The trouble with dog flu

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The dog flu can leave vets and pet owners guessing as to whether a vaccination is necessary. File photo The dog flu can leave vets and pet owners guessing as to whether a vaccination is necessary. File photo

It’s flu season for us humans, and the advice really couldn’t be more simple. Get your flu shot this year and every year. We are social animals who spend most of our day in relatively close physical proximity, and it’s very easy for us to get each other sick. Vaccination may provide an imperfect defense, but any step taken to reduce the total misery inflicted upon the population every year is just being a good neighbor.

Canine influenza shares much in common with its human counterpart. It brings the familiar symptoms of coughing, sneezing and fever and almost always resolves without treatment. Just like human flu, it can leave behind a stubborn cough for a few weeks and can become legitimately dangerous for an unlucky few.

There are currently two common strains of canine flu. The first one (charmingly dubbed H3N8) jumped over from horses around the turn of the millennium and a vaccine has been available for several years. The second strain (H3N2) may have come from birds or pigs and was responsible for a large outbreak in 2015 in Chicago. A new vaccine was released for this strain last year. Despite originating in other species, these viruses have not yet been observed to infect people, although the newer virus may have some ability to infect cats.

Without specific testing, the flu is indistinguishable from any of the other kennel cough infections that we see in dogs on a regular basis. But that testing is rarely performed. Since these cases all progress similarly regardless of the underlying cause, patients are usually managed conservatively, with more aggressive options reserved for unusually severe disease. This makes it very difficult to get a handle on local outbreaks and what might be causing them. We are, more often than not, guessing.

Given the similarities with human flu, you’d think it would be easy to recommend blanket vaccination of the entire population against canine influenza. But that isn’t the case. There are important differences to consider in the way the flu spreads in people versus dogs and in the way available vaccines work against them.

For one thing, human influenza strikes a significant fraction of the population every year. If you didn’t get the flu this season, you almost certainly know someone who did. It is everywhere. Even a partially effective human vaccine can spare millions of people from illness and days of lost productivity. In contrast, canine flu tends to pop up unpredictably in small, isolated outbreaks. The benefits of population-wide vaccination simply aren’t as clear.

People also have more compelling options for vaccination than dogs do. The human vaccine is designed to protect against a broad number of flu strains, and is periodically recalibrated to keep up with the virus as it shifts and evolves. Some years it is more successful than others.

Canine flu vaccines are less sophisticated. We have individual vaccines against each of the two strains, but we can’t really predict when or where either one might pop up. Dogs that get one flu shot remain vulnerable to the other strain. You could give both vaccines, but the next big outbreak may well be an entirely new strain.

This is not to say that these vaccines are worthless. The old strains will always be out there, and they can still make dogs sick. Even flawed protection may be justified in high-risk situations like shelters and kennels, but our expectations should be tempered and people need to know the limits of what canine influenza vaccines can accomplish.

Maybe our tools will improve in the future, but, for now, I only recommend canine flu vaccines on a case-by-case basis. As for us humans? I plan on keeping myself covered.

Dr. Mike Fietz is a small-animal veterinarian at Georgetown Veterinary Hospital. He received his veterinary degree from Cornell University in 2003 and has lived in Charlottesville since.


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