Canine influenza is back in the news in the US, with close to 50 dogs infected with canine influenza at a dog rescue in Florida.  The outbreak has been going on for over two weeks already.  This is a “surprising but not surprising” scenario to me.

It’s a bit surprising because canine flu activity seems to have been really low to non-existent in the US lately. While there’s no formal surveillance program, I haven’t seen reports of it and haven’t heard any other chatter about diagnoses.

It’s more towards unsurprising though. We were able to eradicate canine influenza (as far as we can tell) in Ontario, Canada, when it hit here in 2018, but that took a lot of surveillance and effort, and we intervened very quickly after introduction of the virus. With widespread disease in the US over the years after multiple introductions from Asia and no broad control plan, it was expected that canine flu would continue to spread on this continent – sometimes insidiously, sometimes dramatically. Since the virus is so transmissible and vaccine coverage is low, it can spread quickly, but disease can also burn out in a particular area if infected dogs don’t continue to meet susceptible dogs.

While we weren’t seeing reports of infections, it seemed optimistic to assume it had completely disappeared, and even if it did, importation of dogs from areas where the virus is endemic poses ongoing risk of reintroduction and spread.

It would be nice to know a few things about the reported outbreak in Florida:

  • How solid is the diagnosis? The reports say “canine influenza” but sometimes people get confused and default to saying “flu” for any respiratory disease, or don’t realize there’s a difference between canine influenza and parainfluenza (a very common respiratory virus in dogs worldwide).
  • Assuming this really is canine influenza virus, what strain is it? Is it our expected H3N2 canine flu strain, or could it be something new? (which would also have huge implications from a vaccination standpoint)
  • How did the outbreak start? The rescue says a dog “from the area” brought it into the facility, but where did that dog likely get infected? Unknown-origin infections suggest there could be more transmission going on than we realize.

If this is H3N2 canine influenza (which is the most likely strain), that leads to questions about vaccination. We have an H3N2 canine vaccine in Canada and the US. (We also have a vaccine that covers H3N8 in dogs, but that strain hasn’t been seen in years.) Canine flu vaccines are like human flu vaccines – they’re designed to reduce the likelihood and severity of disease, but they’re far from perfect and may not do a lot for preventing transmission overall. However, they are still useful.

When I think about use of influenza vaccines in dogs, I consider two main factors:

  1. Likelihood of exposure: Dogs in the area of this rescue would obviously be at increased risk of exposure to the virus. Dogs that have contact with imported dogs (e.g. dogs in a rescue that periodically imports dogs) and dogs that travel to areas where the virus is, or may be, present, would also be at increased risk of exposure.  This dogs are more likely to benefit from vaccination.
  2. Likelihood of severe disease: My 8-year-old otherwise healthy Labrador is unlikely to have a severe outcome if he’s infected with influenza virus. If he was a bit older, had respiratory or heart disease, had other debilitating issues or was a brachycephalic breed (like a bulldog), the odds of him having a severe (or even fatal) outcome from infection would presumably be much higher. Vaccination is therefore more important in dogs in these groups. All the deaths we saw here in Ontario from canine flu back in 2018 were in senior dogs. However, severe disease can still occur in younger dogs. So, vaccination is more important in higher risk dogs, but that doesn’t mean it’s not useful in others.

We’ll have to see how this story plays out, hopefully more information will follow.