Three feline coronavirus (FCoV) isolates KUK-H, M91-266, and M91-267 were examined to elucidate their biological and antigenic properties as well as disease potential in cats. Immune stainings of virus-infected cells by using FCoV type-specific monoclonal antibodies indicated that their antigenic specificity was serotype II. However, antigenic variations among these serotype II FCo. Vs were detected by neutralization assay with hyperimmune antisera against FCo. Vs and canine coronaviruses, and with experimentally infected cat sera; there were two subtypes in serotype II FCo. Vs. The isolates efficiently grew in fcwf-4 cell culture showing lytic CPE enough to form distinct plaques; when measured 48 hr after infection, plaque sizes of both M91-266 and M91-267 were approximately 1 mm in diameter, and a mixture of small (less than 1mm in diameter) and large (approximately 3 mm in diameter) plaques were produced in the case of KUK-H. Strains KUK-H, M91-266 and M91-267 produced feline infectious peritonitis (FIP) in 50%, 67% and 89% of experimentally inoculated kittens, respectively. Furthermore, 80% of the kittens inoculated with the small plaque former of KUK-H developed FIP accompanied by more prominent clinical signs as well as pathological changes when compared with 28.6% of kittens inoculated with the large plaque former. These results suggest that serotype II FIPVs producing smaller size of plaques are more virulent than those producing larger size of plaques.
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