A deadly new form of bird influenza has the potential to cause millions of deaths around the world if its ability to spread from person to person increases. The great threat posed two years ago by the highly infective SARS virus has now receded, however. Dick Ahlstrom reports from Washington
A group of specialists in emerging diseases told the American Association for Advancement of Science meeting in Washington about the worldwide effort to identify and contain outbreaks caused by unknown viruses. Emerging organisms had the potential to spread rapidly from country to country due to air travel.
The current greatest risk is posed by an avian influenza virus known as H5N1, explained Dr Nancy Cox of the US Centers for Disease Control and Prevention. Only 55 cases have been identified so far in the Far East but it has a frighteningly high fatality rate per case, currently running at 76 per cent.
This makes it two and a half times more deadly than small pox or the SARS virus, which have fatality rates of about 30 per cent, and is also more dangerous than the ebola virus, she said. And although the 1918 influenza pandemic killed 20 to 40 million worldwide, its actual fatality rate was only about 1 per cent, Dr Cox said.
"It is frightening to see such a high fatality rate," she acknowledged. She also believes, however, the actual rate is probably lower as only the sickest patients that attended hospitals were identified as having had H5N1.
The dangerous virus is endemic in domesticated poultry, she said. "Some of the birds, particularly ducks, can serve as a silent reservoir for the virus." They are apparently healthy but carry the virus and therefore present a constant risk to humans.
At the moment, the virus does not readily jump across to humans and person-to-person transmission is even rarer. "We don't have a precise risk analysis. That is because we don't know enough about how a virus becomes transmissible from person to person," Dr Cox said.
The danger persists, however. "We have a lot of humans exposed to a lot of virus. With a virus that mutates very readily, the chances that a virus mutant might arise increases with each human case," she stated. "We can't say it is inevitable. It is almost certain there will be a pandemic with H5N1." If the virus did mutate to a form readily transmissible from person to person and it also retained its high fatality rate, then H5N1 would represent a microbiological equivalent of the "perfect storm" in meteorology, she said.
The existing organism would be "at least as dangerous" as small pox and "if a virus arrived that could transmit efficiently, we would expect rapid global spread of the disease". How dangerous it might be remained unclear.
"We really don't know. There are so many unknowns here," Dr Cox said.
A global influenza body was established in 1948 to monitor outbreaks and infections, she said. "We really need to strengthen this global network. We also need to make sure we have pandemic preparedness in place. Preparedness is absolutely essential." This would include having vaccines, even only partially effective ones, available as well as antiviral agents. Drugs, isolation and quarantine might be enough to halt the spread of any outbreak and so head off a pandemic, she said.
This approach had worked well with the highly dangerous SARS virus, an organism that is effectively gone, according to Prof Kathryn Holmes of the University of Colorado Health Sciences Centre. She described the huge scientific effort that followed its appearance and subsequent human to human spread.
"There are relatively few mutations needed to jump" from animal sources, in this case the palm civet, to humans and then to allow its spread amongst humans, she said. SARS cases quickly appeared in Beijing, Singapore and Taiwan.
The comprehensive response to the outbreak meant most experts believed SARS was now gone, except for any accidental release from a research lab.