Submitted to: Pig Veterinary Society International Congress Proceedings
Publication Type: Proceedings
Publication Acceptance Date: July 10, 1996
Publication Date: N/A
Technical Abstract: Three field strains of porcine reproductive and respiratory syndrome virus (PRRSV) were serially passed in cultures of MARC-145 cells to determine if such treatment would reduce their ability to cause reproductive failure in susceptible gilts infected at or about day 90 of gestation. On the basis of clinical signs, all of the strains were attenuated by the 84th cell culture passage, but they all retained the ability to cross the placenta and infect fetuses in utero. Most of the pigs that escaped congenital infection were infected neonatally, presumably early in postnatal life by contact with their congenitally infected littermates. Despite congenital and neonatal infection, however, pigs of infected litters gained weight at a rate similar to controls (i.e. pigs of litters of gilts not exposed to PRRSV) and they had few if any clinical signs. Conversely, most had enlarged lymph nodes and a few had lung lesions at the time they were euthanized and necropsied when they were 21 days old. It was concluded that all of the strains were attenuated by cell culture passage, but none sufficiently to be considered for use as a live-virus vaccine. The study is still in progress and the results of testing the same strains after more than 200 cell culture passages also will be reported.