LONDON - Four people have tested positive for a mild strain of bird flu, British authorities said Friday. The cases — which are not the feared H5N1 strain, but the less dangerous H7N2 subtype — were reported after poultry died at a small farm in north Wales.

Tests were performed on nine people associated with the farm, and the Health Protection Agency said in a news release that four people had tested positive for the H7N2 strain of the virus.

Three of the nine were hospitalized, but have since been discharged, the agency said.

The Welsh health authorities said that in all but one of the four human cases the virus had spread from poultry. The other case may have been transmitted from person to person.

"Although the number of people involved is very small, we could be dealing with some spread of the H7 flu virus within the household setting," said Dr. Marion Lyons, a communicable disease specialist with the Welsh health service. "We are treating this possibility very seriously."

Officials are now following up all close contacts of the people who were ill as a precaution.

The H7 subtype of bird flu is believed to be less virulent than H5, but it has in the past caused human cases, and was responsible for a large outbreak in the Netherlands. In that outbreak in 2003, there were 89 mild cases of H7N7, and one person died.

Most of the cases in the Netherlands were conjunctivitis, a mild eye infection.

"The mortality rate of H7 viruses is not on the order of H5N1," said Dr. Michael Perdue, a

While H7 viruses tend to cause milder illnesses in humans, they still have the potential to ignite a global flu outbreak and should be watched.

"A low pathogenic H7 virus might not be picked up as easily as a highly pathogenic virus," said Perdue, explaining that the mild symptoms of H7 infection might be mistaken for ordinary flu, causing experts to lose valuable time tracking an emerging pandemic.

Like all flu viruses, H7 evolves constantly. Other bird flu subtypes have also caused illnesses in humans. Surveillance in Europe for all bird flu viruses has greatly increased in recent years.

While global health experts have been focused on H5N1 as the strain most likely to cause the next flu pandemic, it is also possible that another subtype, such as H7, could ultimately be responsible. "I"e;d hate to predict anything with bird flu," said Perdue.

The farm came to the attention of British authorities after a number of chickens died.

Dr. Christianne Glossop, the chief veterinary officer for Wales, said in a statement that the chickens had tested positive for H7N2.

A 1.6-mile exclusion zone was put in place around the farm, and the remaining chickens have been slaughtered.

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Associated Press writer Maria Cheng in London contributed to this report.