Bird flu requires tight watch
By Qin Chuan (China Daily)
Updated: 2004-04-05 07:49

Vaccines used in China to combat bird flu in chickens have so far proved effective with no new cases of the deadly virus reported in any of the nation's flocks, experts say.

Strong surveillance measures continue in China to prevent the disease from spreading again. A recent report of the United Kingdom-based New Scientist magazine warns that vaccinations can lead to the evolution of new bird flu strains, increasing the risk of human pandemics.

According to the report, vaccines, especially those for the flu ones, are never 100 per cent effective.

While such vaccines can prevent animals from falling ill, small amounts of virus can still replicate inside creatures' bodies and spread from animal to animal.

Such "silent epidemics," the report said, are very hard to spot, and can cause new outbreaks if unvaccinated animals are exposed or if vaccination programmes end too early.

"Such possibilities do exist if the quality of vaccines used are not good enough," Chen Hualan, chief of the national bird flu reference lab with the veterinary research institute in Harbin in Northeast China's Heilongjiang Province, admits.

But she said the inactivated vaccines used in China have proved so far to be effective. And surveillance measures have shown vaccinated chickens in China do not carry the bird flu virus.

"Vaccination does not mean everything is OK. We must never spare our vigilance," said Jia Youlin, chief veterinarian and spokesman for bird flu control at the Ministry of Agriculture.

According to Chen, surveillance measures include the testing of waste samples from vaccinated chickens to see whether there are any bird flu viruses present.

Chen said another more scientific way of putting unvaccinated "alarm" chickens among vaccinated chickens has been also adopted in the country.

The use of such chickens is also recommended in the New Scientist report.

Once bird flu viruses exist, the alarm chickens will show symptoms, Chen said.

The surveillance part of the equation is vital, said Ilaria Capua of the bird flu reference lab of the World Organization for Animal Health in Legnaro, Italy, according to the New Scientist report.

"The vaccine used without this monitoring can have a boomerang effect, and become a tool to spread the virus, not control it," the report quoted Capua as saying.

In 1995 Mexico stopped an outbreak of severe H5N2 flu by vaccinating chickens. But the virus was still circulating silently, and Mexico is still vaccinating, the report said.

Normally the bird flu virus changes little in chickens, because it rarely persists long enough, but in Mexico the virus has been exposed to vaccinated chickens for years. That encouraged new forms of the virus to evolve, the report quoted David Suarez of the US Department of Agriculture's poultry research lab in Georgia as saying.

China lifted its restrictions on the last two bird flu epidemic areas on March 16, but Jia, while announcing the news, said no efforts should be spared to prevent and control the disease.