Characterization of infection with endemic porcine reproductive and respiratory syndrome virus in a swine herd

Abstract

Summary Infection of 8-week-old pigs with endemic porcine reproductive and respiratory syndrome (PRRS) virus was detected on a farm that had an epidemic of PRRS in 1989. During the 2.5 years since the original epidemic, reproductive performance of the breeding herd had been within acceptable limits, but mortality had periodically exceeded one-fourth of the pigs in the nursery (195 died of 761 weaned, 25.6%). Investigators attempted to determine the age and humoral immune status of pigs infected with endemic PRRS virus on the farm. Serum obtained from 9 groups of 1- to 18-week-old pigs (10 pigs/group) was examined for PRRS virus by virus isolation. Serum was obtained from 8 sows that had farrowed within the preceding 24 hours. Serum from the sows was obtained weekly until litters were weaned at 3 weeks of age. Serum was obtained from 27 newborn pigs (3 to 4 newborn pigs from each of the 8 sows) prior to intake of colostrum and at weekly or biweekly intervals until the pigs were 20 to 21 weeks of age. Isolation of PRRS virus and indirect fluorescent antibody serologic testing were performed on these serum samples. In another study, serum was obtained for serologic testing from 10 sows in each of 6 parity groups. The PRRS virus was isolated from serum of only 3- to 12-week-old pigs. The 8 sows and their 27 pigs were seronegative for PRRS virus during the 3-week lactation period. By 10 weeks of age, 18 of the 27 suckling pigs were still alive and had seroconverted (titers ≥ 1:20). Titers peaked at 13 to 14 weeks of age (range, 1:640 to 1: ≥ 2,560). The prevalence of seropositive sows of all parities was 9 of 60 (15%). Pigs in the nursery and grower were the major reservoir for PRRS virus in the herd of this report. Pigs became infected with PRRS virus in the nursery through contact with older pigs that were infected with PRRS virus, and were not infected by in utero or postpartum exposure to sows infected with PRRS virus. Virus circulation in pigs of finishing age and in the breeding herd was minimal. Although all pigs had been viremic and were seropositive for PRRS virus by 9 to 10 weeks of age, 4 of 18 (22%) were seronegative by 20 to 21 weeks of age, indicating titer decay and reversion to seronegative status can develop within 10 weeks.