Attempted elimination of porcine reproductive and respiratory syndrome virus from a seedstock farm by vaccination of the breeding herd and nursery depopulation

An attempt was made to eliminate the virus of porcine reproductive and respiratory syndrome from a seedstock farm by using the combined strategies of vaccination and nursery depopulation. The breeding herd was vaccinated with a modified-live virus vaccine; all breeding and lactating adult animals were vaccinated twice, with a 30-day interval between vaccinations. All the sows were vaccinated in this way except for those in the third trimester of gestation (66 to 114 days) which were vaccinated on day 7 of lactation and 30 days later. A serological profiling system was developed to assess when the piglets became infected. Pigs from vaccinated sows were profiled at weekly intervals after weaning, using immunofluorescence tests for the detection of IgM and IgG, a serum neutralising antibody test, and virus isolation. After completion of the protocol, the nursery and finishing sites were monitored for 15 months. Evidence of reinfection in the finishing stage was detected 16 months after depopulation, but not in the nursery or the breeding herd. The source of the virus was not determined, but suspected origins included a lack of biosecurity, aerosol transmission from another infected farm or a persistently infected pig.