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 resp iratory syndrome virus (PRRSV) were detected in serum samples by RT-PC R. The viruses were analysed by nucleotide sequencing of the genomic r egion encoding open reading frames 2 to 7. During the ongoing study of Danish isolates of PRRSV by means of nucleotide sequencing, RT-PCR re actions and subsequent nucleotide sequencing showed the presence of Am erican type PRRSV in Danish breeding herds. Most likely, these atypica l viruses originated from boars vaccinated with live vaccine of Americ an type (MLV RespPRRS), which were taken to artificial insemination ce ntres and there brought together with unvaccinated boars already at th e centres. The nucleotide sequences of three Danish viruses of America n type PRRSV were compared to those of known PRRSV isolates. The nucle otide 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 whi ch are the parental virus to the vaccine virus. Phylogenetic analysis including field isolates of American type supports the conclusion chat the introduction of American type PRRSV in Denmark was due to spread of vaccine virus.