Detection of cross-reactivity of antibodies to the N proteins of feline morbillivirus and canine distemper virus in Japanese cat plasma samples.

Feline morbillivirus (FeMV) is a globally emerging virus that has been linked to chronic kidney disease (CKD) in infected cats. Immunological assays, such as enzyme-linked immunosorbent assay (ELISA), are important for studying the virus and monitoring its prevalence. A study using rabbit antiserum demonstrated antigenic cross-reactivity between nucleocapsid (N) proteins of FeMV and canine distemper virus (CDV), suggesting not only the risk of false-positive anti-FeMV antibody detection in ELISAs but also potentially false-positive FeMV antigen detection in Western blotting. To examine whether such cross-reactivity occurs in tests using cat plasma samples, we developed ELISAs using affinity-purified recombinant N proteins of FeMV and CDV with expression in Escherichia coli and tested 100 cat plasma samples collected from veterinary clinics in Japan. Twenty samples were found to be positive for anti-FeMV antibodies, while 6 were positive for anti-CDV antibodies. All these latter 6 samples were double-positive for anti-FeMV antibodies. Western blotting with the purified proteins confirmed the specificity of these antibodies to their target viral antigens. A reverse transcription-quantitative PCR assay with a detection limit of 100 copies failed to detect CDV genomic RNA in these 6 double-positive samples. These results strongly suggest the cross-reactivity between anti-FeMV N protein antibodies in cat plasma samples and the CDV N protein. This antigenic cross-reactivity should be considered in future studies using immunological methods employing FeMV or CDV N proteins, or antibodies targeting them.