Emergence of a virulent porcine reproductive and respiratory syndrome virus in vaccinated herds in the United States

• A virulent type 2 PRRSV emerged in a well-vaccinated farm in Minnesota, USA.
• The PRRSV isolate spread rapidly in the region.
• The complete genome sequence of the isolate showed no recombination.
• The isolate came from existing wild-type viruses circulating in the mid-western US.

In early 2014, a Minnesota sow farm with a solid vaccination history suffered a severe porcine reproductive and respiratory syndrome (PRRS) outbreak with unusually high morbidity and mortality in piglets and sows, as well as anorexia and secondary bacterial infections in nursery pigs. Due to the unusual clinical severity in a PRRS-immune herd, genetic characteristics of the virus were examined to determine if a new PRRSV genotype had emerged. Phylogenetic analysis indicated that the virulent strain (PRRSV2/USA/Minnesota414/2014) was related to virulent strains circulating in the mid-western United States in recent years, and that the nonstructural protein 2 (nsp2) gene of MN414 contained an insertion-deletion pattern typical of existing type 2 virulent strains. We conclude that the MN414 isolate is a recently evolved member of the virulent lineage 1 family of type 2 PRRSV.