Interspecies transmission of equine influenza virus (H3N8) to dogs by close contact with experimentally infected horses

In horse populations, influenza A virus subtype H3N8 (equine influenza virus, EIV) is a very important pathogen that leads to acute respiratory disease. Recently, EIV has emerged in dogs, and has become widespread among the canine population in the United States. The interspecies transmission route had thus far remained unclear. Here, we tested whether the interspecies transmission of EIV to dogs could occur as a result of close contact with experimentally EIV-infected horses. Three pairs consisting of an EIV-infected horse and a healthy dog were kept together in individual stalls for 15 consecutive days. A subsequent hemagglutination inhibition test revealed that all three dogs exhibited seroconversion. Moreover, two of the three dogs exhibited virus shedding. However, the dogs exhibited no clinical signs throughout the course of the study. These data suggest that the interspecies transmission of EIV to dogs could occur as a result of close contact with EIV-infected horses without clinical symptoms. Although the interspecies transmission of EIV is unlikely to become an immediate threat to canine hygiene, close contact between EIV-infected horses and dogs should be avoided during an EI epidemic.