The objective of this study was to evaluate the diversity and persistence of porcine reproductive and respiratory syndrome virus (PRRSV) in a swine herd where American type of modified live vaccine (MLV) was used. The study was carried out on the farrow-to-finish farm with 2 500 sows, which had a history of endemic PRRSV manifested as periodic mini-outbreaks of PRRS. The farm has been endemically infected with PRRSV-EU since 1998. The vaccination programme with the MLV was first applied in 2001 and consisted of vaccinating all sows every six months and all gilts 60 d before farrowing. All incoming gilts were also vaccinated twice with a two-week interval. During the monitoring period, PRRSV-EU type was detected in 28 of 63 samples collected from two to three-month-old piglets. PRRSV-US type was detected in 20 samples. Nineteen samples were positive for both EU and US types of PRRSV at the same time. The results of RT-PCR testing serum samples from 58 sows were negative. ORF5 RT-PCR products from samples containing both PRRSV-EU and PRRSV-US types were sequenced. Phylogenetic analyses, showed a close relationship to the American genotype of PRRSV strains from the monitored farm to the known vaccine strain V2332 present in the American type PRRS MLV. The presence of the PRRSV-US in the two-three-month-old piglets indicates that the American type of vaccine virus has spread from vaccinated sows to the non-vaccinated piglets. Simultaneous presence of both PRRSV-EU and US strains in 19 samples suggests very low cross protection between the American type PRRS MLV and very diverse Lithuanian EU type field strains. In this situation an increased chance for inter-genotypic recombination can be a threat.
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