AMSTERDAM--Antibody tests now show that at least 1000 people contracted an avian influenza virus during a massive poultry outbreak in the Netherlands last year--many more than assumed. In another surprise, those who developed symptoms after being infected passed the virus on to a whopping 59% of their household contacts, according to the study, whose results were published last week by the National Institute for Public Health and the Environment (RIVM) in Bilthoven, the Netherlands.
Almost 31 million poultry were culled in the Netherlands last year before the virus, a strain called H7N7, was contained. One veterinarian died from an infection with the virus, and some 450 people reported health complaints, most of them an eye infection called conjunctivitis. In a paper published in The Lancet in February, RIVM virologist Marion Koopmans and her colleagues reported that they could detect the H7N7 virus--using the polymerase chain reaction or by culturing the virus--in eye swabs from 89 patients.
To gauge the true reach of H7N7, however, Koopmans and her colleagues also tested those at risk for antibodies against the virus. They found antibodies in about half of 500 people who had handled infected poultry; based on the total number of poultry workers at risk, the team concludes that at least 1000 people must have become infected, most of them without any symptoms. Wearing a mask and goggles did not seem to prevent infection; taking an antiviral drug called oseltamivir did, but a quarter of the cullers and half of the farmers did not take the drugs. Among 62 household contacts of conjunctivitis patients, 33 became infected themselves. Having a pet bird at home increased household members' risk of becoming infected, perhaps because the birds replicated the virus, too.
Flu experts are cautious about interpreting the findings, revealed in a report in Dutch but not yet published in a peer-reviewed journal. But if true, they are "another warning signal" about avian influenza's risks, says Klaus Stöhr, head of the World Health Organization's global influenza program. The more humans that an avian virus infects, Stöhr says, the greater the risk that it will morph into a flu pandemic. That's why looking for the avian virus in humans should become standard practice during outbreaks, he says--whether the virus is dangerous to humans or not.