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The Thai government insists that sick birds are culled. Credit: A. WEERAWONG/AP

As Thailand's battle with avian influenza drags on, the government has started to crack down on the illegal use of bird vaccines, with several high-profile arrests last month. But farmers are thought to be relying on black-market vaccines in a desperate attempt to avoid culling their chickens, leaving the country debating whether vaccination should be allowed.

For Thailand, once one of the world's major poultry exporters, the outlook is bleak. More than 300,000 chickens have been destroyed since July, and the bird flu that began this summer remains active in 25 villages.

To farmers trying to protect their livelihoods, vaccination seems like a good idea. It promises to keep healthy birds infection-free and to lessen symptoms in those already infected.

But the unregulated use of vaccines can do harm, says Wantanee Kalpravidh, consultant on avian flu for the United Nations Food and Agriculture Organization in Bangkok. Vaccinated birds that seem healthy can still get infected and spread the virus. And in backyard farms, where chickens run around freely in close proximity to people and other animals, this can spell disaster, she says. “Farmers might not be aware of the risk. It's difficult to keep track of what's happening,” she warns.

Black-market vaccines might also contain viruses that have not been properly inactivated, adds Ilaria Capua, who heads a World Organization for Animal Health (OIE) laboratory in Padua, Italy. “You have no idea what you are buying,” she says. The vaccine virus strain might also interact with any infectious flu strains present in the same bird, possibly leading to the evolution of a more dangerous virus. A long-term study of avian vaccines in Mexico has recently shown evidence of this (C.-W. Lee, D. A. Senne and D. L. Suarez J. Virol. 78, 8372–8381; 2004).

Culling is more straightforward and has proved effective — in developed countries at least. When a Texas chicken farm had outbreaks of avian flu in May and June this year, the area was isolated and all the chickens killed. By 25 August the United States was in the clear.

Experts note that vaccines can stop the spread of disease — if accompanied by strict monitoring and testing. This has been successful in parts of Italy and the United States. But many doubt that Thailand has the necessary resources.

Another strike against vaccines is economic, says Alex Thiermann, president of the OIE's Animal Health Code Commission in Paris. As long as a country is vaccinating, it will not get approval for export. This should change in May 2005, says Thiermann, when the rules are expected to become more flexible for developing countries such as Thailand, allowing them to continue trading when vaccinating safely.

Thailand is now deciding what to do. A technical committee reporting to the deputy prime minister is weighing the evidence for and against vaccination, says Kalpravidh. It is expected to reach a decision in the next few weeks.

In the meantime, police have raided market stalls that distribute vaccines to farmers, handing out heavy fines. An education campaign is also planned to explain the risks and benefits of vaccines. But experts doubt that this will stop the use of these vaccines entirely.