WASHINGTON (Reuters) - The H5N1 bird flu virus may sometimes stick to surfaces or get kicked up in fertilizer dust to infect people, according to a World Health Organization report published on Wednesday.
The WHO team reviewed all known human cases of avian influenza, which has infected 350 people in 14 countries and killed 217 of them since 2003, and found that 25 percent of cases have no explanation.
Most are passed directly from bird to people, they noted in their report, published in the New England Journal of Medicine. And very rarely one person can infect another -- always close relatives via intimate physical contact.
"In one quarter or more of patients with influenza A (H5N1) virus infection, the source of exposure is unclear, and environment-to-human transmission remains possible," the researchers, led by WHO's Dr. Frederick Hayden, wrote.
"For some patients, the only identified risk factor was visiting a live-poultry market."
It could be that small particles of virus-contaminated fluid stuck to surfaces, they said. Or perhaps fertilizer made from infected bird feces somehow carried the virus into people's noses or mouths.
"It is unknown whether influenza A (H5N1) virus infection can begin in the human gastrointestinal tract," they wrote.
"In several patients, diarrheal disease preceded respiratory symptoms, and virus has been detected in feces."
Government and health officials have stressed that well-cooked chicken cannot infect people. "Drinking potable water and eating properly cooked foods are not considered to be risk factors, but ingestion of virus-contaminated products or swimming or bathing in virus-contaminated water might pose a risk," the WHO team of bird flu experts noted.
H5N1 is considered entrenched in parts of Asia, including Indonesia, Africa and the Middle East. It pops up frequently in Europe and has prompted the slaughter of hundreds of millions of chickens.
The researchers noted that people only rarely become infected. The fear is that the virus will mutate into a strain that passes easily from one person to another, setting off a pandemic that could kill millions of people in the space of a few months.
"After exposure to infected poultry, the incubation period generally appears to be 7 days or less, and in many cases this period is 2 to 5 days," the WHO team wrote.
"In clusters in which limited, human-to-human transmission has probably occurred, the incubation period appears to be approximately 3 to 5 days, although in one cluster it was estimated to be 8 to 9 days."
It usually causes severe pneumonia and tests suggest that it rarely or never infects people without causing symptoms.
Avian flu kills on average within nine to 10 days and has killed 61 percent of victims.
Quick use of antiviral drugs can save lives, they noted, although some strains of the virus are more treatable than others with Tamiflu, the drug of choice to treat influenza. It is made by Roche Holdings AG and Gilead Sciences under the generic name oseltamivir.