Influences of biological properties of porcine reproductive and respiratory syndrome on immunoprophylactic efficacy

Porcine reproductive and respiratory syndrome (PRRS) is caused by the PRRS virus (PRRSV) that was identified in Europe in 1991 and in the USA in 1992. The syndrome includes reproductive losses, extensive post weaning pneumoni a, as well as respirators difficulties and increased mortality. Several authors underlined the immunosuppresive or immunomodulating propert ies of PRRSV. This hypothesis was supported by experimentally demonstrated tropism of PRRSV to the alveolar macrophages. Due to this, transient deteri oration of local lung cellular defences is observed. This predisposes pigs to secondary infections of the respiratory tract. Genetic and antigenic differences found between American and European strai ns of PRRSV play an important role in immunoprophylaxis of PRRS, This suppo rts the hypothesis that American and European strains belong to two distinc t types of the virus. The above-mentioned antigenic differences indicate th at vaccines containing heterologous PRRSV antigen may be less efficient tha n the ones containing a homologous virus type. Additionally, taking into account biological properties of PRRSV, it should be stressed that modified live PRRS vaccines are generally thought to be m ore efficient than inactivated ones, despite the fact that adverse effects of these vaccines have been reported. Selection and application of a safe and effective vaccine against PRRS shou ld be preceded by specific identification of the PRRSV tape affecting a par ticular farm.