Can people catch the H5N1 avian flu virus from eating infected poultry? Colin Blakemore, chief executive of the UK Medical Research Council, says the public need not worry. “There is no evidence of transmission to people by eating cooked eggs or chicken,” he said on BBC radio last week, adding that the only food risk he could see was from “drinking swans' blood”.

Blakemore's sound bite came a day after Britain's first case of H5N1 in a wild bird was confirmed — a dead swan found floating in a harbour in Cellardyke, Scotland. And it echoes a slew of recent reassurances by governments worldwide and by the World Health Organization (WHO), all conscious of damaging public confidence and the poultry industry.

Direct evidence of oral infection is lacking, but so too is proof against.

But many flu scientists are concerned that, although the risks are low compared with those associated with contact with diseased birds, there is not enough evidence to say that the virus cannot be transmitted by eating infected poultry. “Oral transmission is an open question,” says Masato Tashiro, a virologist at the National Institute of Infectious Diseases in Tokyo. “Direct evidence of oral infection is lacking, but so too is proof against.”

On 23 March, the European Food Safety Authority (EFSA) published a prominent scientific risk assessment (EFSA J. 74, 1–29; 2006). Its advice is that poultry products are safe to eat and have “not been implicated in the transmission of the H5N1 avian influenza virus to humans”.

H5N1 is present in the meat and eggs of infected birds, and animals have become infected by eating diseased birds. But the EFSA plays down this route in humans, arguing that “humans who have acquired the infection have been in direct contact with infected live or dead birds”.

Don't panic: official advice is that despite bird-flu fears, chickens are safe to eat. Credit: F. VILLA/AFP/GETTY

That overstates the case, says Jody Lanard, a physician and risk-communication consultant based in Princeton, New Jersey, who has recently advised the WHO about pandemic communication. She points out that the report itself acknowledges elsewhere that in many instances there is not enough epidemiological evidence to identify the source of infection, and that poor preparation and cooking of food cannot be excluded as the cause. “Such cases could equally well indicate a likely gastro–intestinal portal of entry,” agrees Menno de Jong, a virologist at the Hospital for Tropical Diseases in Ho Chi Minh City, Vietnam.

The report also dismisses the idea that H5N1 can enter the body via the human gut, concluding that there is “no proof that virus replicates in the human intestine”. Although it mentions the presence of diarrhoea in infected humans, together with the detection of viral RNA in intestines and the virus in rectal swabs, it says these “do not allow one to conclude that the GI tract is a portal of entry or a target organ”.

De Jong, who has treated many of the cited diarrhoea cases, says the report's authors are “formally right” to say there is no proof that the virus replicates in the intestine. But there is no proof that it doesn't either, he says, noting that some of the diarrhoea cases had no respiratory symptoms.

“Available evidence suggests that the gastrointestinal tract in humans is a portal of entry for H5N1,” agrees Albert Osterhaus, a virologist at the Erasmus Medical Center in Rotterdam. He carried out a recent study in which cats became infected with H5N1 after being fed infected chickens (T. Kuiken et al. Nature 440, 741–742; 2006) — evidence also dismissed by the EFSA report as “unproven”.

Of course, to pose a risk the virus must enter the human food chain. The EFSA and other authorities point out that this is unlikely, at least in industrialized countries. But some scientists, including Osterhaus, say it cannot be excluded — for example, if the virus enters poultry a few days before clinical signs appear.

The final argument of the EFSA and the WHO is that even if the virus did enter the food chain, it would be killed by cooking or pasteurization, in the same way as bacterial pathogens such as Salmonella. But Lanard points to the countless infections with Salmonella worldwide, and complains that most risk assessments fail to acknowledge that in reality few people follow guidelines for the safe handling and cooking of poultry. These involve cooking chicken right through to 70 °C and eggs until they are hard, using separate knives and chopping boards for raw and cooked foods, and hand-washing between operations.

An EFSA spokesperson says the agency stands by the report's conclusions. Several scientists are also convinced that avian flu carries no food risks. “Avian influenza has never been and should never have been seen as a food safety issue,” says Les Sims, a consultant for the UN's Food and Agriculture Organization (FAO). Bird-flu concerns over food, he says, “have a devastating impact on the livelihood of millions of farmers globally and demonstrate that risk communication on this has been a total failure”.

But Lanard maintains that to say bird flu is not a food issue is an “overstatement”. She says that such advice shows that little has been learnt about risk communication since the British agriculture minister publicly fed his young daughter a hamburger at the height of the crisis over bovine spongiform encephalitis. A 2005 European Commission poll showed that almost half of European citizens believe authorities favour economic interests over consumer health, she points out. “These over-reassuring statements discount the future — they are set up for public distrust,” she says. “Although there is no direct evidence that transmission can occur through poorly cooked infected poultry, all animal evidence to date unfortunately suggests that this is possible.”