Vaccination of infected flocks could backfire without proper surveillance.

2004 APR 21 - (NewsRx.com & NewsRx.net) -- 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.

China recently declared that 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, reported in the March 27, 2004, issue of New Scientist, 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% 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. "They really have few choices now," said Ilaria Capua of the World Organization for Animal Health reference lab for bird flu in Legnaro in Italy.

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, said David Suarez of the U.S. 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 revealed that "major antigenic ...