If we know one thing about influenza A, it’s that there will always be something new with this virus.


A recent study out of China (Meng et al. 2023) describes what seems to be a new canine flu strain. Is it a concern? It’s hard to say at this point, but having more flu strains in a species with which we have close contact is never a good thing.

This was a surveillance study of dogs in an area of China where there’s a massive amount of pet dog breeding and trading. Our familiar H3N2 canine influenza is endemic there, and avian flu strains circulate in wild birds, creating the potential for spillover of avian flu viruses into dogs and/or emergence of new strains from virus reassortment.

Researchers tested dogs from November 2018 to April 2019, and identified influenza virus in 60 of 534 dogs (11%) by PCR. Follow up testing resulted in isolation of live influenza virus from 12 dogs. Isolation of live virus requires culture techniques. We expect to get less recovery with culture compared to PCR, since PCR will detect lower viral loads and does not require the virus to still be viable by the time of testing.

Five of the flu viruses that were isolated were H3N2.

  • That’s not surprising, since H3N2 is a well established canine flu strain in China.

Interestingly, seven of the viruses were H3N6.

  • Looking at the genetic makeup of the virus, it appears to be a mix of H3N2 canine flu and an H5N6 avian flu strain that was circulating in birds in China in 2017 and 2018.
  • It was hypothesized that this was a result of a dog being co-infected with H3N2 canine flu and H5N6 avian flu, resulting in creation of this new H5N6 strain.

Does this mean there’s a new canine flu strain circulating in China?

That’s unclear. Positive samples were from dogs in one shelter at one time point, which could happen for a few reasons:

They found the first dog with this strain, and picked up transmission in the shelter that burned out, making this a one-off event.

  • It’s very unlikely they would have gotten that lucky and captured the very first emergence of this virus. Some of the in vitro characteristics of this virus suggest it should be well adapted to mammals, so full containment is probably unlikely. There’s also some genetic variation in the H3N6 isolates, which we wouldn’t expect with a single point-source exposure.

This virus is rare in dogs and there was cluster in that shelter at that time.

  • Possible, but odds are low that researchers would pick up a rare event like that. As above, the genetic variation in strains suggests that this was more likely from multiple introductions of the virus into the shelter.

H3N6 is circulating in dogs and this study detected a strain that’s been present in the region for a while.

  • This seems most likely.

The lack of clear sampling information is a big limitation in terms of interpreting the study results (e.g. were the dogs sampled on admission to the shelter? Were the positive dogs housed together? Were they sick?).  That’s very basic information that needs to be in a paper like this, but that weak journals may let slip or don’t think to query.

The main things I take away from this report are:

  • We need more surveillance to see if this strain is still present and where it is distributed. H3N6 is probably a relevant new-kid-on-the-block. It’s probably established given it was found in an area like this with massive dog breeding.
  • Changes in importation rules will reduce the risk of this virus hitting North America, but there are enough loopholes that there is still a very reasonable likelihood that it will be introduced here at some point.
  • We need to continue to watch for influenza A infection in various animal species. That includes dogs, which often fall between the cracks because it’s usually VERY hard to get support for disease surveillance in companion animals (compared to food animal). Lots of agencies want to know the results, but rarely do they want to foot the bill. It’s a significant gap in One Health surveillance.

What will we see if H3N6 flu hits Canada (or another country)?

Assuming it causes disease similar to other flu strains, we’ll see large numbers of dogs with typical flu-like disease (which will be lumped together with routine occurrences of “kennel cough”). The number of cases will be the dramatic thing, not the severity; however, with lots of cases, we’ll see more severe cases just based on percentages.

Since dogs will presumably have no existing immunity to this strain, the main thing I’m looking out for is big outbreaks. We see “kennel cough” outbreaks all the time, caused by our usual suspects like canine parainfluenza virus, Bordetella bronchiseptica and canine respiratory coronavirus. However, if/when a new flu virus hits, it will likely be much more and obvious. Rather than an outbreak that affects a lot of dogs in a group (e.g. kennel, shelter), it will affect almost all of them. Rather than a single outbreak in a town, there will be many. I suspect it will be pretty obvious pretty quickly if this virus makes it here.