Be Afraid (If You’re a Chicken)

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.

According to molecular virologist Earl Brown of the University of Ottawa, the current strain, one of a type known as H5N1, began evolving under these kinds of conditions in the early 1990s. By now, it’s intensely virulent. And, despite the flu deaths or cullings of between 100 million and 200 million chickens since last December, it shows no signs of abating, according to David Swayne of the U.S. Agriculture Department’s Southeast Poultry Research Laboratory. “It kills 100% of infected chickens in less than 24 hours,” Swayne said. “And unlike other avian flu strains, [the 2002 Hong Kong H5N1] kills 75% to 100% of ducks and geese as well.”

This sounds terrifying, especially when 31 people have died from the flu. Several cats from a chicken farm in Thailand also have contracted the disease. This wide infectivity is reminiscent of conditions before the 1918 flu broke out. Jeffery Taubenberger of the Armed Forces Institute of Pathology has analyzed genes recovered from preserved specimens infected with the 1918 virus. Those strains, he said, “have a lot of features that make them look like bird influenza viruses, as well as a lot of features that are not so bird-like.... Maybe it spent time in other animals.”

The current avian flu strains in China may be traveling the same path as their terrifying predecessor. Some molecular biologists insist that all that’s needed for the right lethal and transmissible sequence to emerge are a few mutations. Or, they suggest, someone infected with chicken flu could simultaneously be infected with human flu: The viruses could exchange genetic materials and a new, transmissible, deadly flu variant would be instantly created.

But evolution isn’t a crapshoot. Avian and human flu strains are radically different viruses, evolved in different species. They aren’t just a sequence of parts that can be snapped together randomly. Virulent bird flu is a specialized chicken killer; it has evolved that way. Normal human flu depends for its transmission on keeping its victims walking around so they can sneeze and cough in one another’s faces. To get anything like the killing potential of H5N1 bird flu strains in people, you’d have to “cook” the virus -- adapt it to human beings, which only natural selection in the right environment can do. You’d need, in effect, a disease factory for people.

That’s what 1918 was, particularly on the Western Front of World War I. The crowded trenches and hospitals, the trains and trucks carrying the sick and wounded thrown next to one another, the ships that bore the sick and dying home across the Atlantic, all were perfect factories for infection, as evolutionary biologist Paul W. Ewald of the University of Louisville has pointed out. Flu strains of normal virulence cooked in this deadly environment just as they cook in today’s Asian bird markets. The result was the most lethal human flu strain the world has ever seen. It burned across the world and disappeared, leaving its ghost -- a weakened, attenuated form of the same H1N1 flu -- to wander the planet for generations. When the conditions that evoked its virulence died away, the virulence disappeared, and H1N1 spread the way it always does, through the coughs and sneezes of its mobile if miserable hosts.

Diseases don’t evolve in a vacuum; they evolve to adapt to particular host species, under particular selection pressures. Many molecular biologists tend to forget this -- and they can be quite testy if you point it out to them. But as Brown says, it doesn’t do to forget natural selection and the need of the virus to evolve to fit its host under particular environmental conditions. “It’s the environment, stupid!” he said.

In any case, as Ewald insists, the best way to protect against the evolution of deadly human strains is to prevent them from cycling from person to person under conditions in which people are immobilized by illness. As long as influenza strains don’t enter such a disease factory, the likelihood of a lethal pandemic is slim.

But if a flu strain -- whether avian or human -- were to enter a large, crowded hospital, a prison or refugee camp, or any warren where people are packed together and unable to escape, a virulent human flu strain could evolve. The possibility of a lethal new pandemic developing cannot thus be utterly dismissed.

Natural selection is not mocked: No sustained evolution occurs in its absence, and nothing can prevent the evolution of a deadly disease released unchecked into the right environment.

We forget the power of selection at our peril.