Study shows hidden birdflu cases in Cambodia

By Maggie Fox,

Health and Science Editor

WASHINGTON (Reuters) - The H5N1 bird flu virus can infect people without causing noticeable symptoms, but only rarely, according to a report published on Thursday.

A survey of more than 600 people in Cambodian villages where two children died from the virus shows seven more were apparently infected, but without having known about it.

The study, published in the Journal of Infectious Diseases, also suggests that people may become infected by swimming in ponds where infected birds have dabbled.

"Although these results cannot be considered to be representative without broader confirmation, they show that, in some settings, surveillance may substantially miss H5N1 virus infections," Dr. Sylvie Briand and and Dr. Keiji Fukuda of the World Health Organization wrote in a commentary.

The H5N1 avian influenza virus has been regularly causing outbreaks of disease in birds -- 250 outbreaks in February alone in Bangladesh, China, Egypt, India, Indonesia, Laos, Nepal and Vietnam, according to the U.N. Food and Agriculture Organization.

It only rarely infects people but is often deadly when it does. WHO says it has killed 261 people out of 424 infected since 2003.

The big fear is that is could change into a form that people can pass easily to one another, sparking a pandemic. These fears have been overshadowed at least a little by the near-pandemic of much milder H1N1 swine flu that started in March.

One big question has been whether some people have been infected without knowing it. If this is the case, the fatality rate would go down. With current numbers, the fatality rate appears to be around 60 percent but if there are more than 424 infections it would make for a lower rate.

Sirenda Vong of the Institut Pasteur in Cambodia and colleagues followed up on two deaths of children from H5N1 in 2006. They interviewed villagers and took blood samples.

ANTIBODIES IN BLOOD

"Seven (1 percent) of 674 villagers tested seropositive for influenza H5N1 antibodies and did not report severe illness," they wrote. This means their bodies had at some point fought off an H5N1 infection.

Most were male, 18 or younger, and were more likely than other villagers to have reported bathing or swimming in household ponds. They all lived in wooden houses on stilts with well or pond water as the only water source for the family and none had known contact with the two children who died.

Scientists know that birds can pass influenza viruses in their droppings and ducks, especially, can foul ponds with virus-infected droppings. The virus can live in droppings or water for up to six days.

"During the study period, most participants reported repeated direct and close poultry contact, including feeding or touching poultry (73.3 percent), collecting poultry feces for manure (50.9 percent), plucking feathers of sick poultry (31.1 percent), or collecting sick and/or dead poultry with bare hands (36.8 percent)," the researchers wrote.

But, they added, the findings suggest that transmission from sick bird to human in Cambodia was rare in 2006.

The found genetic material from the H5N1 virus in specimens taken from ponds and pond plants.

"Our results also indicate that swimming or bathing in household ponds could be a risk factor for influenza H5N1 virus infection. These small ponds are common and usually serve as a water source for backyard animals and gardening,' the wrote.

"Ducks usually have access to these ponds and may deposit large amounts of feces in ponds in which children commonly bathe and play."