Avian influenza continued its advance across Europe: Health officials in Poland reported evidence of the virus in a pair of dead swans. And the Bulgarian government pledged to sanitize the area where four infected swans had been discovered. The highly pathogenic H5N1 strain has also turned up in dead swans in Austria, Bosnia, Croatia, Germany, Greece, Hungary, Iran, Italy, Serbia, and Slovenia. Why do we keep finding bird flu in 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.
So, why did we see so many swan deaths in such a short time? (New outbreaks were discovered in many of these countries within just a couple of weeks.) No one knows for sure. At first ecologists thought that a sudden blast of cold weather had flushed a population of infected mute swans out of the Danube delta toward Western Europe. But migrating mute swans travel only 30 miles to 60 miles a day, so it's unlikely that they could have turned up in so many different places in such a short time.
In fact, not all mute swans migrate—those in the Western part of their range tend to be year-round inhabitants. Another explanation for the deaths suggests that cold weather in Eastern Europe pushed other waterfowl toward warmer climates, where they infected these stationary mute swans. (The migratory mute swans from the east also seem to have brought along the disease as they moved.)
Of course, you'll also get more reports of dead swans with each new discovery of the virus in a dead swan. As the well-publicized cases of the flu piled up in Europe, wetlands officials must have started paying special attention to their already conspicuous swan populations.
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Explainer thanks Christopher Brand of the USGS-National Wildlife Health Center and Jan Slingenbergh of the United Nations Food and Agriculture Organization. Thanks to reader Lambert Teeuwissen for asking the question.