Key result
An outbreak of an atypical Feline calicivirus strain in adult cats caused severe flu-like symptoms, diarrhea, and anemia, forming a new phylogenetic lineage distinct from known FCVs.
Why the study?
An outbreak of severe flu-like disease with atypical symptoms was reported among stray cats on Aoshima Island, prompting molecular characterization of the isolated feline calicivirus strains.
Observational (n=9)
No
An outbreak of severe flu-like symptoms in adult cats on a remote Japanese island was caused by a novel, atypical virulent strain of Feline calicivirus.
• Epidemic of atypical Feline calicivirus (FCV) in Aoshima Island, Japan. • Adult cats showed severe flu-like symptoms rarely accompanied by anemia and diarrhea. • Aoshima virulent strain likely mutated from typical FCV. • Aoshima virulent strain showed no serological cross-reactivity with an Osaka isolate. • Implications for FCV pathogenicity in cats. In November 2020, a volunteer group reported an outbreak of an infectious disease with a high fatality rate and flu-like symptoms among stray cats in Aoshima, a remote island in Ehime, Japan. Nine adult cats with severe symptoms were hospitalized. Feline calicivirus (FCV) was isolated from pharyngeal swabs of six hospitalized cats. An outbreak of virulent systemic FCV (VS-FCV) infection was initially suspected because of obvious flu-like symptoms in adult cats; however, no symptoms typically associated with VS-FCV, such as skin ulcers on the limbs, edema, or viremia, were observed. Notably, two of the hospitalized cats that showed severe disease had diarrhea and anemia, and died or had a prolonged illness. These cases reveal atypical symptoms of FCV infection that have not been previously reported. We further isolated typical strains from western Japan (Osaka, Kumamoto, and Ehime) and analyzed the viral genes along with virulent strains from Aoshima. Phylogenetic analysis showed that the Aoshima strain formed a new lineage distinct from known FCVs. The Aoshima strains isolated in the initial outbreak before December 5, 2020, and those isolated after the end of the outbreak, which are suspected pathogenic and typical non-pathogenic strains, respectively, were located in the same cluster and shown to be very similar in sequence. The virulent Aoshima strain, which causes atypical FCV infections in cats, may have been derived by acquiring several mutations from a typical strain that chronically infects cats on a remote island.
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Nishisaka et al. (2025) conducted an observational in Feline calicivirus (FCV) infection (n=9). Atypical Feline calicivirus (FCV) infection was evaluated on Clinical symptoms and viral phylogenetic characteristics. An outbreak of an atypical Feline calicivirus strain in adult cats caused severe flu-like symptoms, diarrhea, and anemia, forming a new phylogenetic lineage distinct from known FCVs.
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