PRRSV is an enveloped single-stranded RNA virus of the genus Arterivirus. It is divided into the two species PRRSV 1 (formerly European genotype) and PRRSV 2 (formerly North American genotype). Both species are highly mutagenic and have innumerable subtypes and strains. A highly pathogenic (HP) variant was detected in China in 2007 and resulted in high mortality, including in sows.
Within hours of infection, the virus is detectable in its target cells, the alveolar macrophages. In addition to colonising macrophages in the nose, throat and tonsils, the virus is detectable in the blood within 12 hours. The viraemic phase lasts an average of 28 days. Purchased infected pigs as well as transport vehicles and visitors can transmit the virus from herd to herd.
The clinical symptoms are very varied and strongly depend on the virus strain or strains in the respective herd, the immunity status of the pigs, the age group affected and concomitant diseases. PRRSV infected animals may become infected with other bacterial and viral diseases that may be more severe due to the existing infection with PRRS.
Respiratory symptoms occur mainly at weaning (and fattening) age and are due to interstitial pneumonia. They manifest as dyspnea, increased respiratory rate, increased susceptibility to other infectious diseases, and increased mortality. Initially, conjunctivitis, decreased general condition and inappetence may occur.
Abortions occur mostly after infections in the last third of gestation. There may be increased stillbirths, birth of weak piglets, and increased piglet mortality.Clinical signs in boars are moderate and can be easily overlooked. At times, semen quality is decreased.
Particularly virulent and HP strains can cause outbreaks with high mortality in all age groups. Clinical signs include respiratory signs and abortions, as well as erythema, petechiae, fever, anorexia and tremors, and skin cyanosis, especially of the ears, proboscis disc, tail, scrotum and distal limbs.
Since PRRSV has been categorized as a D + E disease by the European Animal Health Law (AHL), measures must be taken to prevent its spread in connection with the movement of animals and animal products. No specific therapy is available. Priority is given to minimising the risk of pathogen introduction into farms through appropriate monitoring and surveillance programmes and hygiene and biosecurity measures. Eradication of stocks is costly and only promising if a new pathogen introduction can be avoided.