WASHINGTON, Feb. 2 (UPI) -- Health experts said Monday they are concerned that human-to-human transmission has begun from a strain of bird flu virus currently centered in Southeast Asia.
Such an event could mean the new virus, prevalent in poultry across the continent, has adapted to humans and could turn into a worldwide pandemic.
"We're still in a position of not knowing what the future is because there's still tens of thousands of birds -- if not millions -- that need to be culled in a safe way," Dick Thompson, spokesman for the World Health Organization in Geneva, told United Press International. "Until that's done and we see an end to human cases, we won't really feel very comfortable."
The new bird flu strain, which first surfaced in humans in Vietnam last month, has been linked to 10 deaths in Vietnam and Thailand. Preliminary investigations indicate the virus may have been circulating in poultry long before that, but so far authorities have been unable to pinpoint where it originated.
Until recently, all the human cases could be traced back to contact with a bird infected with the flu virus, which appears to be a new strain of the H5N1 subtype of influenza virus.
WHO officials had said four cases in the Thai Binh province -- among one Vietnamese family of two sisters, their brother and his wife -- could not be traced back to an infected bird. This led them to conclude "that limited human-to-human transmission ... is one possible explanation." All the infected family members died except for the brother's wife, who fully recovered.
The U.S. Centers for Disease Control and Prevention in Atlanta said it has sent a team of investigators to Asia to look more closely at the possibility the virus can be spread from person to person.
"The big question we want to answer right now is whether it's being transmitted from human to human," CDC spokesman Tom Skinner told UPI. "The report out of the WHO of possible human-to-human transmission is very concerning but we need to learn more about those cases," he added.
"It's obviously a concern because what we're all afraid of is the virus jumping from avian species to humans and becoming human adapted," said Dr. Martin Blaser, vice president of the Infectious Diseases Society of America in Alexandria, Va.
"Chances are it will die out, but all it would take is for one mutation to occur and we could have SARS all over again," Blaser, chair of medicine at New York University, told UPI.
Even if the Vietnamese family represents a bona fide case of human-to-human transmission, authorities said they took some comfort in the fact the infected family members did not pass on the virus.
"The most important point ... is that they didn't pass the disease to other people," Thompson said. "The thing we'd be worried about is the emergence of a strain that would move easily from person to person, and obviously that didn't happen from this cluster."
The new strain of bird flu is highly lethal in chickens, killing nearly 100 percent of those it infects, and it may have a similarly high mortality rate in humans. Of the 13 infected people detected so far, 10 have died.
This is not the first time a strain of bird flu has jumped species and infected humans. The first documented instance happened in 1997 in Hong Kong, resulting in the death of six people. Since then, bird flu varieties have jumped to humans several times in Hong Kong and other countries, so world health authorities are well-versed in controlling and limiting the spread of such viruses.
The primary means of containment is killing all infected poultry herds, and farmers already have begun slaughtering millions of chickens across Asia.
Officials have had difficulty containing this latest outbreak, however, because it has hit several countries at once. So far, the virus has been detected in poultry in Cambodia, China, Indonesia, Japan, Korea, Laos, Thailand, Vietnam and a single peregrine falcon in Hong Kong.
"The trouble is we've never faced a situation where we've had an outbreak in so many different countries at the same time," Thompson said.
The Hong Kong outbreak in 1997 was limited to that country so it was relatively easy to eliminate, he added.
Another problem is the current outbreak has hit poorer countries, such as Thailand and Cambodia, "where governments have limited resources, so it's going to be harder for them to deal with it," he said.
Thompson said these countries also do not have enough money to offer sufficient financial incentives for chicken farmers to kill their flocks -- noting that for some of the smaller farmers the flocks could represent a lifetime investment.
The WHO last week issued an appeal for other nations to contribute money to assist with reimbursement efforts.
Steve Mitchell is UPI's Medical Correspondent. E-mail email@example.com