LONDON (AFP) - Four people have tested positive for bird flu linked to a low-risk strain found in chickens which died on a farm in north Wales, a senior health official said Friday.

Doctor Christianne Glossop, Chief Veterinary Officer for Wales, said Thursday that the chickens died from the H7N2 low pathogenic avian influenza strain, not the most virulent H5N1 strain.

The Health Protection Agency (HPA) "confirmed infections in four" of the samples taken from nine people who were associated with the infected or dead birds and reported flu-like symptoms, HPA´s chief executive Pat Troop said.

"These test results confirm that human infection with the avian flu virus has occurred. The cases so far have been associated with the infected birds," Troop said.

She stressed that the H7N2 avian flu remains largely a disease of birds.

"The virus does not transmit easily to humans. Worldwide, almost all human H7N2 infections documented so far, including those associated with this most recent incident, have been associated with infected poultry," she added.

Three of the nine people were treated in hospital but were later discharged.

The National Public Health Service for Wales and HPA officials are following up all close contacts of the individuals who have been ill as a "precautionary measure."

Glossop said Thursday that the source of infection by the H7N2 strain was being investigated.

Fifteen Rhode Island Red chickens, 22 weeks old, were bought by the farm two weeks ago, bringing its total number of birds to 45 chickens and two geese, Glossop said.

Chickens had since been dying at the small holding, she said.

Samples were first sent for testing on May 17.

"Contingency plans have been activated and in line with these the farm has been placed under restrictions," she said.

In the restriction zone, poultry and poultry products cannot be moved and bird gatherings can only take place under licence.

Poultry keepers have to observe strict biosecurity measures, but they have not been asked to house their birds.

It is the first confirmed case of bird flu in Wales.

The last case of bird flu in Britain was an outbreak of H5N1 in February at a turkey plant in eastern England.

Nearly 160,000 turkeys were culled as a precaution in the country´s first major outbreak of the potentially lethal virus.

The H5N1 strain of bird flu has infected at least 306 people worldwide and killed around 185 of them, mostly in southeast Asia, since the end of 2003, according to World Health Organisation figures.