The four presumed human H7N2 bird flu cases identified in Britain last week are a reminder that the next flu pandemic could be sparked by a virus other than the feared H5N1 strain, experts say.
While the global health community's attention in recent years has targeted the H5N1 virus, which has killed at least 186 people worldwide since 2003, some experts worry that attention is being diverted from seemingly less dangerous bird flu subtypes like H7.
"There may be a bit of complacency when it comes to recognizing the pandemic potential of H7 viruses," World Health Organization bird flu expert Dr. Michael Perdue said Monday.
Last week, British authorities confirmed that four people apparently tested positive for H7N2, a mild strain of bird flu, after 15 chickens at a small farm in Wales died. Health officials are currently investigating 36 people who may also be infected, of whom 11 have symptoms of flu or conjunctivitis.
Having so many human cases at once is a potential concern. In Asia, where H5N1 has circulated most widely, millions of people have been exposed to millions of infected birds, resulting in about one new infection per week.
"Here, we're talking about a small number of birds and yet we still have four cases," Perdue said. "Unless there's something unusual about the contact with birds, that suggests the virus is finding new ways of getting into humans."
The H7 subtype has previously sparked human outbreaks. In a large outbreak in the Netherlands in 2003, 89 human cases were reported, mostly of conjunctivitis, as well as one death. There were also at least three likely instances of human-to-human transmission involving family members of poultry workers. In the case of the single fatality, officials noticed that particular virus had about 10 mutations.
British officials have been quick to reassure the public that the "low pathogenic" H7N2 virus - in comparison to the "highly pathogenic" H5N1 virus _ poses little risk to the population. Indeed, H7N2 appears to cause only mild symptoms such as eye infections.
But low pathogenic viruses can quickly morph into highly pathogenic ones, sometimes within weeks. Too little is known about flu viruses to predict with any certainty which ones are most lethal for humans.
"The pandemic risk from low pathogenic avian viruses is almost as bad as that from highly pathogenic avian viruses," said Dr. Angus Nicoll, an influenza expert at the European Centre of Disease Prevention and Control.
"When people say low pathogenic or highly pathogenic, that only refers to how unpleasant the disease is for birds," Nicoll explained. "That's almost irrelevant for humans."
Like all flu viruses, low pathogenic viruses mutate rapidly, and could theoretically transform into a pandemic strain without the warning signals of a more virulent strain, which would leave many dead chickens - and perhaps humans - in its wake. Experts also worry about the possibility of a bird flu virus mixing with a human flu virus to create a new pandemic strain.
"If you have an H7 virus causing mild symptoms, that might give the virus the chance to reassort into a more dangerous virus before anybody notices," Perdue said. And for health officials hoping to quash a pandemic in its emerging stages, it might be too late to contain a global outbreak without an early warning.
Most experts believe that the preoccupation with H5N1 as the most likely pandemic candidate is justified.
"The situation with H5N1 is very intense," said Dr. Anthony Fauci, director of the National Institute of Allergy and Infectious Diseases in the United States. "We know that H7 can cause outbreaks in chickens and that it can occasionally jump the species barrier, but it has not done it nearly to the extent of the H5N1 virus."
Unlike many other bird flu subtypes, which disappear off the radar after a short period, H5N1 has remained entrenched in the environment, and continues to spread to new areas.
Still, while no bird flu virus can be ruled out when it comes to igniting the next pandemic, some clues may exist. Though H5N1 has several worrying characteristics, other flu subtypes are also in the running for the pandemic title.
"The last two flu pandemics were the result of a human flu virus recombining with low pathogenic avian viruses," said Perdue. The H7N2 recently detected in Britain would fall into that category. "Given that historical context, perhaps we should concentrate our efforts a little more in that direction."