PERSISTENCE OF PORCINE REPRODUCTIVE AND RESPIRATORY SYNDROME VIRUS ININTENSIVE FARROW-TO-FINISH PIG HERDS

An epidemiological study of porcine reproductive and respiratory syndr ome (PRRS) within pig herds was conducted in 8 intensive farrow-to-fin ish pig farms, Persistence of PRRS virus (PRRSV) in pig herds was demo nstrated by regular postmortem examination on 2 farms for a period of 2 y. Virus isolation and serum neutralization (SN) tests were performe d on the sera collected from 9 groups of pigs (10 pigs/group) of vario us ages on 8 pig farms, Except for 1 farm, isolation rates of PRRSV re ached the highest level of 70 to 100% of pigs 6 to 8 wk of age, which coincided with the lowest levels of maternal immunity, In 1 pig herd, sows (39 in total) with SN titers of less than or equal to 1:2, 1:4-1: 8, and greater than or equal to 1:16 were designated as groups 1, 2, a nd 3, respectively, Sera were obtained from their progeny (3 pigs rand omly selected from each litter) at various ages from 0 to 22 weeks. A positive correlation (r = 0.377, P < 0.001) between the SN titers of s ows and those of their progeny (1-week-old piglets) was observed, Pigs at the age of 6 wk, only 7.9% of group 1 pigs compared to 72.4% of gr oup 3 pigs were seropositive. it significant difference (P < 0.01) in the percentage of pigs with PRRSV viremia among the 3 groups was obser ved, with the lowest level found in group 3 pigs, The isolation rates of PRRSV from serum reached the maximum at the age of 9 wk for all 3 g roups, The results indicated that passively acquired serum antibodies conferred a protective effect for piglets; however, loss of passive im munity at various ages of pigs produced susceptible pigs that resulted in PRRSV persistence in the pig herds, Pigs 6 to 9 weeks old were the major reservoir for PRRSV in farrow-to-finish pig herds.