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An ounce of prevention: Why bird flu has put health officials on alert

By David Brown
The Washington Post

	PAULA BRONSTEIN / GETTY IMAGES Thai military members carry chickens to be stuffed in fertilizer bags, then buried alive yesterday in Song Phi Nong, Thailand. Bird flu has ravaged poultry farms in the world's fourth-largest chicken exporter.
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WASHINGTON — The metaphor that public-health officials invoke when talking about a global flu epidemic is the same one that lies at the heart of the scariest horror movies.

It is the idea of a small and deadly thing that is poking and prodding for a weak spot in whatever is protecting its intended victims. It is patient because it knows it will eventually succeed. When it does, a horrible metamorphosis makes it huge and unstoppable.

This scenario was deadly in 1968, 1957 and, most notoriously, 1918, when pandemic influenza killed about 50 million people worldwide. Today, virologists fear an encore could be under way in East Asia.

Over the past month, a strain of bird flu that has killed thousands of chickens has broken through the "species barrier" to claim a few human victims. This time, though, the public-health community hopes to write a different end to the script.

"There is a chance that something can go wrong," Klaus Stohr, head of the World Health Organization's flu program, said Friday in Geneva. "But it looks as if we act decisively and timely now, there is a window of opportunity here to control the disease before it takes global proportions."

That action consists of exterminating chickens carrying the virus, protecting people in contact with the birds from infection and understanding the pathogen at the molecular level — all as quickly as possible. Cross-border traffic of live birds and poultry products has stopped in much of the region, and there is talk of vaccinating millions of chickens.

So far, there is no evidence that this bird flu can be passed from person to person — a trait it would need to acquire to be a global threat.

If evidence of person-to-person contagion appeared, the response would be much like the one mounted against severe acute respiratory syndrome (SARS) a year ago.

The stakes would be much higher, however, because the flu virus, once fully adapted in humans, can spread with a speed and ease that SARS never showed. A lot has to happen, though, for bird flu to gain that capacity. The trouble is that in influenza's world, a lot can happen very quickly.

Variations of bird flu can tear through chicken flocks with a mortality approaching 90 percent. But virologists did not think it could infect people — at least, not until 1997.

That year, 18 people in Hong Kong became infected with the strain identified as H5N1 — the first time direct bird-to-people transmission had been seen. Six died, most of them healthy young adults — a disturbingly high percentage.

Previously, scientists believed that to infect and kill a person, a bird-flu virus would first have to acquire at least a few genes from the flu viruses that circulate in human populations. That is possible because unlike viruses whose genes reside on a single unbroken strand of RNA or DNA, flu carries its genetic information on eight strands. Under the right conditions, it can trade one or more of them with another flu virus.

Virologists once believed these "reassortments" occurred only in pigs because that species was capable of being infected by both human and avian flu. With the 1997 Hong Kong cases, however, it was clear reassortment also might occur in a person simultaneously infected by both.

The chance of that occurring depends on how much avian flu is around. What scares scientists this winter is that it is all over the place in Asia.

How the H5N1 virus — the one that afflicted Hong Kong in 1997 — became so widespread this time around is not known. The urgent chore is to get rid of the animals harboring it.

It is also important to protect the workers culling the flocks from getting human flu, lest they themselves become the "mixing vessels" in which a reassortment occurs. WHO is urging they be vaccinated and, if possible, be given preventive medicines.

Even without reassortment, it is possible that avian influenza could become a pandemic strain, but that is far less likely.

Curiously, Hong Kong, where the first human cases of bird flu occurred, is reporting no H5N1 now.

That region changed its poultry-marketing practices after the 1997 outbreak. Waterfowl, which can carry H5N1, were separated from chickens. Quail, also viral hosts, were banned. Markets were also were required to be cleaned twice a month.

Still, H5N1 continued to turn up occasionally until last year, when a poultry vaccine began to be used widely, said Robert Webster, a top U.S. influenza expert. Today in Hong Kong's markets, he said, "every chicken has had its shots."

Copyright © 2004 The Seattle Times Company