Scientists find way to spread bird flu

GENEVA -- Five genetic tweaks made a deadly strain of bird flu that can infect humans spread more easily, according to a study that the U.S. government had first sought to censor on concerns it could be used by bioterrorists.

The genetic changes made the H5N1 virus airborne among ferrets, the mammals whose response to flu is most like that of humans, researchers from the Netherlands wrote in the journal Science yesterday. The likelihood of those changes occurring naturally is difficult to estimate but there is "no fundamental hurdle to that happening," said Derek Smith, a University of Cambridge researcher who led a second study.

Publication of the paper was delayed after a U.S. biosecurity panel in December asked the scientists to censor some parts of their work to prevent it being used by bioterrorists. Researchers meeting at the World Health Organization in February agreed the full findings should be published to help scientists design vaccines and drugs, and public health officials prepare for a pandemic.

More than 600 people have been infected with H5N1 since 2003, and almost 60 percent have died, according to the Geneva-based WHO. Most had direct contact with infected poultry, prompting scientists to question what it would take for the virus to become easily transmissible between humans.

While influenza viruses mutate constantly in a process called antigenic drift, the flu pandemics of the past century, including the 1918 Spanish flu that killed as many as 50 million people, have all been triggered by so-called antigenic shift, the mixing of human and animal flu viruses to create new pathogens to which people have no pre-existing immunity. Scientists led by Ron Fouchier at Erasmus Medical Center in Rotterdam set out to test whether H5N1 could become more transmissible by antigenic drift alone. The answer: yes.

Fouchier and colleagues examined mutations in viruses responsible for previous flu pandemics, and made three such changes to a strain of H5N1 from Indonesia, which they used to infect a ferret, then 10. The virus developed the ability to replicate in the animals' respiratory tract, suggesting the potential for airborne transmission that the scientists then proved. In addition to the three genetic changes introduced by the scientists, they identified two other mutations that enabled the virus to spread, the researchers wrote. Those mutations are now the subject of further research.