Submitted to: Research Workers in Animal Diseases Conference Proceedings
Publication Type: Abstract Only
Publication Acceptance Date: 11/12/2000
Publication Date: N/A
Technical Abstract: The study was made to determine the possible role of genetic recombination in the in vivo emergence of new strains (quasispecies) of porcine reproductive and respiratory syndrome virus (PRRSV). Sixteen pigs were each exposed simultaneously to 2 X 10**6 median cell culture infective doses of each of 5 attenuated strains of PRRSV for which the entire base sequence of all structural genes (open reading frames [ORF] 2-7) was known. At weekly intervals for 6 weeks thereafter blood samples were obtained and tested for the presence, number, and identity of strains contained therein. Initial testing was done by virus isolation and restriction fragment length polymorphism (RFLP) analysis. Isolates from samples that were obtained from 8 pigs at either 4 or 6 weeks after exposure, and that appeared by RFLP analysis to contain a single strain of the virus (many samples contained multiple strains), were sequenced (either ORF 3-5 or ORF 2-7) to determine whether genetic recombination had taken place. All were shown to be recombinants of either 2 or 3 of the strains to which the pigs were initially exposed. To test for relative virulence, two groups of pigs were exposed to either a selected recombinant or a known virulent field strain of PRRSV. Lesions and virus replication were more extensive in pigs exposed to the known virulent strain. Our results indicate that under the conditions described above, recombination is common and recombinants can predominate in the circulation of infected pigs. The latter may be explained by relative replication rate, or immune selection, or both. We are currently investigating these possibilities.