May 26 (Bloomberg) -- Mild bird flu strains circulating in North America have gained some ability to infect human cells, and should be monitored for dangerous mutations, government researchers said.

The virus family, called H7, is genetically different than the H5N1 strain that has killed millions of birds and hundreds of people, said Terrence Tumpey, a U.S. Centers for Disease Control and Prevention scientist in Atlanta. More mutations in the H7 strain could make it dangerous to humans, he said.

People don't have natural immunity to many strains of flu spreading in birds, allowing these viruses to cause severe infections when they enter human cells. Some strains of H7 have increased their ability to stick to proteins on the surface of human lung cells, a key step in infection that may at some point allow its spread from one human to another, Tumpey said in a study in the Proceedings of the National Academy of Sciences journal.

``This underscores the importance of continued surveillance so we can be best prepared for early response to a pandemic threat,'' he said in a telephone interview.

At least 241 people have died of H5N1 bird flu since 2003, most of them in Asia. A worldwide network of laboratories collects and analyzes samples for mutations that might allow the virus to spread quickly from person to person.

Tumpey analyzed H7 viruses that infected poultry and people from 2002 through 2004. One was an H7 strain that caused an outbreak in the Netherlands in 2003, infecting about 80 people and killing one person.

Eye Infections

That strain wasn't well adapted to human lung cells, and most human infections were in the eye, Tumpey said. His analysis showed the virus prefers attaching to a molecule in birds' intestines, called alpha 2-3.

Other H7 strains circulating at the same time in Canada and the U.S., however, had the ability to attach to a cell surface molecule called alpha 2-6. That molecule is found in the breathing tissues of humans and animals, and is a common target for seasonal flu viruses that cause annual outbreaks and spread quickly through the population, he said.

One H7 strain that infected a New York man in 2004 was easily transmitted among ferrets, the study showed. Ferrets and humans are susceptible to many of the same flu viruses.

While the finding is important, other characteristics probably contribute to the ability of viruses to spread and make people ill, said Albert Osterhaus, a virologist at the Erasmus Medical Center in Rotterdam. Other factors, such as whether the virus grows in the human nose and throat, rather than deep in the lungs, may allow it to spread quickly, he said.

Respiratory Tract

``Those that replicate in the upper respiratory tract are more likely to be transmitted between mammals,'' he said in a telephone interview. The ability to bind to human cells ``is not the whole story.''

While these H7 viruses are ``low pathogenic,'' meaning they rarely cause deaths, they nonetheless pose a threat, Tumpey said.

``We have to be aware of these viruses just like we're aware of H5 viruses,'' he said. ``They have the potential to mutate to high pathogenic and they are in our backyard.''

Three influenza pandemics occurred last century in 1918, 1957 and 1968. The most lethal by far was the 1918 ``Spanish flu'' that killed as many as 50 million people worldwide.

To contact the reporter on this story: John Lauerman in Boston at jlauerman@bloomberg.net.

Last Updated: May 26, 2008 17:00 EDT