What species is believed to have first transmitted the H3N8 influenza virus to dogs?

The H3N8 influenza virus is believed to have first transmitted to dogs from horses. This particular strain of the virus is primarily associated with outbreaks in equine populations, particularly in the context of racetracks and stables where horses are in close contact. The transmission likely occurred when the virus adapted from its original host in horses to infect dogs.

Understanding the evolutionary history of influenza viruses is critical, as they can jump from one species to another, particularly in settings where different species are in close proximity. The process of adaptation to a new host species, such as dogs, showcases how cross-species transmission can lead to the emergence of new diseases.

Considering other options, avian species and cats have their own strains of influenza; however, they are not the origins for the H3N8 strain found in dogs. Wild rodents were not implicated in the initial transmission either. Therefore, the involvement of horses, particularly in their role as the original host species, makes this the correct answer.