Thursday, March 09, 2006

Bird Flu and Swans

Slate's Daniel Engber speculates about why so many cases of H5N1 in wild birds seem to be swans.

They seem to be susceptible to the disease, and it's easy to notice when they die. Scientists don't think swans are being infected at a higher rate than other waterfowl. In fact, migratory ducks are considered somewhat more likely than other birds to carry the disease. But ducks are also less likely than swans to exhibit symptoms of the flu.

Migrating birds often share space in shallow bodies of water where a bird-flu infection can be passed around with ease. An infected duck, for example, can shed the virus in its droppings and contaminate the water. (The virus can survive for quite some time in a cold environment.) Another bird—a mute swan, for example—might ingest some of that water and develop the disease. Since epidemiologists think the effects of the flu are more acute in swans, the swan would most likely die first.

But the sudden rash of swan deaths might have as much to do with the size and color of the bird as its susceptibility to the virus. Health officials find out about possible bird flu cases only when someone notices a dead animal. And people are more likely to notice a large, lifeless, white swan than a little dead brown duck.