Genetic, geographical and temporal variation of porcine reproductive and respiratory syndrome virus in Illinois

Porcine reproductive and respiratory syndrome virus (PRRSV) ORF5 gene seque nces were generated by RT-PCR from 55 field isolates collected in Illinois and eastern Iowa. Spatial and temporal patterns of genetic variation in the virus were examined on a local geographical scale in order to test the hyp othesis that the genetic similarity of PRRSV isolates (measured as their pe rcentage pairwise ORF5 nucleotide similarity) was positively correlated wit h their geographical proximity. Levels of genetic variability in the Illino is/eastern Iowa PRRSV sample were similar to levels of variability seen acr oss broader geographical regions within North America. The genetic similari ty of isolates did not correlate with their geographical distance. These re sults imply that the movement of PRRSV onto farms does not generally occur via distance-limited processes such as wind or wildlife vectors, but more t ypically occurs via the long-distance transport of animals or semen. Geneti c distances between PRRSV isolates collected from the same farms at differe nt times increased as the time separating the collection events increased. This result implies rapid movement of new genetic types of PRRSV into and o ut of farms. PRRSV ORF5 displayed a pattern of third-codon-position diversi ty bias that was not evident in a geographically comparable sample of pseud orabies virus (a swine alphaherpesvirus) gC gene sequences. This result pro vides evidence that PRRSV ORF5 is experiencing stabilizing selection agains t structural novelty. Despite high genetic variability at all geographical levels, PRRSV ORF5 nevertheless contained potentially antigenic regions tha t were invariant at the amino acid level. These regions should make effecti ve vaccine targets if they prove to be immunogenic.