What is the most likely explanation for how H3N2 canine influenza emerged in the United States?

• A. It was transmitted from wild waterfowl to dogs in the Chicago area.
• B. It arrived in dogs rescued and imported from Asia.
• C. It was transmitted to dogs because of the practice of feeding raw horse meat to racing greyhounds.
• D. It was transmitted directly from humans to dogs and then adapted to dogs and spread.

Answer: (C)

A virus can jump species when it finds an exposure route that brings it into a new host and allows it to replicate there. In the case of H3N2 canine influenza emerging in the United States, the route that best fits the context is exposure of dogs to virus present in horse tissue through the practice of feeding raw horse meat to racing greyhounds. In that setting, a virus circulating in horses or horse tissues could be introduced directly to dogs, giving the virus the chance to adapt to a canine immune system and respiratory tract. Once the virus adapts to dogs, close contact in kennels and the high-density environment of dog racing facilitate efficient dog-to-dog transmission, allowing the outbreak to spread.

Other scenarios—such as direct transmission from wild waterfowl, importation of dogs from Asia, or transmission from humans to dogs—are less consistent with the observed pattern of emergence and spread in this particular outbreak. The raw horse meat route directly ties the exposure to a common practice in a high-risk setting and explains how the virus could move from an equine-associated source into a canine population and become established there.