Be Afraid (If You're a Chicken)

ATLANTA — Avian flu, in its current form, is a deadly disease. It has sprung up in poultry flocks all across Southeast Asia in the last two years, despite the slaughter of millions of birds in efforts to contain it. This virulent form of bird flu invades deep tissues. It can be spread through the air, as well as in the feces of affected poultry. It causes capillaries to leak blood, quickly turning squawking, active chickens into bloody heaps. And 43 people have contracted avian flu, almost all from direct contact with infected chickens, since January, with 31 fatalities. The threat of avian flu, combined with the critical shortage of this year's human flu vaccine -- an entirely unrelated problem -- has generated something of a panic not seen since the swine flu scare in 1976.

Could we be facing the next great pandemic? Could the 31 deaths be a harbinger of another 1918, when as many as 40 million people died worldwide? Flu experts are worried. As a recent editorial in the New York Times put it: "All it would take to set off a raging global pandemic would be for the virus to mutate into a form that is readily transmissible among humans."

How concerned should you be?

Terrified. If you happen to be a chicken.

Without a doubt the strain of avian flu ravaging poultry flocks in Asia is a highly pathogenic virus. But many strains of avian flu don't cause disease. They spread among wild ducks through feces shed into water. Migratory birds carry flu strains from one area to another, leaving infected droppings and moving on. Strains like these cannot be killers, because the ducks would die. And dead ducks can't fly. Natural selection, which affects all living things, pushes the strains toward mildness, and those strains that kill ducks kill themselves off as well.

But what happens when harmless duck flu is introduced into the chicken farms of Asia? The Asian chicken industry is enormous, raising more than 70 billion birds a year. Farms, both peasant-run and commercial, raise chickens under densely crowded conditions, and cram them into small wire cages for sale in live-animal markets. These circumstances produce chicken disease factories in which a benign virus can quickly turn lethal because transmission from one bird to another is so easy. Natural selection, which kept the wild duck strains benign, now turns the equation around: Fiercer viruses, which can exploit more of the host chicken's body, no longer pay a price for their virulence. A disease that was transmitted through feces now invades the respiratory tract and other organs. It's just a short hop to the next chicken's beak, and a new killer strain is born, a strain that grows more and more virulent as it races through the farms and markets of Asia.