An Asian strain of canine influenza virus that was first seen in the U.S. in early 2015 re-emerged in 2017, and has been spreading widely across the nation throughout the past year. Recently it has caused repeated flare-ups in a number of cities in the U.S. around San Francisco earlier this year, and in New York in May and June. During those outbreaks the virus was able to gain an initial foothold by infecting dogs that came together in kennels, animal shelters, and doggy day cares. From those locations it may infect dogs in households, but generally does not spread for very long among household dogs before the epidemic dies out. Researchers at the Baker Institute provide a detailed background to understanding the emergence and spread of these important new canine pathogens in a new paper published in the Journal of Virology.

“It is now clear that dogs can be a good host for influenza viruses, and that long distance transport of infected dogs can move the infection from one region of the world to another, introducing the virus into new locations. We clearly need to be vigilant for influenza in dogs, and to develop new strategies for protecting dogs from infection, and for controlling the disease once it breaks out in a community” said Dr. Colin Parrish, John M. Olin Professor of Virology at the Baker Institute.

Dog flu outbreaks caused by the canine influenza virus are a fairly recent occurrence in the U.S., with the first case described in 2004 caused by the H3N8 type of virus that had transferred from horses. Researchers estimate that the H3N2 type of canine influenza virus, which entered the U.S. in early 2015, initially jumped from birds into dogs in 2005. It circulated through dogs in China and Korea for almost a decade before coming to the U.S., where it has caused repeated flare-ups in different cities, most of which have subsequently faded out.

To understand the dynamics causing these flare-ups, the researchers analyzed 64 newly collected samples of the H3N2 virus by sequencing their complete genomes, and combining that with comprehensive flu surveillance data in dogs throughout the U.S. They discovered that the outbreaks stemmed from repeated introductions of the virus from Korea and China, most likely with infected dogs imported from those countries. Numerous changes in the genomes of the strains suggest that they are continuing to evolve within the U.S. The surveillance data, however, shows that each dog infected with the H3N2 virus passes it to only 1 or 1.5 other dogs, on average, indicating that outbreaks will most likely be relatively short-lived.

Taken together, the findings suggest that the strains in dogs are still relatively weak, and have not evolved into more contagious viruses so far, which limits the virus to outbreaks in animal shelters and other high-density dog populations, from which it spreads to infect household dogs in the same city. However, this appears to have been a rather inefficient process overall, and without additional reintroduction from Asian dogs, the virus may fade out in the U.S.

Based on the recent outbreaks in San Francisco and New York, Dr. Parrish advises vaccination against the H3N3 virus. Vaccinating your dog once an outbreak has started in your area may not be as successful as vaccinating before the outbreak starts. Consult with your veterinarian to discuss the best options for you and your dog.