Genetic diversity and possible avenues of dissemination of porcine reproductive and respiratory syndrome virus in two geographic regions of Mexico

Objective: To determine the range of porcine reproductive and respiratory syndrome virus (PRRSV) genetic variation and possible avenues of dissemination in geographically separated swine herds in Mexico. Methods: Pooled serum samples (n=33) from suspected viremic sows and nursery and finisher pigs from 13 sites in the state of Sonora (northwest Mexico) and six sites in Puebla (south-central region) were subjected to PRRSV polymerase chain reaction amplification, purification, and sequencing. Results: Thirty-four PRRSV open reading frame 7 (ORF7) sequences were generated from a total of 31 isolates from 13 sites in Sonora and six in Puebla. The nucleotide similarity of ORF7 among isolates ranged from 86 to 100%. Phylogenetic analysis revealed no obvious geographic, temporal, or other characteristics that accounted for the observed distribution of isolates. Implications: DNA sequence analysis revealed unexpectedly high levels of genetic variation among and within swine-rearing regions in Mexico, suggesting independent PRRSV introductions. Re-isolation of a vaccinal strain of PRRSV suggests importation of vaccinated animals or semen or surreptitious use. The diversity and distribution of viral genotypes indicates that the mechanisms of geographic spread were complex, such that effective control may present a substantial epidemiological challenge.