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TUESDAY, Jan. 27 (HealthDayNews) -- The virus behind the so-called bird flu that has been devastating Asia's poultry industry could be the source of a long-feared human influenza pandemic, health experts warn.

So far, there have been eight confirmed human deaths from the rapidly spreading virus, six in Vietnam and two in Thailand. All the victims apparently handled sick chickens, according to news reports, which would indicate that transmission has occurred only from animals to humans.

But health officials worry that human-to-human transmission might ensue, resulting in a potential pandemic.

"This is scary stuff," said Dr. Arnold S. Monto, professor of epidemiology at the University of Michigan School of Public Health. "It has the potential to become a pandemic. It's not good."

"It [the virus] seems to be more lethal to chickens," Monto added. "It seems to be infecting additional species such as ducks that were not infected [in previous bird flu outbreaks], and certainly is transmitting at a frequency that we've never seen before."

The worry now is that a person who is infected with a human flu virus will also become infected with the bird flu germ, allowing the two viruses to exchange genetic material. This new variant germ could pose a significant health threat because people wouldn't have had a chance to develop immunity to it.

Dr. Shigeru Omi, the World Health Organization's regional director for the Western Pacific, said at a press conference in Hanoi, Vietnam, Tuesday that if bird flu jumps the species barrier, it could kill millions of people, The New York Times reported.

"Flu viruses recombine all the time," explained Dr. Susan McLellan, associate professor of medicine in the infectious diseases section at Tulane University School of Medicine in New Orleans. "Little bits of one go off and recombine with little bits of another. That happens when one individual [animal or human] is infected with two strains and they swap a few genes."

"You could end up with a pathogenic form of bird flu attached to genetic material from human flu that makes it easy to transmit from human to human," McLellan says.

Complicating matters, World Health Organization officials say a vaccine for bird flu is at least six months away because the disease keeps mutating. The agency also is worried because the virus appears to be resistant to cheaper antiviral drugs used to treat regular influenza.

But the good news, McLellan added, is that "you have to have someone first infected with both [bird flu and the human versions]. And we don't appear to be having that many people infected with this bird flu."

Dr. Donald J. Kennedy, professor of internal medicine at St. Louis University, said, "The present data supports that this virus was acquired from birds as opposed to humans."

At a press conference Tuesday, Dr. Julie Gerberding, director of the U.S. Centers for Disease Control and Prevention, emphasized that the United States did not appear to be in any immediate danger from bird -- or avian -- flu.

"In the U.S. we do not have avian flu in chickens or any other bird. There are no human cases," she said. "Right now the risk here appears to be very low."

U.S. public health officials are working on developing a vaccine, but Gerberding said one was not expected for "weeks or months." That makes containing the virus the first priority, she said.

The last pandemic to strike the United States was the Asian flu epidemic of 1957, which killed nearly 70,000 Americans.

On Tuesday, Thailand confirmed that a second boy in two days had died from bird flu. Thai officials said they're awaiting lab results on five other people whose deaths may be linked to the virus.

Vietnam has already confirmed six human deaths from the disease.

The number of countries reporting avian flu cases among fowl rose to 10 Tuesday as Chinese officials said the virus had turned up in poultry in that country. So far, tens of millions of infected birds, primarily chickens, also have been found in Cambodia, Indonesia, Japan, Laos, Pakistan, South Korea, Thailand, Taiwan, and Vietnam.

But only Thailand and Vietnam have reported human cases of the virus, which has been designated H5N1.

Bird flu has surfaced sporadically in Asia in recent years. Six people in Hong Kong were killed when an avian flu spread through the human population in 1997.

Viruses tend to be adapted to one species, with limited transmissibility to other species. However, flu viruses mutate and can acquire the ability to cross-infect another species. So, the greater the number of infected chickens, the greater the potential for human infection.

To contain the outbreak and prevent a human epidemic, officials in affected nations have ordered the slaughter of tens of millions of chickens. Thailand, the world's fourth largest exporter of chickens, has already killed more than nine million of the birds.

Bird flu isn't necessarily that much more virulent than the human flu. It's just that people don't have any natural immunity to it.

Most of the human strains that have circulated over the past century have been H1, H2 or H3 (the H refers to a protein on the outer surface of the virus). The current avian flu strains are H5 and H7.

"If your H number is different, then you don't develop immunity," said Kennedy, of St. Louis University. "Since humans aren't normally infected [with these strains], we have no cross-immunity and the disease is liable to be severe."

Added the University of Michigan's Monto: "Even if it doesn't spread to humans, which is the ultimate worry, it's devastating the economy of a number of countries in Southeast Asia."

More information

The CDC and the World Health Organization have more on bird flu.

(SOURCES: Donald J. Kennedy, M.D., professor of internal medicine, St. Louis University; Susan McLellan, M.D., associate professor of medicine, infectious diseases section, Tulane University School of Medicine, New Orleans; Arnold S. Monto, M.D., professor of epidemiology, University of Michigan School of Public Health, Ann Arbor; Jan. 27, 2004, teleconference with Julie Gerberding, M.D., director, U.S. Centers for Disease Control and Prevention, Atlanta; The New York Times)

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