Cambodia has reported two cases of bird flu infection in people, a father and a 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 has acquired the ability to spread between people and could trigger another pandemic. But the World Health Organization said Friday that 11 of the girl’s contacts, four of whom have flu-like symptoms, have tested negative for infection with the H5N1 flu virus.
Cambodian authorities said Saturday that the father was infected with a viral variant that is endemic in the country and unrelated to outbreaks in birds in the United States and Europe. The 49-year-old man was not displaying any symptoms, according to the Ministry of Health.
WHO is working closely with the Cambodian government to determine if the father and daughter contracted the virus through direct contact with infected birds, the most likely possibility, or if they infected each other.
The experts noted that there have 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 adapted to humans.
Person-to-person transmission is “very, very rare, compared to a common source of infection,” said Richard Webby, an avian flu expert at St. Jude Children’s Research Hospital in Memphis and a WHO adviser.
But people should be careful to avoid contact with wild birds that may be infected, Dr. Webby said.
“The risks of this virus for the average person on the street right now are very low, but they are not zero,” he said. “And that’s mainly because there are so many more birds that are infected 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 geese in China in 1996 and in people in Hong Kong in 1997.
Since then, there have been almost 1,000 cases in people in 21 countries, but the vast majority have resulted from direct and prolonged contact with birds. H5N1 does not yet appear to have adapted to spread efficiently between people.
“At the end of the day, this is a continuum of the same outbreak that started in 1996,” said Dr. Malik Peiris, head of virology at the University of Hong Kong, who helped monitor responses to several bird flu outbreaks in the Southeast Asian. “Really, it never left.”
H5N1 is usually transmitted by waterfowl, such as ducks, which can transmit the virus to domestic poultry through feces, saliva, or other secretions.
The current version of the virus has been unusually widespread, causing the largest bird outbreaks in recorded history. Europe and in the United States, affecting 58 million farm birds in the last It is now considered endemic in several countries in Asia and Europe, according to Dr. Webby.
The virus has also taken a heavy toll on wild birds, causing mass die-offs, and has spread to mammals, especially scavengers such as foxes, which might feed on infected carcasses.
Any reports of infection in people warrant investigation to confirm that the H5N1 virus has not yet adapted to person-to-person transmission. Six more cases of H5N1 have been reported in people since September, according to the WHO. The death of the 11-year-old girl this week is the first death from bird flu in Cambodia since 2014.
Experts have been closely monitoring H5N1, especially since an outbreak at a Spanish mink farm in October suggested the virus could spread efficiently among some mammals. Samples of virus isolated from transported mink a genetic mutation which is known to help flu reproduce better in mammals.
No human infections were detected. But a mink-adapted version of the virus could be one step closer to efficient transmission between people.
The WHO is “updating a bank of vaccine candidate viruses that are suitable for manufacturing, should the need arise,” the agency said in a statement. WHO also provides antiviral drugs from an available stock.
Genetic analysis can reveal whether H5N1 has acquired mutations that help it spread from person to person.
“That should give us a good clue as to whether or not the virus has really gone a step further,” said Dr. Shayan Sharif, an avian immunologist at the Ontario College of Veterinary Medicine at the University of Guelph in Canada.
But it will be more difficult to determine how the two family members in Cambodia became infected. That’s because the 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 they were both infected by the same group of chickens, they will be infected with very similar viruses,” he said. It may be more informative for scientists to trace the path of the virus by examining the nature of contact between infected people.
The virus poses the greatest risk to people who are in direct contact with birds, such as poultry farmers. Safety measures on poultry farms and processing plants, including the use of personal protective equipment by workers, can help reduce the risks of infection.
To contain local outbreaks, infected herds are usually culled and farms quarantined. But the virus is now so widespread in birds that experts are beginning to consider whether broader measures, such as vaccinating poultry, might be necessary.
Vaccination has not traditionally been used to control avian influenza in poultry in the United States or Europe. But officials are reconsidering that stance, and bird flu vaccine trials are clandestine.
“I really don’t think we should panic right now,” Dr. Sharif said. But “as we see all these different pieces of the puzzle coming together,” he said, “I think we really need to be prepared for an emergency.”