Mild H5N1 cases weren't missed in Cambodian outbreak

Mar 27, 2006 (CIDRAP News) – Researchers who looked for mild or asymptomatic human cases of H5N1 avian influenza following an outbreak in Cambodia last year didn't find any, challenging the view that human cases have gone undetected, according to findings presented last week.

The research described Mar 20 at the International Conference on Emerging Infectious Diseases in Atlanta implies that surveillance for human cases might be more effective than some experts assumed, according to a story by the Canadian Press (CP). However, the findings also imply that the case-fatality rate for avian flu is higher than some experts thought.

Dr. Philippe Buchy and his colleagues at the Institut Pasteur in Phnom Penh last spring tested blood samples from 351 residents of a Cambodian village where poultry and one person had died of avian flu, the CP reported. No signs of antibodies to H5N1 were found in the samples, indicating the residents had not suffered even mild cases of avian flu.

Some of the people tested had "significant" exposure to poultry or infected people, the story said. For example, a doctor who inserted a tube down an H5N1 patient's windpipe without wearing any protective gear did not show any antibodies indicating infection. The same was true for other healthcare workers, including two veterinarians who had autopsied H5N1-infected birds. The healthcare workers did not know at the time they were dealing with avian flu cases.

"We didn't find any cases of H5N1, so nobody seems to have been asymptomatic or with mild symptoms during this outbreak in Cambodia," Buchy told the CP.

On the bright side, Buchy was quoted as saying the data indicate the virus still finds it difficult to jump from poultry to humans.

Dr. Nancy Cox, head of the influenza branch at the US Centers for Disease Control and Prevention (CDC) in Atlanta, concurred with Buchy, but added: "On the other side of the coin, it means that the case-fatality rate is still very high. And that is a negative thing."

Experts have suggested that the current case fatality rate for avian flu, about 56%, could be inaccurate because milder or asymptomatic cases have not been identified.

"The work in Cambodia is extremely important because it shows that we really aren't missing that much," Cox told the CP. However, she added that it is important to conduct research on a larger scale to determine whether mild or asymptomatic cases are occurring, and said such studies are planned.

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