The H5N1 bird flu virus has mutated to infect people more easily, although it still has a way to go to transform into a pandemic strain, U.S. researchers claim.
Dr. Yoshihiro Kawaoka of the University of Wisconsin-Madison says his team has identified a "specific change" that appears to allow bird flu to grow in the upper respiratory tract of humans. His research is published in the Public Library of Science journal, PLoS Pathogens.
The H5N1 strain of avian flu infects mostly birds, though it has, since 2003, infected 329 people in 12 countries, killing 201 of them.
So far, it has passed from one person to another only rarely, but if it acquires the ability to do so easily, it may be the source of the next influenza pandemic, for which the globe is thought to be well overdue.
All flu viruses are continuously evolving. Scientists have some ideas about what mutations are needed to change a virus from one that infects birds easily to one more contagious in humans.
Kawaoka says the mutation his team has discovered is related to the body temperature the virus can withstand in order to replicate.
Birds usually have a body temperature of 106 degrees F; humans have a body temperature of 98.6 degrees F. The human nose and throat, where flu viruses usually enter, is usually around 91.4 degrees F.
The bird flu doesn't usually grow well in the nose or throat of humans, but the mutation his team has found allows H5N1 to live well in the cooler temperatures of the human upper respiratory tract.
Kawaoka believes that the viruses circulating now in Africa and Europe, which are descendants of the virus that began in China, are closest to becoming a virus that can easily pass to humans.
He says recent samples of the virus taken from birds in Africa and Europe all carry the mutation.
He adds that those viruses don't have more of the mutations that are needed to change the virus into a highly pathogenic human form. But he adds it's unclear still how many more mutations are needed for it to become dangerous to humans.
Since the H5N1 virus re-emerged in Asia in 2003, outbreaks have been confirmed in 60 countries and territories, according to data from the World Organisation for Animal Health.
The strain made the first known jump into humans in Hong Kong in 1997, infecting 18 people and killing six. The government ordered the destruction of the territory's entire poultry flock, ending the outbreak.
