LAUSR

Abstract Background Reemergent local outbreaks of classical swine fever (CSF) occurred simultaneously in multiple pig farms on CSF‐free Jeju Island, South Korea, in 2014 because of inadvertent injection of a commercial CSF (LOM) vaccine into pregnant sows. The LOM virus has since spread across the island and has become endemic in Jeju herds, raising concern about possible reversion to the virulence of the LOM vaccine. We previously isolated LOM‐derived field CSF virus (CSFV) strains with unique insertion‐deletion (INDEL) mutations in the 3′‐untranslated region (UTR), designated LOM‐derived Jeju 3′‐UTR INDEL variants, from CSF‐recurrent swine farms on Jeju Island in 2019. Methods The present study conducted animal experiments to investigate whether a 2019 emergent LOM 3′‐UTR INDEL variant, KNU‐1905, has reverted to a pathogenic form in conventional pigs (n = 10). Results Experimental animal infection showed that pigs inoculated with the commercial LOM vaccine strain developed no adverse effects compared to the sham‐infected pigs. However, KNU‐1905 displayed pathogenic characteristics in pigs, including clinical symptoms (e.g., lethargy, conjunctivitis, nasal discharge, and diarrhoea), weight loss, and gross lesions. Moreover, viremia, virus shedding in faeces and nasal fluids, and viral loads in various tissues of all the KNU‐1905‐infected pigs were highly significant, in contrast to those of the LOM‐infected group in which CSFV RNA was detected only in the serum, nasal, and tonsil samples of one identical pig. Conclusions Overall, the LOM‐derived field isolate with molecular variations induced clinical adverse events in pigs, which commonly shed considerable amounts of CSFV. This study provides evidence that the genetic evolution of the LOM‐derived CSFV circulating on Jeju Island might have allowed the LOM vaccine to recover its primary prototype and that these variants might have induced chronic or persistent infection in pigs that can shed CSFV in field farms leading to a risk of transmission among pigs or farms in this former CSF‐free region.