(Recasts with the death of the patient, details)

By Ho Binh Minh

HANOI, July 31 (Reuters) - A Vietnamese woman who was seven months pregnant has died of bird flu, the country's third human death from the virus this year, doctors said on Tuesday.

"She died on Saturday," said Doctor Tran Thuy Hanh, chief of the Bach Mai hospital where the 22-year-old woman was taken last week from a farm in the northern province of Ha Tay, the largest poultry supplier to Hanoi.

Hanh gave no further details of the woman, who had been confirmed to have contracted the H5N1 virus by laboratory tests in Vietnam.

The World Health Organisation (WHO) has not confirmed she had bird flu, which killed a 28-year-old woman from northern Vietnam last month.

A doctor in Ha Tay province said there were no bird flu outbreaks in the area where the latest victim lived.

"It made us difficult to pin down how she became infected," he said. "Earlier she bought a chicken leg of unknown origin for a meal in the family but nobody else got sick."

Ten people who came into contact with the woman have been monitored for any symptom and given anti-viral Tamiflu as a preventive measure while her house was disinfected, the doctor from the provincial Preventive Medicine Centre told Reuters.

Bird flu has killed 45 of the 99 confirmed cases in Vietnam since late 2003, including the latest death.

Globally, the H5N1 virus has killed 192 people out of 319 known cases, according to a WHO tally. Hundreds of millions of birds have died or been slaughtered.

The H5N1 virus remains mainly a virus of birds, but experts fear it could mutate into a form easily transmitted from person to person and sweep the world, killing millions.

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