Oct 20, 2004 (CIDRAP News) – Avian influenza probably infected at least 1,000 people during a major outbreak in the Netherlands in 2003, many more than originally thought, Dutch researchers say.

Results of antibody tests of poultry workers suggested that at least 1,000 people were infected with the H7N7 avian flu virus, according to research conducted by the Dutch National Institute for Public Health and the Environment (RIVM).

The RIVM findings, which have drawn considerable media attention, are almost entirely written in Dutch, but an abstract and some parts of the report are available online in English. The research has been submitted for publication in a journal, but has not yet been published, which prevents RIVM from freely distributing full written reports, an RIVM spokeswoman told CIDRAP News by e-mail.

During the outbreak, 453 people complained of health problems, chiefly conjunctivitis but also influenza-like illness. This represented about 10% of the estimated 4,500 people who potentially were exposed to infected poultry, the report says. However, further research has found that many infected people were asymptomatic.

Researchers found an unexpectedly high rate of transmission of the H7N7 strain to people who handled infected poultry, as well as evidence for person-to-person transmission, according to the report.

To identify the virus, researchers used a combination of culturing of the virus from eye swabs and conducting polymerase chain reaction tests. They also tested those who potentially exposed to the virus for antibodies, according to an Oct 18 story by Science Now online. Antibodies were found in about half of 500 poultry handlers tested.

Given the total number of poultry workers at risk, RIVM researchers estimated that at least 1,000 people had been infected, most without symptoms, Science Now reported. Antibodies were found in 59% of household members of infected people who developed symptoms, according to the report.

The outbreak among poultry began in late February of 2003 in the province of Gelderland and spread to 255 farms. About 30 million chickens, or 28% of chickens in the Netherlands, died or were killed to control the disease. At the time of the outbreak, 83 people were confirmed to have the H7N7 virus, and one veterinarian died. Poultry cullers and veterinarians had the highest estimated attack rates, the RIVM report says.

Poultry farmers and workers didn’t comply sufficiently with the preventive measures, researchers noted. Such measures varied in efficacy as well: the antiviral drug oseltamivir protected against infection, but masks did not, they wrote.

The outbreak also affected mental health, the researchers found. One third of farmers whose holdings were cleared of poultry described stress reactions, fatigue, and symptoms of depression.

The report concludes by noting that human attack rates in the outbreak far exceeded earlier outbreaks, but it wasn’t clear why. The unique properties of the virus, specifics of the handling of poultry, and the active case finding were three possible factors, researchers said.

Although safety measures were in place within a week of confirmation of the initial human case, 1,000 people were infected anyway, the report says. Researchers concluded that “if a more effectively spreading variant had arisen, containment would already have been extremely difficult” by that time.

“We see this as a strong reinforcement of the need for pre-tested pandemic preparedness plans, including the stockpiling of essential components such as vaccine and antivirals,” they wrote.

Bosman A, Mulder YM, de Leeuw JRJ, et al. Avian flu epidemic 2003: public health consequences. National Institute for Public Health and Environment, The Netherlands, 2004 (Abstract)