I’ve had a couple of reports today about an apparent H1N1 influenza outbreak in dogs in the US. Note that I said influenza in dogs, not canine influenza – there’s a reason for that, explained below…

The situation revolves around a respiratory disease outbreak in a dog kennel in California, and PCR testing of some of the sick animals identified H1N1 influenza. Clinically, it sounds like a moderate to severe outbreak, with a reasonable number of infected dogs.

We generally see two types of influenza in dogs:

• “Canine flu” is caused by dog-adapted strains of influenza A, and they are maintained through spread in the dog population.  The two main canine flu strains that we know about are H3N8 (which seems to have disappeared) and H3N2 (still sporadically present in the US and endemic in some parts of Asia).
• Spillover infections in dogs with flu viruses that are primarily adapted to other species are also detected occasionally.  Most often this involves stains of influenza A that are adapted to humans, when a dog gets infected from its owner. We usually assume that these are “dead-end” infections, in that the dog doesn’t pass the virus on any further, because it’s not a canine flu virus so the infected dog likely doesn’t produce enough virus to infect others.

Back to the H1N1 in California.  We periodically see human (previous pandemic) H1N1 virus in dogs, which they catch from their owners. However, there can be different strains of the virus even within a single flu “type” like H1N1. If we look at H3N2, we have human H3N2, canine H3N2, swine H3N2, and so on. While they are all H3N2, they are adapted to a specific animal species and don’t infect others as readily. The important question in this case is, what type of H1N1 influenza is involved?  Finding a single case of human H1N1 flu in a dog wouldn’t surprise or concern me. But detecting a whole outbreak is a different story.  More information is needed, since this could range from an interesting story to a serious canine disease threat.

Here are the big questions:

Is the diagnosis confirmed/solid?

• Is this really on outbreak of H1N1 or is it a different flu strain?
• Is it an H1N1 outbreak, or was there an outbreak of something different and some incidental H1N1 infection was detected in the process of testing? (Unlikely since it seems like at least a few dogs were diagnosed with H1N1.)

Is this “human” H1N1 in dogs?

• Is this outbreak due to spillover infection from humans, or is it from a different source? Presumably someone’s sequencing the virus, which will help answer that question. If it’s actually H1N1, hopefully it’s just an oddball scenario with a cluster of human H1N1 flu infections that will die out, versus an indication that we have a new canine H1N1 flu strain, or a human strain that is now more adept at infecting dogs.

Is this virus a “canine” flu virus?

• It’s too early to say. Hopefully not. We don’t want a new flu strain in dogs for lots of reasons.  A new strain could spread easily through the dog population because no dogs would have any immunity to it. That can cause a significant amount of disease.  There would also be potential zoonotic concerns with a new strain. H3N2 and H3N8 canine flu viruses haven’t been significant zoonotic risks. They’ve stayed in dogs and haven’t spread to people, as far as we can tell. However, flu viruses like to adapt and change, and we just don’t want any more influenza viruses floating around, in terms of their potential for direct infection of people or the potential for recombination with other human flu viruses which could make more new flu strains (to which we might not have any immunity).

At this point, I’m interested and curious but not worried. Hopefully this situation is being investigated thoroughly (I assume it is).