Sep 21, 2006 (CIDRAP News) – South Korea said last week that five workers who helped cull poultry nearly 3 years ago showed evidence of past infection with H5N1 avian influenza though they had never been ill.
The Sep 15 report by South Korea's Yonhap news agency said the five workers had tested positive for antibodies to the H5N1 virus but had never had symptoms.
Samples from the workers were tested by the US Centers for Disease Control and Prevention (CDC), according to a Sep 15 Reuters report.
"The five did not develop major illnesses and have no strain to transmit bird flu," Reuters quoted Korea's Center for Disease Control and Prevention as saying.
Four other South Korean poultry workers were previously found to have H5N1 antibodies without having been ill, bringing the total to nine, according to Reuters.
South Korea had outbreaks of H5N1 disease in poultry in December 2003 and early 2004, but no human cases were reported. About 400,000 birds were infected and about 5 million were destroyed to contain the disease, the Reuters report said.
At the time, the government sent samples from 318 poultry industry workers to the US CDC for testing, which identified the four workers who had antibodies, the story said. Korean officials reported last February that the four had never been ill.
But the findings prompted Korean officials to send samples from another 2,109 workers to the US for testing, which led to identification of the five additional cases, according to Reuters.
No outbreaks of H5N1 avian flu have been reported in South Korea since March 2004, say reports the government filed with the World Organization for Animal Health (OIE).
Researchers have suggested that the H5N1 strain that struck Korea in 2003 and 2004 was less pathogenic for humans than the strains that infected people in Vietnam and Thailand.
In a report in the March 2005 Journal of Virology, US and South Korean scientists said they had found differences between the Korean and Vietnamese H5N1 isolates in all eight viral genes. In addition, Korean and Thai strains of H5N1 differed in their surface protein (hemagglutinin and neuraminidase) genes, the scientists said. The researchers also found that the Korean strain had a low level of pathogenicity in mice.
A few asymptomatic and mild human cases of H5N1 infection have been reported previously. When the virus first infected humans in Hong Kong in 1997, a small number of poultry cullers, household contacts of patients, and healthcare workers tested positive despite having no serious illness, according to reports in medical journals.
In addition, two elderly relatives of H5N1 patients in Vietnam tested positive for the virus in March 2005, according to news reports at the time. And in January of this year, a World Health Organization official reported that two young Turkish brothers tested positive but were not sick, according to news services.
However, recent serologic surveys of healthy people with a history of exposure to H5N1 have found almost no one with evidence of infection. Most recently, researchers reported that among 351 Cambodian villagers who had extensive contact with infected poultry, none had antibodies to the virus. Their study was published this month in Emerging Infectious Diseases.
March 2005 Journal of Virology article on H5N1 isolates from South Korea
Sep 7 CIDRAP News story "Cambodian study suggests mild H5N1 cases are rare"
Jan 11, 2006, CIDRAP News story "WHO says Turkish cases may yield new findings on H5N1"
Sep 29, 2005, New England Journal of Medicine article with information on serologic surveys in people exposed to H5N1
Oct Emerging Infectious Disease article on Cambodian serologic surveys