Tuesday, October 04, 2005

Dog flu

First bird flu, now dog flu. Dog flu is causing consternation in the US because people love their dogs more than they love their neighbors. As we saw in Katrina, sometimes they love their dogs more than they love themselves. The Reveres have a dog. We understand.

So now we are faced with dog flu. Influenza A/H3N8 to be exact. Originally found in racing dogs (greyhounds) in Florida and Massachusetts, it has now been detected in "companion dogs" (aka pets) in Florida, Massachusetts and New York City (CDC Media Advisory). It is undoubtedly present elsewhere as well. The disease is a (usually mild) respiratory disease that is indistinguishable from "kennel cough," itself a description of a general flu-like respiratory infection in dogs, with cough, fever and nasal discharge prominent. Many kennels require vaccination against the most common causes of kennel cough (Bordetella bronciseptica and parainfluenza virus), but the kennel cough vaccine will not protect against the new canine influenza virus. Dog flu can be fatal to dogs, but there seems to be some dispute as to the mortality rate, with the scientists who discovered it estimating 6% to 8%, while others say it is much less, perhaps in the 1% to 2% range. There is insufficient data at present to fix it precisely.

There are no reports of human infection with this virus, but there are some lessons for public health nonetheless. This influenza A subtype has been known for 40 years as a horse influenza virus and it was a great surprise to find that it had jumped directly (it appears) to dogs. Sequencing of the hemagglutinin protein of the virus (the H3 part of H3N8) showed a number of amino acid changes from the horse virus, but this remained a horse virus, i.e., it did not seem to be a reassortment. This underlines once again that a species jump can take place without the reassortment that WHO continually invokes as the herald of a change in avian virus that can transmit efficiently in humans. Reassortment simply is not needed for this. The species barrier that has been a convention of influenza epidemiology is not as high or as difficult to cross as believed and dog flu is a good example.

Perhaps we can gain a little sympathy for the plight of the Indonesians who regard their pet birds as we do our pet dogs. If CDC were to give the order for a "mass cull" of dogs because they carried influenza virus (as is being advocated for birds in Indonesia), my guess is it would be a non-starter, even if it made eminent good sense (which it probably doesn't). Only a part of managing a pandemic flu problem is "medical." As our Best Friend would be quick to tell us with a bark and a nuzzle.

Update, 10/5/05): Add California to the list.