Laboratory Investigation of PRRS Virus Infection in Three Swine Herds

This report describes the clinical and pathological effects of porcine reproductive and respiratory syndrome (PRRS) virus infection in three pig herds, showing that PRRSv infection increased the incidence of other common diseases in each case.
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Key Messages

  • This report describes the clinical and pathological effects of PRRS virus infection in three herds and the laboratory investigations realised on samples submitted.
  • Clinical signs on all farms began in 1991 and continued into 1992.
  • All three cases had both respiratory syndromes in growing-fattening pigs and reproductive signs in sows.
  • PRRSv infection was confirmed but also several other concurrent bacteria (some are non-typical pathogens) were found.

Article Brief

This report describes the clinical and pathological effects of PRRS virus infection in three herds. Clinical signs on all farms began in 1991 and continued into 1992.

Herd 1

A 120-sow farrow-to-finish swine herd started with a respiratory outbreak in growing and finishing pigs with moderate morbidity (35 per cent) and low mortality (one per cent). In a few months, a reproductive failure syndrome with high-incidence affected pregnant sows and continued for two months. The clinical picture in sows was:

  • two to three days of anorexia
  • one-week of premature farrowings
  • increase of stillborn pigs
  • higher mummies
  • weak or “thumping” piglets, and
  • higher piglet mortality

At that time, there was no virus was isolation but all 30 sow-samples had PRRS antibodies. Later, PRRSv was isolated retrospectively from two piglets.

Herd 2

A 2,800-sow farrow-to-finish swine herd started with smaller litter size, mummified piglets, and small weak born piglets. Initially, abortions were not part of the syndrome. After this initial outbreak, a high-morbidity low-mortality respiratory outbreak affected 2,000 pigs in the grower/finisher phase. Two months later, another respiratory disease occurred in the grow-finishing pigs (around 2,000) but in this case, it affected close to 100 per cent of the pigs and the mortality was five per cent.

The pathogens detected were salmonellosis, Actinobacillus pleuropneumoniae and Pasteurella multocida. The respiratory disease also moved into the nursery and neonatal pigs where eight per cent of piglets were “thumping”.

Twelve week-old pigs had severe pneumonia with pleurisy and again Salmonella sp. and Actinobacillus pleuropneumoniae were found and later, Actinomyces pyogenes was isolated from a hog with mild purulent exudate. Although initial virus isolation was negative, later attempts resulted positive.

Herd 3

This 120-sow farrow-to-finish swine farm had and increased incidence of respiratory problems in nursery pigs. Later, this outbreak affected grower, finishing pigs and also farrowing rooms. Three months later, a syndrome appeared in piglets and sows that showed vomiting and diarrhoea, and 15 per cent of the sows had abortions and stillbirths.

Different submission of tissues were done and the main pathogens found were: Mycoplasma hyopneumoniae, Haemophilus parasuis, Streptococcus suis and Actinomyces pyogenes from the lungs of different affected animals, also Actinobacillus lignieresi was isolated from a pig with pneumonia and pleurisy. The lungs were positive for PRRS virus by both FA testing and virus isolation.

The clinical picture indicates that PRRSv infection increases the incidence of other common diseases in all three cases.


D. Zeman, R. Neiger, M. Yaeger, E. Nelson, D. Benfield, P. Leslie-Steen, J. Thomson, D. Miskimins, R. Daly, M. Minehart. 1993. Laboratory investigation of PRRS virus infection in three swine herds. J. Vet. Diagn. Invest. 5:522-528.

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October 2015

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