Vaccinating chickens may be the only way out of the bird flu nightmare in Asia. But it could also lead to the evolution of new strains, the latest research shows, increasing the risk of a human pandemic. Only intensive surveillance can stop this happening, but experts say the countries affected do not have the necessary systems in place.
Last week China declared its bird flu outbreaks had ended. Health officials are vaccinating millions of the birds that escaped slaughter. Indonesia is also vaccinating, and other Asian countries hit by the H5N1 bird flu are considering the same strategy.
But the H5N1 virus is almost certainly still circulating among the vaccinated birds, and the fear is that in this abnormal setting it may evolve into a form that is not only fatal to people, like the current one, but can also spread from person to person. Research in Mexico has shown for the first time that under these conditions bird flu evolves at an unprecedented rate, with unpredictable consequences.
Veterinary scientists usually prefer to control livestock epidemics by destroying sick and exposed animals, instead of vaccinating. The reason is that vaccines, especially flu ones, are not 100 per cent effective. While they prevent animals falling ill, low numbers of viruses can still replicate inside their bodies and spread from animal to animal. Such 'silent epidemics' are very hard to spot, but can cause new outbreaks if unvaccinated animals are exposed or if vaccination ends.
But with H5N1 bird flu now affecting a huge area of Asia, vaccination could help end the outbreaks more quickly. Fewer flocks destroyed would leave fewer small-scale poultry farmers destitute.
There is a precedent - but it is a worrying one. In 1995 Mexico stopped an outbreak of severe H5N2 flu by vaccinating chickens. But the virus is still circulating silently, and Mexico is still vaccinating. Normally the bird flu virus changes little in chickens, because it rarely persists long enough, says David Suarez of the US Department of Agriculture's poultry research lab in Georgia. But in Mexico the virus has been exposed to vaccinated chickens for years and this has encouraged new forms to evolve.
In a report that will appear in the Journal of Virology, Suarez's team reveals that "major antigenic differences" have been found in the bird flu viruses isolated from vaccinated chickens in Mexico since 1995. It is increasingly different to the vaccine strain, which means that infected birds will shed more of the virus and spread the infection more readily.
The H5N1 virus circulating in vaccinated chickens in Asia is likely to evolve the same way. There has already been speculation, notably in New Scientist, that vaccination programmes in China may have led to greater genetic diversity in the virus over the past two years, and perhaps even contributed to the emergence of the current strain.
It is possible, however, to eradicate wild virus like H5N1 from vaccinated flocks. The key is to detect and destroy silent infections. The low-tech way of doing this is to place unvaccinated birds next to vaccinated flocks. If any flu is circulating, these "sentinel" birds will develop obvious symptoms. The weakness of this system is that farmers who want to save their flocks from destruction can cheat by replacing any sick birds.
The high-tech method is to use a marker vaccine that elicits the production of a different set of antibodies to the wild virus. Antibody tests can then distinguish between infected birds and those that have simply been vaccinated. In 2002 Italy became the first country to eradicate bird flu using a marker vaccine and regular testing.
"The vaccine used without this monitoring can have a boomerang effect, and become a tool to spread the virus, not control it," said Ilaria Capua of the World Organisation for Animal Health reference lab for bird flu in Legnaro in Italy.
China does at least have the right vaccines. Its agriculture ministry announced last week that it is launching two marker vaccines. But thousands of vaccinated chickens must still be tested, and infected flocks destroyed, to eliminate the virus. Yet such surveillance systems will be a tall order for the Asian countries that are vaccinating or plan to.