Sequence analysis of porcine reproductive and respiratory syndrome virus of the American type collected from Danish swine herds.

Vaccine-like viruses of American type of porcine reproductive and respiratory syndrome virus (PRRSV) were detected in serum samples by RT-PCR. The viruses were analysed by nucleotide sequencing of the genomic region encoding open reading frames 2 to 7. During the ongoing study of Danish isolates of PRRSV by means of nucleotide sequencing, RT-PCR reactions and subsequent nucleotide sequencing showed the presence of American type PRRSV in Danish breeding herds. Most likely, these atypical viruses originated from boars vaccinated with live vaccine of American type (MLV RespPRRS), which were taken to artificial insemination centres and there brought together with unvaccinated boars already at the centres. The nucleotide sequences of three Danish viruses of American type PRRSV were compared to those of known PRRSV isolates. The nucleotide sequence identities of the atypical Danish isolates were between 99.2-99.5% to the vaccine virus RespPRRS and 99.0-99.3% to VR2332 which are the parental virus to the vaccine virus. Phylogenetic analysis including field isolates of American type supports the conclusion that the introduction of American type PRRSV in Denmark was due to spread of vaccine virus.