Evidence for divergence of restriction fragment length polymorphism patterns following in vivo replication of porcine reproductive and respiratory syndrome virus

Objective-To determine stability of the restriction fragment length polymor phism (RFLP) pattern of a porcine reproductive and respiratory syndrome vac cine virus and patterns of other viral strains as they replicate in pigs. Sample Population-Field samples of porcine reproductive and respiratory syn drome virus (PRRSV) and samples from 2 weaned pigs, 2 nursery-age pigs, and 5 gilts experimentally infected with PRRSV. Procedure-PRRSV was isolated from field samples, experimentally infected pi gs, or pigs that were in contact with experimentally infected pigs. For eac h virus, RNA was isolated from infected cells, and RFLP patterns were deter mined. Results-61% of field samples had 2-5-2 RFLP patterns characteristic of the vaccine virus, 32%.had field virus RFLP patterns, and 7% had intermediate R FLP patterns that indicated a virus with a close relationship to the vaccin e virus. Viruses isolated from experimentally infected pigs had no change i n RFLP patterns after up to 13 weeks of in vivo replication and transmissio n to contact pigs. Conclusions and Clinical Relevance-RFLP patterns distinguish the vaccine an d field strains of PRRSV; however, as the vaccine virus spreads among a swi ne population, the RFLP pattern can change to a related intermediate patter n. A glycine at residue 151 of open reading frame 5 is another marker for t he vaccine virus; this glycine is rapidly lost and eventually replaced with arginine as the vaccine virus replicates in pigs.