Study: Swine flu easily overtakes other strains

WASHINGTON – Put swine flu in a room with other strains of influenza and it doesn't mix into a new superbug — it takes over, researchers reported Tuesday.

University of Maryland researchers deliberately co-infected ferrets to examine one of the worst fears about the new swine flu. But fortunately, the flu didn't mutate. The researchers carefully swabbed the ferrets' nasal cavities and found no evidence of gene-swapping.

The animals who caught both kinds of flu, however, had worse symptoms. And they easily spread the new swine flu, what scientists formally call the 2009 H1N1 virus, to their uninfected ferret neighbors — but didn't spread regular winter flu strains nearly as easily.

In other words, it's no surprise that swine flu has become the world's dominant strain of influenza. It's not under evolutionary pressure right now to mix and mutate while it has a clear biological advantage over other kinds of flu, concluded the Maryland team led by virologist Daniel Perez.

The Maryland study, funded by the National Institutes of Health, reinforces concern about how easily swine flu may sweep through the country.

"The results suggest that 2009 H1N1 influenza may out-compete seasonal flu virus strains and may be more communicable as well," said Dr. Anthony Fauci, director of NIH's National Institute of Allergy and Infectious Diseases. "These new data, while preliminary, underscore the need for vaccinating against both seasonal influenza and the 2009 H1N1 influenza this fall and winter."

Seasonal flu vaccine is available around the country now, and swine flu vaccine is expected to arrive in mid-October.

The U.S. has closely watched how swine flu rapidly dominated the Southern Hemisphere's winter, as authorities here prepare a fall resurgence. In Australia alone, eight of every 10 people who tested positive for influenza had the new pandemic strain. While it seems no more deadly than seasonal flu, it claims different victims: Seasonal flu kills mostly people over 65. The new swine flu spreads most easily in children and young adults, and so far has killed mostly people in their 20s, 30s and 40s.

The study is posted on PLoS Currents: Influenza, a Web site operated by the Public Library of Science to rapidly share scientific flu information.