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Cambodian study hints at subclinical H5N1 cases
Jan 25, 2008 (CIDRAP News) ? A recent study in Cambodia suggests that some human cases of infection with the H5N1 avian influenza virus escape detection because symptoms are mild or absent, according to a report from an international avian flu conference this week in Bangkok.
The meeting drew about 500 experts from 40 countries to discuss research and ideas on a wide range of topics. Some other topics discussed included the idea of stockpiling vaccine adjuvants to prepare for a pandemic, the use of engineered human antibodies as a defense against the H5N1 virus, and the high H5N1 case-fatality rate in Indonesia.
Cambodian study
The Cambodian researchers tested 674 people in two villages who were exposed to the virus and found that seven of them, all between the ages of 4 and 18, had antibodies signaling previous infection, according to a Jan 24 Bloomberg News report.
The finding contrasts with previous serologic studies of people in areas affected by H5N1 outbreaks. A review published Jan 16 in the New England Journal of Medicine (NEJM) said the few serologic studies since 2003 of people with potential exposure to H5N1 suggest that asymptomatic or mild cases are rare. The studies involved people living with backyard poultry, workers in live-bird markets, and healthcare workers.
More cases of mild disease might suggest that the virus is improving its ability to spread among humans, while becoming less virulent. Based on the current global count of 353 cases with 221 deaths, the case-fatality rate is almost 63%.
The Cambodian researchers, led by Sirenda Vong of the Pasteur Institute of Cambodia in Phnom Penh, conducted their study in early 2006, according to the Bloomberg story. The researchers asked villagers about their exposure to poultry and tested their blood for antibodies to H5N1.
The median age of the seven people who had antibodies was 12 years, compared with 27 years for those who had no antibodies, the story said.
Vong and colleagues had conducted a similar study of 351 Cambodian villagers in 2005 and found that none had antibodies to the virus. The study was published in Emerging Infectious Diseases in 2006.
Malik Peiris, a microbiology professor at the University of Hong Kong, told Bloomberg that the latest study supports findings from the 1997 H5N1 outbreak in Hong Kong, in which human cases were first reported. The virus infected 18 people, 6 of whom died. Peiris said children were less severely affected than adults and had a better survival rate, Bloomberg reported.
"Most of the children diagnosed in Hong Kong in 1997 had a very mild course of infection; they basically had a mild flu-like illness and they recovered," Peiris was quoted as saying. "I don't think there is any evidence to say the situation has changed."
The recent NEJM review said H5N1 infections involving febrile upper respiratory illnesses without pneumonia in children have been reported more often since 2005, but early antiviral treatment may account for this.
Poultry-to-Human H5N1 Transmission, Southern Cambodia, 2005
