Cambodia has reported two cases of bird flu infection in people, a father and daughter in a village in Prey Veng Province. The 11-year-old girl died earlier this week.
The cases, the first reported in Cambodia since 2014, raised fears that the virus had acquired the ability to spread among people and may trigger another pandemic. But the World Health Organization said on Friday that 11 contacts of the girl, four of whom have flulike symptoms, had tested negative for infection with the H5N1 flu virus.
The 49-year-old father who has tested positive was not showing any symptoms, according to the Health Ministry. The W.H.O. is working closely with the Cambodian government to determine whether the father and daughter both caught the virus from direct contact with infected birds — the most likely possibility — or whether they infected each other.
Experts noted that there had been hundreds of sporadic cases of H5N1 infection in people since the virus was first identified and that there was no evidence that it had become adapted to humans.
Transmission among people is “very, very rare, versus a common source of infection,” said Richard Webby, a bird flu expert at St. Jude Children’s Research Hospital in Memphis and an adviser to the W.H.O.
But people should take care to avoid contact with wild birds that may be infected, Dr. Webby said.
“The risks from this virus to your average person on the street right now is very low, but it’s not zero,” he said. “And that’s primarily because there’s just so many more infected birds around right now.”
Bird flu, or avian influenza, is a group of flu viruses that are primarily adapted to birds. The particular virus in these new cases, called H5N1, was first identified in 1996 in geese in China, and in people in Hong Kong in 1997.
Since then, there have been nearly a thousand cases in people in 21 countries, but a vast majority have resulted from prolonged, direct contact with birds. H5N1 does not yet seem to have adapted to spread efficiently among people.
“At the end of the day, this is a continuum of the same outbreak that started back in 1996,” said Dr. Malik Peiris, chief of virology at the University of Hong Kong, who has helped oversee responses to several bird flu outbreaks in Southeast Asia. “Really, it never went away.”
H5N1 is typically carried by aquatic birds, such as ducks, that can transmit the virus to domestic poultry via feces, saliva or other secretions.
The current version of the virus has been unusually widespread, causing the largest ever bird outbreaks in Europe and in the United States, affecting 58 million farmed birds in the latter. It is now considered endemic in several countries in Asia and Europe, according to Dr. Webby.
The virus has taken a heavy toll on wild birds too, triggering mass die-offs, and it has been spilling over into mammals, especially scavengers, like foxes, that might feed on infected carcasses.
Any reports of infection in people warrant investigation to confirm that H5N1 has not yet adapted itself to human-to-human transmission. There have been six other cases of H5N1 reported in people since September, according to the W.H.O. The death of the 11-year-old girl this week is Cambodia’s first bird flu death since 2014.
Experts have been closely monitoring H5N1, especially since an outbreak on a Spanish mink farm in October suggested that the virus could spread efficiently among some mammals. Samples of the virus isolated from the mink carried a genetic mutation that is known to help flu replicate better in mammals.
No human infections were detected. But a mink-adapted version of the virus might be one step closer to efficient transmission among people.
If the version of H5N1 identified in Cambodia were found to be closer to the one seen in Spain than to those in previous Asian outbreaks, scientists would be concerned, Dr. Peiris said. “It is important to try to understand exactly what has gone on” in Cambodia, he added.
The W.H.O. is “updating a bank of vaccine candidate viruses that are suitable for manufacturing, should it be needed,” the agency said in a statement. W.H.O. is also providing antiviral drugs from an available stockpile.
Genetic analysis can reveal whether H5N1 has acquired mutations that help it spread among people.
“That should give us a good hint as to whether or not the virus has really jumped one step further,” said Dr. Shayan Sharif, an avian immunologist at the Ontario Veterinary College at the University of Guelph in Canada.
But it will be more difficult to determine how the two family members were infected in Cambodia. That’s because H5N1 samples from the father and daughter are likely to be nearly identical regardless of whether the virus was acquired from a person or from the same infected birds, Dr. Webby said.
“If both of them were infected from the same set of chickens, they are going to be infected with very similar viruses,” he said. It may be more informative for scientists to chart the path of the virus by examining the nature of the contact among infected people.
The virus poses the biggest risk to people who are in direct contact with birds, such as poultry farmers. Security measures on farms and poultry processing plants, including the use of personal protective equipment by workers, can help reduce the risks of infection.
To contain local outbreaks, infected flocks are generally culled and farms are put under quarantine. But the virus is now so widespread in birds that experts are beginning to consider whether broader measures, such as the vaccination of poultry, might be needed.
Vaccination has not traditionally been used to control avian influenza in poultry in the United States or Europe. But officials are rethinking that stance, and trials of bird flu vaccines are underway.
“I don’t really think that we should panic at the moment,” Dr. Sharif said. But “as we see all of these various different bits and pieces of the puzzle coming together,” he said, “I believe we need to get really seriously ready for an emergency.”