On an almost weekly basis, it seems as if I’ve been seeing reports of the “new” form of dog flu (H3N2) causing illness in parts of the country that were previously unaffected. There are a couple of reasons why H3N2 is spreading more rapidly than the “old” form of dog flu (H3N8), but first some background.


According to the American Veterinary Medical Association:


The canine H3N2 strain… emerged in Asia in 2006-2007 among dogs suffering from respiratory disease. This strain in Asia likely arose through the direct transfer of an avian influenza virus – possibly from among viruses circulating in live bird markets – to dogs. The new canine virus spread widely among dogs in South Korea and in several regions of China, and caused an outbreak of respiratory disease among dogs in Thailand in 2012. In 2015, a canine H3N2 that was genetically almost identical (99% identical) to the Asian strain was detected in the United States. Although rumors have circulated that the virus was introduced to the U.S. through dogs rescued and imported from Asia, there is no evidence to confirm these rumors. [National Public Radio reports “The virus apparently was brought into the country through O'Hare International Airport by an infected dog from South Korea.]


Zoeitis, the maker of one of the two canine H3N2 vaccines that are currently available under conditional licensure, describes the spread of dog flu this way:


In the contiguous United States from 2006 to 2014 [8 years], CIV [canine influenza virus] H3N8 was reported in 36 states. In the short period from March 2, 2015, to September 31, 2015 [7 months], CIV H3N2 was found in 25 states.


I believe the count must be up to at least 27 by now since the recent reports I’ve been seeing of dogs infected in Montana and Washington State are not included in the Zoeitis data.


Why is H3N2 dog flu spreading so quickly in the United States? Two factors are primarily responsible.


First of all, the H3N2 form of dog flu has been present in the U.S. for less than a year, so the great majority of dogs have no immunity to it. Preventative vaccination is now available through veterinarians, but the inoculations are only given to dogs considered to be at significant risk for contracting the disease. This would include individuals who live in H3N2 hotspots and/or those who board, attend doggy daycare facilities, show, etc.


Secondly, dogs who are infected with H3N2 flu can shed the virus and infect other dogs for a longer period of time than do those who contract the H3N8 form of the disease. Veterinarians have traditionally recommended isolating dogs with an infectious respiratory disease for two weeks to limit its spread. With H3N2 dog flu, it appears that a three week quarantine is more appropriate.


Even though the new form of dog flu is spreading rapidly, the decision of whether or not to vaccinate should be made on a case by case basis. Most dogs develop a mild form of the disease and recover over the course of a couple of weeks. Some cases, however, progress to pneumonia and may prove fatal. Talk to your veterinarian to determine if vaccination is in your dog’s best interests.



Dr. Jennifer Coates