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Summary Jeju island is the biggest island in Korea. The imports of pigs or their relatives from mainland Korea to this island has been banned since 1998. With this unique geographical and epidemiological context, epidemiology of porcine reproductive and respiratory syndrome virus (PRRSV) was investigated on the island. While all investigated farms showed 100% of seropositive rate for PRRSV, pigs on 37.2% (16/43) of the farms had viremia with type II PRRSV. The seropositive and viremia-positive rates for PRRSV in 30- to 60-day-old pigs were significantly higher in the western area ('swine farm complex' area) than the eastern area ('Scattered swine farm' area) of Jeju island. When 21 ORF5 sequences obtained from viremic sera were phylogenetically analysed, lineage 5 and Kor (newly termed in this study) of type II PRRSV were only found in Jeju island without changes from a previous report (2002-2003). Because other lineages of type II PRRSV (lineage 1 and 3) and type I PRRSV have recently emerged in mainland Korea, the banning of pigs' movement might be effective to protect the island from the introduction of these new PRRSV genotypes. Under this epidemiological condition (no introduction of new strains except for the modified-live vaccine (MLV) strain), the positive selection sites were analysed based on ORF5 of the virus. The amino acid 58 of GP5 (located on the hypervariable region) was predicted as a strong positive selection site. Although 51.2% (22/43) of the investigated farms had applied MLV, field strains of type II PRRSV were still circulating with strong positive selection. However, the restricted population of type II PRRSV (lineage 5 and Kor) without introduction of type I PRRSV or the other lineages of type II PRRSV indicate that the island has an effective quarantine system, which would allow PRRSV eradication. [ABSTRACT FROM AUTHOR]