When flu flies the coop: a pandemic threatens

Science News, Sept 10, 2005 by Ben Harder

When a nasty strain of influenza first jumped from poultry to people in Hong Kong in 1997, government officials there ordered the slaughter and cremation of more than a million domestic birds. That action squelched the human outbreak, but the virus didn't go away. Six years later, that flu, known as avian influenza A H5N1, again began felling people and large numbers of birds, and the trend continues. This time, it's not confined to one country but is spreading across Asia.

So far, this virus has rarely if ever passed directly from one person to another, as the annual human influenzas do. But each new host, regardless of its species, is like a lottery ticket for the virus, giving it yet another opportunity to evolve the characteristics that would enable it to spread person to person. Many scientists say that it's only a matter of time before that happens.

The consequences of this influenza spreading among people could be disastrous on a global scale. The H5N1 virus is currently more deadly to people than were the viruses that caused past pandemics, or global outbreaks, of influenza that killed millions of people.

The worst flu pandemic on record, in 1918 and 1919, killed at least 20 million worldwide. That flu strain was lethal in about 2.5 percent of cases, giving it a fatality rate far higher than that of annual, run-of-the-mill flu viruses, says virologist Robert G. Webster of St. Jude Children's Research Hospital in Memphis.

Those typical influenzas are lethal to people over 65 years old in only about 0.1 percent of cases, and they kill less than 0.0001 percent of infected children, who constitute another relatively vulnerable group.

Since reemerging in people in late 2003, the H5N1 flu has killed about half of the 100-some people it has infected, and it has been lethal to people in all stages of life. "If it does go human-to-human and maintains anything like [a fatality rate of] 50 percent, it would be devastating," says Webster.

According to an estimate published in June by the nonprofit organization Trust for America's Health, even a pandemic strain with a fatality rate of about 20 percent could kill half-a-million people in the United States and send 2 million more to the hospital.

As governments around the world scramble to arm themselves with flu-fighting medicines and to develop public health protocols, some scientists are taking stock of the evolving threat. In addition to tracking the spread of the virus, they're studying different outbreak scenarios to determine the best strategy for responding when a pandemic strain finally strikes.

If an outbreak began today, it would catch the world unprepared.

FOWL DEEDS Among birds, the H5N1 virus is spreading at an alarming pace. Early this spring, it appeared to be confined mainly to poultry in southern China and Southeast Asia, where hundreds of millions of domestic birds have died or been culled. But during May and June, an outbreak around Qinghai Lake in central China killed more than 1,000 wild waterfowl.

In a report in the July 14 Nature, Webster and eight colleagues in China describe that outbreak and the genetic traits of the viral strain responsible for it. Influenza strains show slight genetic differences that researchers use to determine the pedigree of a virus. Such analysis suggests that the Qinghai Lake strain originated in a single bird that picked it up, the team says, "most probably from poultry in southern China." The infected birds at Qinghai Lake included bar-headed geese, gulls, and other migratory species, so the virus maybe as mobile as they are. This was the first report of transmission of the virus among migratory birds.

In another line of research, George F. Gao of the Chinese Academy of Sciences in Beijing and his colleagues tested the Qinghai strain's virulence by experimentally infecting eight chickens and eight mice. All the birds died within 20 hours, and the rodents survived no longer than 4 days, the scientists report in the Aug. 19 Science. Those findings indicate that the virus retained its lethal nature during its migration.

Gao's team notes that migratory birds that spend the summer at Qinghai Lake tend to overwinter not only in Southeast Asia but also in India and Tibet, where the virus hadn't been identified when they first published their report online on July 6. Carried by migrating waterfowl, H5N1 may well cross the Himalayas this fall and infect wild and domestic flocks in the Asian subcontinent, the scientists say.

Since the Qinghai outbreak, the virus has been identified or suspected in birds in Kazakhstan, Mongolia, Siberia, and Tibet. The virus has turned up almost as far west as Asia's boundary with Europe.

"It's probably going to spread across Russia to Europe," says Webster.

Russian officials have indicated that they also consider Azerbaijan, Georgia, Iran, Iraq, and Ukraine likely to acquire the virus from birds migrating out of Russia. United Nations representatives have also labeled some Mediterranean and central European countries at risk.