A study of duck hunters in Iowa is believed to be the first to show people can catch bird flu - a non-threatening kind - from wild birds.

Previous cases of people being infected with any form of bird flu have involved domestic poultry, like chickens.

The type of bird flu seen in the study is not H5N1, the deadly form that emerged in Asia which has sparked concerns of a potential worldwide flu epidemic. Rather, it is H11N9, another form seen in ducks and other waterfowl that has not been associated with human illness.

In the Iowa study, one hunter and two state environmental workers tested positive for the virus, though none of the three men got sick, said Dr. James Gill, the University of Iowa researcher who led the study.

Federal health officials say the finding is noteworthy. Domesticated birds - like chickens - are an established source of bird flu transmission to humans, but this is believed to be the first documented case of a person getting such a virus from a wild bird.

"This study is evidence that transmission of avian influenza virus from wild birds to humans can occur," said Tom Skinner, spokesman for the U.S. Centers for Disease Control and Prevention.

However, it´s not clear from the paper whether the virus was spread through blood, water or by other methods.

"But we want to make clear this type of influenza virus should not be confused with H5N1," he said noting that domestic birds are a bigger worry when it comes to the deadly bird flu, Skinner added.

The study is published in the August issue of a CDC journal, Emerging Infectious Diseases.

University of Iowa researchers went to a state-managed duck hunting area in October 2004, and asked visiting hunters to take part in the study.

Blood from 39 hunters was tested for antibodies that would reveal prior infection to any of about a dozen types of bird-based influenza. Several hunters had antibodies to H1, H2 and H3 forms of bird flu, which have adapted to humans and are now routinely seen in people. But one hunter also tested positive for H11N9, which is not seen in humans.

The hunter, 39, was a healthy Iowa man who had been hunting birds for 31 years and kills or handles hundreds of birds a year, Gill said.

"So he has had a lot of exposure," said Gill, zoonotic disease specialist at Iowa´s Hygienic Laboratory in Iowa City.

In addition, H11N9 antibodies were seen in two of 68 Iowa Department of Natural Resources workers who were studied. The men, ages 52 and 53, have placed leg bands on ducks for years.

None of the hunters or DNR workers infected had a history of working with domesticated birds, Gill said.

The researchers took fecal swabs from ducks that were shot the day the hunters were studied, but testing and typing of those samples is not yet completed, Gill said.

Gill advised hunters to wear gloves while handling birds, and to disinfect their hands afterward with hand sanitizers or soap and water.

H5N1 had not been detected in any wild or domestic birds in the United States. That form of bird flu swept through poultry populations in many parts of Asia beginning in 2003 and also jumped to humans, as well as to other regions. Since the beginning of 2003, 232 human cases have been reported, including 134 deaths, according to the World Health Organization.