Infectivity and pathogenicity of canine H3N8 influenza A virus in horses

BACKGROUND Equine H3N8 influenza A viruses (EIVs) cause respiratory disease in horses and circulate among horses worldwide. In 2004, an outbreak of canine H3N8 influenza A virus (CIV) occurred among dogs in Florida and has spread among dogs in the United States (US). Genetic analyses revealed that this CIV is closely related to the recent EIVs. Although CIV-infected dogs could be the source of H3N8 influenza A virus for horses, it remains unclear whether the CIV circulating in the United States still maintains its infectivity and/or pathogenicity in horses. To address this, we investigated the infectivity and pathogenicity of CIV in horses and the receptor binding specificity of CIV. MATERIALS AND METHODS Three horses were inoculated with A/canine/Colorado/30604/2006 (CO06, H3N8). Clinical signs and nasal swabs were recorded or collected every day. We also evaluated the virus binding to α2-3-linked 5-N-acetylneuraminic acid (NeuAcα2-3Gal) and 5-N-glycolylneuraminic acid (NeuGcα2-3Gal) receptor analogues. RESULTS Although all the three horses inoculated with CO06 seroconverted, they showed only mild clinical signs and two of them showed no virus shedding. CO06 had reduced binding to NeuGcα2-3Gal. DISCUSSION Our results demonstrated that CO06 had reduced proliferation ability and pathogenicity in horses. As the recognition of NeuGcα2-3Gal by EIV is known to be essential for binding to the equine respiratory system, the decreased binding of CO06 to NeuGcα2-3Gal may be one of the important factors that reduces the proliferation ability and pathogenicity of CO06 in horses.