Cessation of porcine reproductive and respiratory syndrome (PRRS) virus spread in a commercial swine herd

We report a case of porcine reproductive and respiratory syndrome (PRRS)virus infection that appeared to spontaneously stop spreading. We compared serologic profiles of this 250-sow farm (herd A) with those of another 300-sow farm (herd B), both with a previous history of PRRS. In November 1992, we collected serum samples from pigs of different age groups (from I week to 3 years old) from both herds. We tested PRRS virus antibody titers by an indirect-fluorescent antibody (IFA) method. IFA titers of ~/: 16 were detected in 20 of 129 pigs (/5.5%) from herd A and 73 of 158 pigs (46.2%) from herd B. In April 1993, none of 30 pigs between 4-26 weeks of age from herd A were seropositive, while all 60 pigs of the same age group from herd B were seropositive. A group of 15 gilts in herd A, introduced in january 1992 from a farm showing no clinical signs of PRRS, remained seronegative. No PRRSvirus was isolated in April 1993 from 30 serum samples from pigs 5-10 weeks old in herd A, while virus was isolated from 4 of 28 sera from pigs of a similar age in herd B.PRRS virus stopped spreading in herd A, while the infection is still endemic in herd B.