancement of bacterial disease.
• PRRS virus affected nursery pigs on two farms that had normal reproductive for more than 2.5 years following an outbreak.

Article Brief

After the initial outbreaks of PRRS, two farms were chosen to investigate the evolution of the endemic course of the disease.

On December 1991 from each farm, eight “typical” acutely affected nursery pigs of six to eight weeks old were selected to collect blood samples and to perform a complete necropsy (four pigs).

Microscopic examination and laboratory tests for common agents (bacteria and virus) were completed for all piglets.

Farm 1 was 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.

Three 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 per cent in average for the 11 months before the PRRS infection, and it was 7.4 per cent for the 34 months after the outbreak. Since PRRS appeared, the mortality in the nursery had five to six months of low mortality (3 to 5 per cent) and then five to six months of high mortality (7 to 16 per cent).

Piglets were lethargic at the beginning with anorexia and skin cyanosis. After two days, they showed dyspnoea and had a poor response to broad spectrum antibiotics.

Farm 2 was a 1,000-sow farrow-to-finish farm with all-in/all-out in the farrowing room and continuous flow in nursery, growing, and finishing rooms.

Also three 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.

Reference

G.W. Stevenson, W.G. Van Alstine, C.L. Kanitz, and K.K. Keffaber. 1993. Endemic porcine reproductive and respiratory syndrome virus infection of nursery pigs in two swine herds without current reproductive failure. J. Vet. Diagn. Invest. 5:432-434.

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July 2015