Apr 8, 2009 (CIDRAP News) – The recent series of H5N1 avian influenza cases in Egyptian children yet very few in adults has raised concern that some Egyptians may be getting infected without getting sick, according to a Reuters news report published today.

John Jabbour, a World Health Organization (WHO) emerging diseases specialist based in Cairo, said the Egyptian government and the WHO are planning a study to find out if subclinical or asymptomatic cases have been occurring, according to the story.

Jabbour said the occurrence of asymptomatic cases would be worrisome because it could give the undetected virus more time to mutate in human hosts, Reuters reported.

"If there is any subclinical case in Egypt, the aim is to treat immediately to stop the reproduction of the virus," he told Reuters. "Because whether [through] mutation or reassortment, this will lead to the pandemic strain."

The story said that all but two of the 11 Egyptians infected with H5N1 this year have been children under age 3 (the official WHO count for Egypt reached 12 today). In the same period last year, most of the seven Egyptian case-patients were adults and older children, the report said.

Jabbour said the string of cases in children without similar cases in adults had prompted the questions whether adults were being infected without falling ill.

He said the Egyptian study would involve testing the blood of people who were potentially exposed to infected birds but had not gotten sick.

Tim Uyeki, MD, a medical epidemiologist in the Influenza Division at the Centers for Disease Control and Prevention, told CIDRAP News that limited studies in several countries over the past few years have found relatively little serologic evidence of asymptomatic or unrecognized cases of H5N1 virus infection.

When the H5N1 virus first infected humans in Hong Kong in 1997, there were two studies in which investigators looked for H5N1 antibodies in people who had potentially been exposed to the virus, Uyeki said. One study focused on 293 poultry cullers, of whom only nine (3%) were found to have H5N1 antibodies, indicating past infection. In the other study, involving 1,525 poultry workers, an estimated 10% had antibodies.

More recent studies "have either found no evidence of H5N1 virus infection or very low seroprevalence of H5N1 antibodies, around 1% or less, among different exposed populations," Uyeki said. "Since H5N1 virus strains continue to evolve, there's a continued need for these studies in people who have contact with poultry in countries where H5N1 virus strains are circulating in birds, since the risk of transmission to exposed persons could change."

Uyeki also noted that subclinical infection with seasonal influenza virus is known to occur. "But how that relates to H5N1 is unclear," he said.

See also:

Jan 25 2008 CIDRAP News story "Cambodian study hints at subclinical H5N1 cases"