Another strain of Bird Flu, sub-type H7 seems to have adapted to more easily spread among humans and other animals.
Scientists said that a few strains of the H7 type virus have caused minor, untransmissible infections in people in North America between 2002 and 2004 have increased their affinity for the sugars found on human tracheal cells.
Subsequent tests in ferrets suggested that these viral strains were not readily transmissible.
But one strain of the H7N2 virus, a low pathogenic avian flu strain isolated from a man in New York in 2003, replicated in the ferret's respiratory tract and was passed between infected and uninfected ferrets suggesting it could be transmissible in humans.
The investigators said the evidence suggests that the virus could be evolving toward the same strong sugar-binding properties of the three worldwide viral pandemics in 1918, 1957 and 1968.
"These findings suggest that the H7 class of viruses are partially adapted to recognize the receptors that are preferred by the human influenza virus," said Terrence Tumpey, a senior microbiologist with the US Centers for Disease Control and Prevention in Atlanta.
The authors said that if the viruses continue to evolve in this direction, the avian flu viruses could travel more easily between other animals and humans. They called for strict surveillance of avian flu viruses and continuing federal preparations for a possible future pandemic.
The study appears in the Proceedings of the National Academy of Sciences.
Among the H7 viruses the CDC scientists studied were H7N3 viruses recovered from the two British Columbians infected during an outbreak in the poultry farm-dense Fraser Valley in 2004. More than 17 million chickens were destroyed in the efforts to stop that outbreak.
Of all the H7 viruses studied for this work, the New York man's seemed most adapted to humans. It bound more easily to the receptors found in the lining of the human upper respiratory tract and had decreased binding to bird receptor cells. And when ferrets were inoculated with the virus, it spread from the infected animals to healthy animals placed in the same cages.
But in general H7 viruses from North America that have been isolated from about 2002 onwards seem to have developed an increasing affinity for the human-type receptors, said Dr. Terrence Tumpey, the CDC scientist who led the work.
Tumpey also said in his statement that the study shows that while there is no direct indication that the H7 virus is about to aquire potentially damaging mutations, it is vital that global inspection and research covers this virus class as well as the more obvious H5N1.
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Wednesday, May 28, 2008 at 10:01PM 
