Vietnam’s biggest city, home to 10 million people, began slaughtering its ducks on Wednesday in an increasingly desperate fight to halt the spread of the deadly bird flu virus that has killed 13 people in the last month.

Health workers and inspectors in Ho Chi Minh City, accompanied by police, headed to farms to collect ducks, which can carry the H5N1 virus without showing symptoms, as well as pigeons being raised for food.

The birds would be killed by burning or being buried alive, an animal health official said a day after a Vietnamese doctor confirmed the death of the first Cambodian in waves of outbreaks that have now killed 45 people over the past year.

“After the killing, duck raising will not be allowed for one year,” said the official at the city’s Animal Health Department.

World Health Organization (WHO) officials have headed to the Cambodian province of Kampot, which abuts Vietnam across a porous border, to investigate the area the Cambodian woman came from.

Relatives of the dead woman said the chickens had died and they had cooked and eaten them - these were the same chickens that the dead women ate. The relatives later complained of respiratory problems, raising concern of a more widespread outbreak among poultry.

Tests results on the dead birds were due on Thursday, officials said, although the relatives had turned out to be in the clear.

“I have just received the test results from the Pasteur Institute and it was negative, not H5N1,” Sok Touch of the Health Ministry’s communicable disease department told Reuters.

Fowl in the area were slaughtered and the area was sprayed, officials said, although the WHO has expressed fears the virus might already have spread to Cambodia, Laos and Myanmar, all poor countries where surveillance systems are rudimentary at best.

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The virus kills about 80 percent of people it infects and almost all its victims have caught it from infected poultry, although a pair of Thai sisters are believed to have got it from prolonged contact with the dying daughter of one of them.

Experts say it could mutate - if it infects a person with ordinary flu or an animal, such as a pig, which can also harbor a human flu virus - into a form that could cause a global pandemic in which millions would die.

The virus, believed to have been brought to Asia in late 2003 by migrating wildfowl, erupted in a major fashion for the third time in southern Vietnam’s Mekong Delta in December.

Ho Chi Minh City next door to the delta set a Sunday deadline for the slaughter of its more than 210,000 ducks, half of which are raised outdoors on small farms, allowing a potentially rapid spread of the virus.

While Ho Chi Minh City has not had any human cases of bird flu infection, six people from nearby provinces have died in its hospitals from the virus, which produces a high fever, coughing and acute pneumonia.

Most of its victims so far have been Vietnamese. The virus has also killed 12 Thais and the Cambodian woman.

The experts say it is now endemic in parts of Asia and the death toll is likely to rise.

On Tuesday, officials confirmed a 24-year-old Vietnamese man had bird flu, the 18th confirmed case since the virus erupted anew in December.

He was recovering in a Hanoi hospital where three other men have been treated for the virus. Two recovered.

By January 31, the H5N1 virus had killed or resulted in the slaughter of more than 1 million poultry in 32 of Vietnam’s 64 provinces, the Agriculture Ministry said.

Last year, the epidemic wiped out 17 percent of Vietnam’s poultry stock of 250 million.