Porcine reproductive and respiratory syndrome
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.
To maintain PRRSV-negative herds (especially breeding herds and boar stations), targeted monitoring in the herds as well as quarantine and examination of newly housed pigs is necessary.
Due to the viraemia lasting about 28 days, blood samples are equally suitable for virus genome detection by PCR and for antibody detection by ELISA. Frequent testing (at least quarterly) of a representative number of samples (approx. 13 samples per herd, irrespective of herd size) can lead to rapid detection of pathogen introduction into the herd, even if no clinical symptoms occur.
To clarify the cause of respiratory symptoms in weaners and fattening pigs as well as gilts, blood samples of typically sick pigs can also be examined by PCR. Antibody testing in older groups of piglets can confirm the diagnosis. In case of dissections, tonsils, lungs as well as lung lymph nodes can be used for detection by PCR. Detection from saliva is also possible. Longer-standing infections can be detected by PCR from tonsillar scrapings, as the virus can be detected in the tonsils for the longest time.
In the case of abortions, detection by PCR from the placenta and fetal organs is suitable for establishing the diagnosis.
PRRSV RNA can also be detected in semen, but a negative result does not mean that an infection can be ruled out, as excretion in semen is intermittent.
Last updated: 02.02.2022