Submitted to: American Journal of Veterinary Research
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/29/1998
Publication Date: N/A
Interpretive Summary: Porcine reproductive and respiratory syndrome (PRRS) virus is a pathogen of pigs causing one of the most costly diseases currently faced by the swine industry worldwide. Since severe outbreaks of the disease can occur in vaccinated herds, the origin of PRRSV from diagnostic samples is often in doubt. We have identified genetic markers in the RespPRRS/Repro vaccine virus that distinguish it from field strains of PRRSV. Using this test on PRRSV field isolates, we have found PRRS viruses that were derived from the vaccine but have slightly altered genetic markers which is consistent with the persistence of attenuated PRRSV vaccine in swine herds. This information will assist pork producers, veterinary practitioners, and diagnosticians in controlling the disease.
Technical Abstract: Restriction fragment length polymorphism (RFLP) patterns based on ORF 5 amplicons were evaluated to determine the stability of these patterns as PRRS viruses replicate in pigs. RFLP patterns were determined for 221 North American PRRSV field isolates. In addition, 2 weaned pigs, 2 nursery-age pigs, and 5 gilts were experimentally infected with PRRSV; and RFLP patterns were analyzed from viruses subsequently recovered from the same animal or from contact pigs. Sixty-one percent of the field samples had a 2-5-2 RFLP pattern characteristic of the RespPRRS/Repro (TM) vaccine virus, 33 percent had field virus RFLP patterns, and 7% had intermediate RFLP patterns that indicated a close relationship to the vaccine virus. Thus, RFLP patterns can distinguish the RespPRRS/Repro (TM) vaccine virus from other North American PRRSV strains, but as RespPRRS/Repro (TM) replicates in a swine population the RFLP pattern can change to a related or intermediate pattern. For 9 experimentally infected pigs, there was no change in the RFLP patterns even with up to 13 weeks of in vivo virus replication and transmission of virus to contact pigs. Analysis of the ORF 5 nucleotide sequence data from selected field isolates and from PRRS viruses recovered from experimentally infected pigs indicated that glycine at residue 151 is another marker for the RespPRRS/Repro (TM) virus. As the vaccine virus replicates in pigs, this glycine (G) is rapidly lost and eventually replaced with an arginine (R).