IMMUNE-RESPONSE AND PERSISTENCE OF THE PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS IN INFECTED-PIGS AND FARM UNITS

The kinetics of the serum antibody response to porcine reproductive an d respiratory syndrome (PRRS) virus and the persistence of the virus a fter infection were determined in experimentally and naturally infecte d pigs. In an experimental study, four specific pathogen free (SPF) so ws were infected with a French strain of PRRS virus at 90 days of gest ation, and their piglets (the test piglets) were monitored for 29 week s from birth. In one litter, antibodies against PRRS virus were absent before the piglets had ingested colostrum. Four days after birth, pas sive antibodies were present in the serum of these piglets, but they h ad disappeared by three weeks (just before weaning) when clinical sign s were observed in a minority of the pigs. In a second litter, most of the piglets had no detectable antibodies until they were four weeks o ld, and clinical signs were observed during their second week of life. By eight weeks, antibodies were detected in all the pigs, and they pe rsisted until observations ceased at 29 weeks. Two groups of three SPF pigs were placed in direct contact with the test piglets when they we re four weeks old and a group of five SPF pigs was placed in indirect contact when they were 13 weeks old. The first two groups showed clini cal signs and seroconverted but the third group did not. At 22 weeks o ld, two of the test piglets were subjected to movement stress and were given exogenous corticosteroids, after which the in-contact SPF pigs developed clinical signs and seroconverted. An epidemiological survey was carried out on three farms for six months, when similar serologica l profiles were observed. Pigs that did not seroconvert shortly after weaning developed clinical signs and seroconverted five months later. Antibodies to PRRS virus diminished to undetectable levels within the study period in a number of the pigs which had seroconverted soon afte r the onset of disease on the farm. It is concluded that the PRRS viru s can persist on farms for extended periods after an initial infection .