As many as 2,000 people got bird flu (search) during the 2003 Netherlands outbreak, a Dutch investigation shows.

That's way up from the 69 cases previously reported. Even more ominous is the finding that person-to-person spread was vastly more efficient than ever before seen with bird flu. Nearly 60 percent of infected poultry workers' household contacts showed signs of infection.

The ongoing bird flu outbreaks in Asia and the 2003 Netherlands outbreak were caused by different strains of the virus. The Dutch bird flu (search) virus is far less deadly to humans than the bird flu virus in Asia. There was only one death in the 2003 Netherlands outbreak. The Asian bird flu (search) has killed dozens of people so far.

Experts say it's only a matter of time before these viruses evolve to the point where they spread easily from human to human. That hasn't yet happened, notes Arnold Bosman, MD, of the National Institute for Public Health and the Environment (RIVM) in Bilthoven, Netherlands. But Bosman, who led the RIVM investigation for the Dutch government, says he's surprised by how easily the bird flu virus spread from chickens to poultry workers, and from workers to their families.

"If we look at the infection rate we found in poultry workers and their household contacts, it is very high considering other studies of avian flu," Bosman tells WebMD.

In a summary report on their findings to the European Union's Eurosurveillance Weekly web site, Bosman and colleagues suggest that bird flu may not have too much further to evolve to trigger a worldwide flu pandemic. They also note that human-to-human spread was common despite the best efforts of health authorities.

"Both [the Dutch and Asian] avian influenza outbreaks illustrate that crossing the species (search) barrier is less rare than previously recognized, that avian influenza virus adaptation [to humans] occurs rapidly, and that if such jumps between species occur, human behavior in the broad sense may accelerate dissemination," Bosman and colleagues write.

Findings Controversial

Why did it take so long to find out so many people were infected with bird flu? Bosman says the blood test used to detect previous bird flu infections was faulty. His team developed a new test and found vastly more infections.

At first, Bosman says, he thought the new test had to be wrong. But positive results on the test correlated with the main human symptom of bird flu: the viral eye infection called conjunctivitis or redeye.

"We are pretty confident that the results really reflect true infection," Bosman says.

Walter Orenstein, MD, says it's hard to know what to make of these findings until it's clear that the test Bosman and colleagues used is truly valid. Orenstein is director of the vaccine policy and development program and associate director of the Vaccine Center at Emory University in Atlanta.

"This report is very intriguing. But it is hard to know what it means at this point. We need to know how valid this test is," Orenstein tells WebMD.

But if the test is valid, Orenstein says, it suggests that the 2003 bird flu outbreak spread far beyond poultry workers and their immediate household contacts.

"It would seem to me with that high a household secondary infection rate -- 59 percent -- you would think it would have gotten out into the community more," he says. "If it turns out to be a valid finding, it is of concern that there was a lot more spread than previously thought."

Even so, Orenstein notes that there's no reason to think that the far more deadly Asian bird flu is spreading as the bird flu did in the Netherlands.

"If true, this report suggests a lot of transmission to humans of a nonvirulent strain. That's very different from the situation in Asia, where you have very limited transmission to humans of a very virulent strain," he says.

Bosman says it's important to find out for sure whether a different test for the Asian bird flu virus would yield different results.

"We now need to assess whether there are more sensitive tests available to look more closely at the possibility of higher prevalence of other flu viruses," he says.

By Daniel J. DeNoon, reviewed by Michael W. Smith, MD

SOURCES: Bosman, A. Avian Flu Epidemic 2003: Public Health Consequences, RIVM report, December 2004. Bosman, A. Eurosurveillance Weekly, Jan. 6, 2005. Arnold Bosman, MD, epidemiologist, National Institute for Public Health and the Environment, Bilthoven, Netherlands. Walter Orenstein, MD, professor of medicine and pediatrics; director, vaccine policy and development program; associate director, Vaccine Center, Emory University, Atlanta.