Bird Flu May Not Spread Easily
Bird Flu Virus May Need Complex Genetic Changes to Spread Easily Among People
Aug. 1, 2006 -- The bird flubird flu virus may have its work cut out for it if it's going to learn to spread easily among people.
Simple genetic changes may not be enough to make the H5N1 bird fluflu virus spread easily among people, according to a new study done on ferrets.
But the study -- published in the online early edition of Proceedings of the National Academy of Sciences -- may not mean that bird flu is no threat to people.
The study "does not mean that H5N1 cannot be transmitted," CDC director Julie Gerberding, MD, MPH, told reporters in a teleconference.
"It means that more than simple genetic changes are necessary," she continues. "It does not mean that H5N1 cannot develop into a pandemic strain."
Gerberding warns against getting complacent about bird flu.
"We are far from out of the woods on H5N1," she says. "Avian influenzainfluenza virus constantly evolves. We don't know where these changes will take us."
Bird Flu in People
The H5N1 virus currently spreads much more easily among birds than among people.
Since 1997, there have been more than 200 lab-confirmed cases of H5N1 infections in people, according to background information in the new study.
The World Health Organization's web site notes 232 reported cases of bird flu in people worldwide since 2003, including 134 deaths, as of July 26, 2006. None of those cases has occurred in North America or South America.
The vast majority of bird flu cases occurred in people who had direct contact with dead birds, note CDC researcher Taronna Maines, PhD, and colleagues.
"Despite limited instances of probable human-to-human transmission, H5N1 viruses have not yet acquired the ability to transmit efficiently among people," the researchers write.
Maines and colleagues studied H5N1 transmission in ferrets, which served as a model for people.
First, they gathered genetic material from a 1997 strain of the H5N1 virus and the H3N2 human fluflu virus.
Next, the scientists added a little of the H5N1 genetic material to the H3N2 virus. Basically, they created a viral remix, taking care that the remixed virus couldn't escape.
Three male ferrets with the remixed virus were put in a cage. Three healthy ferrets moved into the cage next door 24 hours later. They didn't get sickened by their ill neighbors.
Later, two or three other ferrets sickened by the remixed virus were put in a cage. A healthy ferret joined them in the same cage one day later. It didn't get sick.
In short, the remixed virus didn't appear to spread very easily among the ferrets, the study shows.
The CDC's Jacqueline Katz, PhD, who worked with Maines and other scientists on the study, spoke to reporters in the CDC teleconference.
"Our results cannot be generalized to contemporary H5N1 strains now circulating," says Katz, who works with the CDC's influenzainfluenza branch.
"The virus used in the study was a 1997 strain," she adds. "But we did see that with an avian-human virus, we did not see transmission from infected to healthy animals."
The researchers aren't done with their tests.
"We are doing similar types of experiments, working with viruses that are more contemporary," Katz says. "We can create more recent hybrids."