Identification and clinical assessment of suspected vaccine-related field strains of porcine reproductive and respiratory syndrome virus
Mengeling, W.L.; Vorwald, A.C.; Lager, K.M.; Clouser, D.F.; Wesley, R.D.
American Journal of Veterinary Research 60(3): 334-340
1999
ISSN/ISBN: 0002-9645 PMID: 10188816 Document Number: 505991
An apparently uncommon restriction endonuclease digestion site in a commercial vaccine strain of attenuated porcine reproductive and respiratory virus (PRRSV) was tested for stability and prevalence under defined conditions. Selected field strains of PRRSV, with or without the restriction-site marker, were tested in pigs for virulence and for the ability to replicate competitively in pigs simultaneously given the vaccine. Under experimental conditions, the restriction-site marker was stable during long-term infection of pigs. It was not detected in any of the 25 field strains of PRRSV that were isolated before use of the vaccine or 21 of 25 field strains that were isolated after use of the vaccine but that, on the basis of previous testing, were believed unrelated to the vaccine strain. Conversely, it was detected in 24 of 25 field strains that were isolated after use of the vaccine and that, on the basis of previous testing, were believed to be direct-line descendants of the vaccine strain. Putative vaccine-related strains caused more severe pathological changes than did the vaccine strain alone, and they predominated during replication in pigs also given the vaccine strain. In some herds, the vaccine strain may have persisted and mutated to a less attenuated form. The potential for persistence and mutation of specific strains of virus should be an important consideration when designing vaccination programmes involving attenuated PRRSV.