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PRRS is an acronym (porcine reproductive and respiratory syndrome) for a viral disease characterized by two overlapping clinical presentations, reproductive impairment or failure in breeding animals, and respiratory disease in pigs of any age. PRRS is the most economically significant disease to affect US swine production since the eradication of classical swine fever (CSF).


The PRRS virus is an enveloped RNA virus in the genus Arterivirus, classified in the virus family, Arteriviridae. 


  • Once transmission of virus to the tonsil or upper respiratory system has occurred, primary replication occurs in lymphoid tissues. Viremia follows and may persist for several weeks. The virus has a predilection for lymphoid tissues (spleen, thymus, tonsils, lymph nodes, Peyer’s patches).

  • It infects and compromises the function of pulmonary alveolar and intravascular macrophages resulting in interstitial pneumonia. PRRS appears to increase susceptibility of the lungs to other pathogens.

  • Clinical signs may include a period of anorexia, fever, lethargy, depression, and perhaps respiratory distress or vomiting. Mild cyanosis of the ears, abdomen and vulva has been reported in some outbreaks. Reproductive problems, often the most obvious signs, include a decrease in the number of dams that conceive or farrow.

  • There is usually an increase in premature farrowings, late term abortions, stillborn or weak piglets and mummified fetuses. Preweaning mortality is high. Nursing pigs may have dyspnea (“thumping”). The period for reproductive signs varies with herd size but is usually two to three months in duration. A slow improvement in reproductive performance then begins. In larger operations, signs may be cyclical, especially if naïve gilts or sows continue to be introduced into the herd.

  • Any tentative clinical diagnosis should be confirmed by detection of the PRRS virus. This can be by virus isolation (VI), detection of PRRS antigen by fluorescent antibody tests (FAT) or immunohistochemistry (IHC), or detection of PRRS virus genome by polymerase chain reaction (PCR) and be coupled with presence of typical lesions. Serology provides indirect evidence of infection but does not determine if there is actual disease caused by PRRS virus.