Pathogenicity of a Canine-Derived Feline Panleukopenia Virus in Cats and NS1 Evolution Associated with Adaptation to Dogs

copies/g), particularly in the ileum, jejunum, and lungs. Viral DNA levels in the lungs, ileum, caecum, and rectum were significantly higher in the feline-derived group. Sequence analysis identified four NS1 mutations, 115I, 132L, 247Q, and 595Q, which showed stepwise evolutionary accumulation and signatures of positive selection. These findings indicate that canine-derived FPV retains infectivity in cats but exhibits attenuated pathogenicity and reduced replication fitness, highlighting NS1 as a potential determinant of host adaptation.