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

Feline panleukopenia virus (FPV) is a major feline pathogen, but canine-derived FPV variants have recently been identified. Here, we compared the pathogenicity of a canine-derived FPV strain in cats with that of a lethal feline-derived FPV strain and evaluated the evolutionary significance of its NS1 mutations. Kittens infected with the canine-derived strain developed only mild, self-limiting diarrhea without fever or mortality, whereas those infected with the feline-derived strain developed severe disease and reached humane endpoints by 9 dpi. The canine-derived strain caused prolonged fecal shedding from 6 to 38 dpi but only low tissue viral loads (101-103 copies/g), while the feline-derived strain reached markedly higher loads (103-106 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.