Pathogenicity of genotype 2.1 classical swine fever virus isolated from Japan in 2019 in pigs

Classical swine fever (CSF) re‐emerged in Japan in 2018 for the first time in 26 years. The disease has been known to be caused by a moderately pathogenic virus, rather than the highly pathogenic virus that had occurred in the past. However, the underlying pathophysiology remains unknown. This study conducted an experimental challenge on specific pathogen‐free (SPF) pigs in a naïve state for 2, 4, and 6 weeks and confirmed the disease state during each period by clinical observation, virus detection, and pathological necropsy. We revealed the pathological changes and distribution of pathogens and virus‐specific antibodies at each period after virus challenge. These results were comprehensively analyzed and approximately 70% of the pigs recovered, especially at 4‐ and 6‐week post‐virus challenge. This study provides useful information for future countermeasures against CSF by clarifying the pathogenicity outcomes in unvaccinated pigs with moderately pathogenic genotype 2.1 virus.