Emergence of a PRRSV Strain Recombined From Two Modified‐Live Virus Vaccines and Its Elimination From a Breeding Herd

Porcine reproductive and respiratory syndrome (PRRS) is one of the most challenging diseases for swine production. The PRRS virus (PRRSV) is an RNA virus that replicates via an RNA-dependent RNA polymerase (RDRP) mechanism, which is prone to high mutation rates. Recombinations are characterized by the exchange of genetic material across two or more viruses. Modified live virus (MLV) vaccines produce an immune response to PRRSV after replicating in pigs, similar to natural exposure. Here, we report the emergence of an MLV-like recombinant strain, its associated production impact, and its disappearance trajectory from a breeding herd. The emergent virus was identified and successfully eliminated from a 9248-sow breed-to-wean herd. Accidental usage of two distinct MLVs in the herd led to the recombination and emergence of a new strain. The clinical presentation was mild compared to current wild-type strains, with the associated production loss amounting to 549 weaned piglets per 1000 sows. Production levels returned to normal within 7 weeks. Transitory, no significative production loss in the wean-to-market phase was identified. Immunization of the herd and tightening of biosecurity and biocontainment practices were able to eliminate the virus from the herd, without evidence of broad regional spread.