Which species is thought to have first transmitted the H3N8 influenza virus to dogs?

• A. Avian species
• B. Cats
• C. Wild rodents
• D. Horses

Answer: (D)

The main idea is how influenza can jump from one species to another and how the source of a canine-strain can be traced. For H3N8 canine influenza, the evidence shows that the virus originated in horses. The equine influenza virus H3N8 was circulating in horse populations, and it crossed over to dogs, with early outbreaks in racing greyhounds linked to exposure to infected horses or horse-environments. Genetic analyses show the canine H3N8 virus is highly similar to the equine H3N8 strain, supporting a horse-to-dog transmission event. Over time, the virus adapted to dogs and established itself in canine populations.

The other animals listed aren’t the source for this particular H3N8-to-dogs transmission. Avian influenza can give rise to other influenza strains in different hosts, but this specific canine H3N8 lineage comes from horses, not birds. Wild rodents and cats aren’t identified as the original transmitter for this strain.