H5N1 influenza was recently found in two wild fox kits in St. Marys, Ontario. It’s a pretty noteworthy event given the scope of the current H5N1 highly pathogenic avian influenza (HPAI) outbreak across Canada, and the fact this is the first identification of H5N1 influenza in wild mammals in Ontario. The fox kits were submitted by a wildlife rehabilitation centre; these centres are great sources of information about emerging wildlife diseases as they are on the front lines and often bear the brunt of such issues. One of the fox kits was found dead and the other had severe neurological disease and died shortly after admission. Influenza virus was found in their brain tissue and was likely the cause of death. Further characterization of the virus identified it as an H5N1 strain of influenza A, more specifically of the A/goose/Guangdong/1996 (Gs/GD) lineage.

Is this surprising?

Not really. We don’t recognize much spillover of flu viruses into wild canids, but when you consider how widespread this virus currently is, it’s not a shock that spillover would happen. (It’s maybe more of a shock that we’d actually find it.)

We know that foxes, like other canids, are susceptible to various flu viruses. In an experimental study (Reperant et al. 2008), researchers were able to infect foxes by feeding them carcasses of infected birds, though the infected foxes were only mildly sick, at most. In a more recent study (Rijks et al. 2021) found two naturally infected red fox kits in the Netherlands. Those foxes also had neurological disease, like the one Ontario fox kit and as is often seen with severe infections in certain types of birds. In April 2022, avian influenza was also reported in a fox found dead in Japan; the strain was not reported in this case, but is likely related to the H5N1 strains circulating around the globe in migratory wild birds.

Why foxes and not other canids?

It’s not clear (to me, at least) whether foxes are predisposed to infection with influenza, or whether it’s a matter of there being more foxes than other canids in affected areas that get tested. I suspect it’s a numbers and surveillance bias rather than foxes being predisposed or particularly susceptible to infection.

Have more wild mammals been infected in Ontario than these two fox kits?

It’s hard to say, but it’s likely. These were only identified because they happened to be presented to a rehab facility AND they were submitted for testing. That doesn’t happen with most sick or dead wild animals. So, we don’t know if this was a very lucky identification of a very rare event, or a result of something bigger happening in the wildlife population of which we are as of yet unaware.

Let’s back up a bit… What do we know about influenza in canids in general?

There are two main situations to consider when it comes to influenza in canids: 1) infection with flu strains that are adapted to canids (including domestic dogs, foxes, coyotes, wolves, etc.) and 2) spillover infection with other non-adapated flu strains in canids.

“Canine flu” strains are influenza A strains that effectively circulate in the dog population. Currently the most common canine flu strain is an H3N2 that’s endemic in Asia and causes sporadic outbreaks in the US, often associated with imported dogs.  Canine H3N2 influenza was introduced into Canada this way back in 2018, but as far as we know was rapidly eliminated.  That’s not the strain we’re dealing with here (at least at this time).

Dogs (and other canids) can also get “spillover” infections of influenza from other species. For example, human-to-dog transmission of seasonal human influenza strains can occur. It’s probably more common than we realize, because dogs don’t usually get very sick and testing is uncommon, but it still seems to be a pretty uncommon event. Spillover of equine H3N8 influenza has also been reported.

Most of the time, spillover events are sporadic and a dead end for the virus, as the animal gets infected but doesn’t effectively spread a strain that isn’t adapted to that host, so it dies out in that individual. However, that’s not guaranteed in every case. Canine H3N8 is believed to have originated in horses, with subsequent adaptation to dogs to become a true canine flu virus.

Why do we care about H5N1 influenza in a couple of foxes?

From a dog health standpoint, spillover events are not a big deal because they are rare and don’t typically cause severe disease. However, the foxes in this report died, so we can’t assume all infections will be benign.

The main big-picture concern with spillover infections is the potential for emergence of a new flu variant, through adaptation of that strain to the new species, or (more critically) recombination of influenza viruses within the new host, which can happen when two different flu viruses infected an individual at the same time and swap genes. If the new variant that emerges is still able to infect a particular species (like humans) and is highly transmissible, yet different enough from the original virus that we have little immunity from previous infections or vaccination, it’s a recipe for a new pandemic virus. Dogs are unlikely to be the source of a new strain, but the more flu viruses that can infect them, the greater the risk. Since dogs can be infected by strains of human flu, dog flu and spillover of avian flu, there’s a theoretical chance that a dog could be infected with two different flu viruses at the same time. That’s why we want to control and eliminate flu viruses in dogs (and other species) as much as possible. Realistically, the recombination risk is probably greater in other species (including people), but we’d rather not roll the dice unnecessarily.

How did the fox kits get infected?

Presumably it was from eating an infected bird, especially since we know that can occur based on the previous experimental study. Whether these kits both got infected from the same infected bird, whether one got infected and then infected the other, or whether both were infected by a littermate or their vixen is impossible to say. I guess we also can’t rule out they got it from another mammal that acquired it from a bird, but then we’d be talking about a spillover from a spillover, which is quite a stretch.

What should the average dog owner do about influenza in dogs during the current outbreak in birds?

Step 1: Relax. Yes, this virus currently wreaking havoc in wild birds and domestic poultry around the globe can presumably infect domestic dogs. So can lots of other viruses (including many that are more serious, and yet we don’t panic about them either). Don’t ignore the issue, but let’s keep things in perspective.

Step 2: Use some common sense measures to reduce the risk of dogs being exposed to influenza-infected birds.

Step 3: Use some common sense measures to reduce the risk of dogs being exposed to influenza from other species.

Here are some examples of easy and practical measures:

  • Keep dogs away from sick and dead birds.
  • If avian flu is reported in your region, stay away from areas where infected birds have been found and keep your dog under control so it can’t wander off and snack on a recently dead bird.
  • Remove bird feeders to reduce congregation of birds, reduce the potential for bird-to-dog contact, and reduce exposure of dogs (and other animals, including people) to potentially influenza-contaminated bird poop.

Likely the most important thing the average person can do to reduce the risk of their dog getting a spillover influenza infection is for that person to get a flu shot, to reduce the risk of getting infected themselves and exposing their dog to flu.

Is there a flu vaccine for dogs?

Yes, but it won’t help us here. Flu vaccines aren’t great at providing cross protection against other strains. The canine vaccines are for H3N2 and/or H3N8, not the strain we’re currently dealing with in wildlife.