The bird flu time bomb

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The bird flu time bomb

Mar 30 2004

 

- Asian countries vaccinating against bird flu will be

carrying out an uncontrolled experiment in viral

evolution that could ultimately lead to a human

pandemic, according to an article in New Scientist.

 

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.

 

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