Submitted to: American Journal of Veterinary Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 1, 1998
Publication Date: N/A
Interpretive Summary: Porcine reproductive and respiratory syndrome (PRRS) is presently one of the most costly infectious diseases faced by the swine industry. As an aid in its control, attenuated PRRS virus (PRRSV) vaccines have been developed and are now widely used in the United States. Although it is generally believe that the judicious use of such vaccines can be beneficial there are certain side effects that are important to keep in mind when designing a vaccination program. In this manuscript we identify such side effects and discuss the means by which their negative impact can be minimized. The proper use of PRRS vaccines leads to more profitable pork production and lower cost pork products for the American consumer.
Technical Abstract: Objective: To determine the origin and clinical relevance of selected strains of porcine reproductive and respiratory syndrome (PRRS) virus (PRRSV). Procedure: A seemingly uncommon restriction endonuclease digestion site in the RespPRRS vaccine strain of attenuated PRRSV was tested for its stability during replication in pigs under experimental conditions and for its prevalence in field strains of PRRSV isolated befor and after the use of RespPRRS. Selected field strains of PRRSV, with or without the putative marker, were subsequently tested in pigs for virulence and for their ability to replicate competitively in pigs simultaneously exposed to RespPRRS. Results: The RespPRRS marker was found to be stable during long-term (greater than or equal to 7 weeks) infection of pigs. It was absent from all of 25 field strains of PRRSV isolated before the use of RespPRRS and from 22 of 25 field strains of PRRSV isolated after the use of fRespPRRS, but believed unrelated to RespPRRS. Conversely it was present i 24 of 25 field strains of PRRSV that were isolated after the use of RespPRRS and, on the basis of additional information, believed to be direct-line descendants of RespPRRS. The suspected RespPRRS-related strains caused more pronounced pathologic changes than did RespPRRS alone and they predominated during replication in pigs also exposed to RespPRRS. Conclusions: In some cases RespPRRS may have persisted in vaccinated pigs, or been transmitted from vaccinated pigs to naive contact pigs, or both, and through presumed mutational changes evolved to a less attenuated form. Clinical Relevance: The potential for persistence, transmission, and mutation should be an important consideration in the design and application of any live PRRSV vaccination program.