H5N1 Flu Does Not Pass Easily to Humans, Study Finds

By Charlene Porter
Washington File Staff Writer

Washington – Hundreds of Cambodian villagers tended sick birds around their homes, but showed no evidence of infection with the H5N1 virus when tested by an international team of researchers, according to a study published in the October edition of Emerging Infectious Diseases, a publication of the U.S. Centers for Disease Control and Prevention (CDC).

“This study provides evidence of the low transmissibility of the H5N1 virus from infected poultry to humans, even in circumstances in which human-poultry interactions are regular and intense,” says the study conducted by researchers at the Institut Pasteur in Cambodia, the Food and Agricultural Organization of the United Nations, the World Health Organization, Australia National University, CDC and others.

The H5N1 virus is a highly pathogenic avian influenza strain that has spread to pandemic proportions among Asian birds, with more than 200 million dead. It has been found in domestic and wild birds in more than 50 countries and in regions other than Asia, and has killed more than 140 humans. Because this particular flu strain previously infected humans only rarely, international health officials warn that if H5N1 becomes easily transmissible among humans, an influenza pandemic could sweep the world.

The researchers focused their study on a Cambodian village in Kampot province where a 28-year-old man was infected with the H5N1 virus in March 2005.  Within a week after the man’s death, the researchers conducted surveys of families within a 1 kilometer radius of the patient’s household to determine how widely the illness might have appeared in birds and people in the same area.

The man who succumbed to avian influenza was a farmer who was known to have handled sick birds and eaten poultry that had died of suspected H5N1 illness. The farmer died within a week of his first symptoms, and his infection with the H5N1 virus was confirmed after testing of samples collected during his hospitalization.

In interviews with neighbors after the man’s death, the research team found evidence that “an H5N1 outbreak among numerous chicken flocks in the village” in the weeks before the farmer became ill. The villagers themselves gave blood samples to test for H5N1 antibodies to determine whether they had been exposed to the virus.

“Despite frequent direct contact with poultry suspected of having H5N1 infection, none of 351 participants from 93 households had neutralizing antibodies to H5N1,” the report says. None of the participants recalled having any respiratory illness during the 12-month period prior to the survey, despite regular, close contact with poultry, and in some cases, pigs.

The investigators report that they cannot come to a definitive conclusion on why only one person developed illness when so many others reported similar poultry exposures, though they acknowledged a small chance that previous H5N1 levels might have been missed in the antibody testing.

The findings do provide the basis for suggesting that transmission to humans occurs in persons who have “unique host susceptibilities and a predisposition to an abnormal inflammatory response that results in severe and fatal outcomes,” the study says.

The villagers also had very different ways of handling their birds, the research team found, a variance that could be another factor in their vulnerability to infection. Some families kept their birds closer to the house, potentially reducing their contact with wild birds that might carry the virus, in comparison to families that allowed their birds to roam broadly.

The researchers further noted varying practices in tending poultry with some villagers cleaning stalls and cages more regularly than others, and removing potentially infectious materials.

“These findings may highlight the value of educating farmers about hygienic animal-handling practices,” the study said.

For ongoing coverage of the disease and efforts to combat it, see Bird Flu (Avian Influenza).

See the research in Emerging Infectious Diseases.