Canine influenza viruses (CIVs) haven't generated the same level of interest and concern that avian and swine flu strains have, mostly because we haven't seen any evidence of them infecting, and sickening, humans.
But like all flu viruses, CIVs continue to evolve - and occasionally reassort with other viruses - making past performance hardly a guarantee of future results.
The relatively short known history of CIV began about 12 years ago, when an equine H3N8 virus (which only jumped to horses about 50 years earlier), mutated enough that it jumped from horses to greyhounds at a racetrack in South Florida (see EID Journal article Influenza A Virus (H3N8) in Dogs with Respiratory Disease, Florida).
As this canine H3N8 virus was a 96% match to the equine H3N8 virus, this canine adapted virus was believed to have jumped directly from horses to dogs without any reassortment.
The canine influenza story evolved again in 2008, when the EID Journal reported on the 2007 Transmission of Avian Influenza Virus (H3N2) to Dogs in Korea. Analyses showed that the HA and NA genes of the A/canine/Korea/01/2007 (H3N2) isolate were closely related to those identified in 2003 from chickens and doves in South Korea.
So instead of horses, the H3N2 CIV virus had an avian origin.
Over the next few years this Korean H3N2 virus spread into China, and in 2011 we learned from a study that appeared in the Journal of General Virology, that in 2010 it was found to have infected cats, as well (see Korea: Interspecies Transmission of Canine H3N2).
As with the equine and canine strains of H3N8, we’ve not seen any evidence of human infection with this canine H3N2 virus.
But what we have seen is a further mixing of canine influenza viruses with human, swine, and avian flu viruses.
In late 2012, in China: Avian-Origin Canine H3N2 Prevalence In Farmed Dogs, we saw a study that found more than 12% of farmed dogs tested in Guangdong province carried a strain of canine H3N2 similar to that seen in Korea. The authors cautioned:
As H3N2 outbreaks among dogs continue in the Guangdong province (located very close to Hong Kong), the areas where is densely populated and with frequent animal trade, there is a continued risk for pets H3N2 CIV infections and for mutations or genetic reassortment leading to new virus strains with increased transmissibility among dogs.
In 2014, a study appearing in the journal Epidemiology & Infection, that found a new reassortment of the canine H3N2 virus – one that had picked up the M (matrix) gene from the 2009 H1N1 pandemic virus (see Canine H3N2 Reassortant With pH1N1 Matrix Gene).
When this H1N1 M gene turned up in reassorted swine variant viruses (H1N1v, H3N2v, H1N2v), The CDC speculated that `This M gene may confer increased transmissibility to and among humans, compared to other variant influenza viruses.’ – CDC HAN 2012
And in early 2015, the Virology Journal a paper where Chinese scientists isolated and identified what appears to be a human/swine combination H3N2 influenza virus in pet dogs from Guangxi, China (see Virology J: Human-like H3N2 Influenza Viruses In Dogs - Guangxi, China).
Going back once again to 2014, in Study: Dogs As Potential `Mixing Vessels’ For Influenza, we looked at a study in the Journal of Virology, that tested the ability of different influenza strains to infect, and replicate in, canine tracheal tissues. An accompanying press release from the American Society for Microbiology warned:
Evolution of Equine Influenza Led to Canine Offshoot Which Could Mix With Human Influenza
WASHINGTON, DC – June 19, 2014 – Equine influenza viruses from the early 2000s can easily infect the respiratory tracts of dogs, while those from the 1960s are only barely able to, according to research published ahead of print in the Journal of Virology. The research also suggests that canine and human influenza viruses can mix, and generate new influenza viruses.
(continue . . .)
All of which brings us to a recent report (alas, behind a paywall) in Virus Genes that describes finding an H3N2 CIV in Korea, in 2015, that had apparently picked up one of its gene segments from the Avian H9N2 virus.
First the abstract, after which I'll have a bit more.
Isolation of a novel H3N2 influenza virus containing a gene of H9N2 avian influenza in a dog in South Korea in 2015.
AbstractWe isolated a serotype H3N2 influenza virus from a dog with severe respiratory distress in an animal clinic in South Korea in 2015 and characterized the sequences of its eight genes.
The following seven genes were derived from canine influenza virus: PB2, PB1, HA, NP, NA, M, and NS. However, the PA gene was derived from avian H9N2 influenza virus that is circulating in poultry in Korea.
These findings suggest that the continued surveillance of the influenza virus in dogs is warranted because humans have close contact with dogs, which may promote viral transmission.
While there is no indication from this abstract that this novel H3N2 virus poses any elevated risk to canine or humans, this is yet another example of CIV's expansion and growing diversity.
The genetic contribution of the PA gene from H9N2 is also noteworthy since this Asian LPAI virus has provided the backbone genes to some of the most successful (and dangerous) novel flu viruses (H5N1, H5N6, H7N9, etc.) to come out of China in the past two decades.
Last November, in EID Journal: Replication Of Avian H9N2 In Pet Birds, Chickens, and Mammals, Bangladesh we looked at just how promiscuous (and successful) this avian virus has become - both on its own, and lending bits and pieces of its genetic structure to other viruses.
None of this is to suggest that CIV is on the verge of jumping to humans, but as the CDC notes in their Canine Flu FAQ, there are good reasons to watch viruses like these very closely.
Can canine influenza viruses infect humans?To date, there is no evidence of transmission of canine influenza viruses from dogs to people and there has not been a single reported case of human infection with a canine influenza virus.
However, influenza viruses are constantly changing and it is possible for a virus to change so that it could infect humans and spread easily between humans. Human infections with new influenza viruses (against which the human population has little immunity) are concerning when they occur. Such viruses could present pandemic influenza threats. For this reason, CDC and its partners are monitoring the canine influenza H3N8 and H3N2 viruses (as well as other animal influenza viruses) closely. In general, canine influenza viruses are considered to pose a low threat to humans.