By Julie Steenhuysen and Meredith Davis

CHICAGO (Reuters) - The strain of avian flu that has been identified in 12 U.S. states and led to the extermination of more than 7 million birds is different from the H5N1 bird flu virus that has spread from birds to humans in the past, an official with the U.S. Centers for Disease Control and Prevention said on Wednesday.

An analysis of the genetic composition of avian viruses circulating in North America, including the H5N2 strain, showed they do not contain genetic markers which in the past have been linked to more severe outbreaks in birds and transmission to humans, Alicia Fry, a medical officer in the CDC's influenza branch, said on a conference call with reporters.

There have been nearly 650 cases of H5N1 human infections, reported from 15 different countries, since 2003, according to the U.S. Department of Health and Human Services.

Two different strains have been discovered in the United States this year. The H5N2 strain is in Arkansas, Idaho, Iowa, Kansas, Minnesota, Missouri, Montana, North Dakota, Oregon, South Dakota, Washington and Wisconsin. It has also been identified on farms in Ontario, Canada. The H5N8 strain has been identified in California and also in Idaho, U.S. Department of Agriculture data shows.

There have been no cases of the highly infectious disease in humans since the outbreak started in the United States at the beginning of the year. This is the most widespread bird flu outbreak in North America in more than three decades.

In the cases involving human infection outside the United States, illness from the H5N1 virus occurred only among people who had direct contact with infected birds.

A principal concern with any new flu virus in birds, however, is that it will mutate to become easily transmissible from human to human. "At this point we don't know very much about these viruses," said the CDC's Fry. "They have only recently been identified."

At present, she said, "it seems the risk for human infection is very low." But she added: "This is a rapidly evolving situation."

Fry said the CDC will monitor any person who has been exposed to the virus in the United States. At least 100 people have been monitored so far.

As more people are exposed, the question of how well the virus can be transmitted to people will become clearer.

Fry said the federal government has developed a "seed strain," a first step toward making a vaccine. Seed strains must then be grown and tested to ensure they will grow well in chicken eggs or cells.

Warmer weather should help lower the number of infections in birds and curtail the virus' spread.

"As we get into spring and summer, sunshine comes and ultraviolet light will kill influenza virus," said David Swayne, director of the USDA's Southeast Poultry Research Lab.

Swayne was reluctant to give a time frame for an end to the outbreak, however.