PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME (PRRS) - A REVIEW, WITHEMPHASIS ON PATHOLOGICAL, VIROLOGICAL AND DIAGNOSTIC ASPECTS

Despite early attempts to control the spread of the disease, porcine r eproductive and respiratory syndrome (PRRS) has now become endemic in many countries including Britain. The occurrence of subclinical herd i nfections, the prolonged circulation of virus within herds and probabl e aerogenic virus spread all mitigated against the success of control measures. The origin of the disease is unknown but the causative agent has been shown to be an arterivirus with shared features to lactate d ehydrogenase virus of mice. There is evidence of extreme genetic and a ntigenic variability between American and European isolates. PRRS viru s has a predilection for alveolar macrophages and does not grow in mos t cell lines. In infected pigs, viraemia can persist for many weeks in the face of circulating antibodies and little is known about the mech anisms by which immunity to infection develops. A wide spectrum of dis ease has been reported from the field, accompanied in some cases by he avy economic losses. Reproductive and perinatal losses were most promi nent when the disease first appeared. In the endemic phase, PRRS may b e more significant as a contributory factor to a post-weaning respirat ory syndrome of young pigs of 3-8 weeks. On-farm techniques have been developed to reduce the recycling of PRRS virus fi om older infected n ursery pigs to the younger newly weaned pig. Vaccines are now marketed for the control of PRRS, but are not licensed for use in Britain. Imp rovements in knowledge of virion composition and antigenic stability a nd in the nature of the immune response of the pig should result in ge netically engineered subunit vaccines becoming available. Diagnosis of PRRS is still difficult as many animals do not show clinical signs an d may only be detected by serology and often only when other respirato ry diseases are being investigated, Now that the infection is widespre ad, serological testing must be properly targeted and interpreted to g ive meaningful results about virus circulation. An increasing arsenal of diagnostic methods are becoming available to detect virus in both f resh and fixed specimens. The pathogenic mechanisms of PRRS remain poo rly defined and more work is needed to reveal the nature of the intera ction between PRRS virus and other factor's in disease.