The United Nations health agency says a study of human bird flu victims in Vietnam indicates that people became ill very soon after exposure to the virus, and their chances of recovery are not good.
The World Health Organization oversaw a study of 10 people stricken with bird flu in Vietnam, where the disease has hit hardest in human terms.
The study results, which were summarized Thursday, revealed that the H5N1 strain of the avian virus moves very quickly, once it infects a person.
The WHO reported that symptoms showed up only two to four days after infection, and most victims died about 10 days after that. The WHO said only one of the 10 people in the study recovered, and one remains in critical condition.
Overall, 14 people in Vietnam are confirmed to have died of bird flu. The disease, in one form or another, has devastated chicken flocks in 10 Asian countries, and tens of millions of the birds have been killed in an attempt to contain the spread.
Dr. Lance Jennings, a virologist at the WHO's Pacific headquarters in Manila, says the Vietnam findings suggest that the chances of recovery from bird flu are slim.
"When it infects humans, it is extremely pathogenic, and the severity of the disease can be indicated by the numbers of very few patients surviving," said Dr. Jennings.
However, Dr. Jennings says there could be milder cases of bird flu in people who never seek treatment, so it is too early to reach any conclusions on the death rate in humans.
The WHO report said high fever, shortness of breath and cough were the main symptoms of bird flu in humans, often followed by pneumonia. None of the victims in the study suffered from runny nose or sore throat.
Maria Cheng, the WHO spokeswoman in Hanoi, says knowing the symptoms will help doctors identify new cases.
"We think it will be helpful information for clinicians, in that, if they do see several of these symptoms presenting in the same patient, who may have had exposure to birds, they will know to look for H5N1," she said.
Because a large majority of the confirmed cases in Vietnam and Thailand have been fatal, some U.N. scientists say this virus might be more lethal than the strain that infected 18 Hong Kong residents and killed six of them in 1997.
While human cases have been limited to just two countries this year, the WHO fears the virus could infect many more people in China, Laos and Indonesia, some of the countries where the disease has spread rapidly in poultry.
China reported seven more outbreaks in birds on Friday, including one case of a swan in Shenzhen, the city that borders Hong Kong. The Food And Agriculture Organization, another U.N. agency, said Friday that the disease was still not under control in Asia.