Dutch scientists have found that more than twice as many people as thought may have been infected during a bird flu outbreak in Dutch chickens in 2003.
The flu is not the same as the one now breaking out again in East Asia, but it shows once again that these viruses are capable of unpleasant surprises.
The 2003 outbreak, which led to the culling of 31 million birds on more than a thousand farms, was of a flu virus called H7N7, after the type of surface proteins it carries. The bird flu that spread across East Asia earlier in 2004, and has broken out again over the past two months, is called H5N1.
H7N7 had not been previously known to cause serious infections in people. But during the Dutch outbreak, 86 people who were in contact with sick birds got either a mild eye infection, or typical flu symptoms. One vet died of pneumonia caused by H7N7.
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More worrying, three more sick people had not been directly in contact with chickens, but only with people who had been working with sick birds, showing the virus is capable of limited human-to-human spread.
Viral antibodies
Since then, Marion Koopmans and colleagues at the Dutch National Institute for Public Health and the Environment (RIVM), have been testing people who had contact with either sick chickens, or chicken workers, but had no suspicious symptoms. At a meeting this week in the Netherlands, they reported that of 419 people who worked with sick chickens, 212, more than half, had antibodies to the virus. Most of them had no symptoms at all, Arnold Bosman of the RIVM told New Scientist.
The researchers also confirmed that H7N7 can spread between people. Of 62 people who had contact with chicken workers, but not chickens, 33 had antibodies to H7N7. The work is due to be published in a few weeks.
Most of the antibodies were caused by an actual infection, not just exposure to large amounts of viral proteins, says Bosman, because taking antiviral drugs, as chicken workers were asked to do, seems to have kept people from developing antibodies to H7N7 – although some people on the drugs did anyway.
Dangerous hybrid
This means H7N7 can infect people more readily than was thought, and without causing symptoms. The fear is that H7N7 might infect someone who is also carrying human flu, allowing a dangerous hybrid to emerge.
That is an even greater fear in Thailand and Vietnam, where a handful of people have contracted H5N1 bird flu since the virus broke out again in poultry in August.
H5N1 is a much nastier virus, killing 29 of the 40 confirmed human cases in Thailand and Vietnam. But, so far, it seems incapable of human-to-human spread.
However, H5N1 can spread undetected in poultry that have been vaccinated against it, posing a constant risk of surprise infection, and possibly undergoing dangerous adaptive changes.
Citing the risk to human health, Thailand decided on Wednesday not to vaccinate poultry against H5N1, unlike China and Indonesia, despite the risk of outbreaks and intense pressure to vaccinate from the cock-fighting industry.
