MONDAY, Jan. 9, 2006 (HealthDay News) -- Many more people have been infected with bird flu than reported, but they've only shown mild symptoms, a new Swedish study finds.
This raises the possibility that the illness may not be as dangerous as thought, some infectious disease experts suggest.
According to the report, there's a connection between direct contact with dead or sick poultry and flu infections in humans, and transmission from birds to humans is probably more common than believed.
The finding about contact with sick fowl seems to confirm new developments in Turkey, where preliminary tests on Sunday showed that two young brothers and an adult in the capital city of Ankara have the deadly H5N1 strain of bird flu.
The cases, not yet confirmed by World Health Organization (WHO) officials, would raise to 15 the number of cases detected in Turkey since Wednesday, heightening concerns that the virus is spreading across that country. Two of three teenaged siblings who died of bird flu in the city of Van last week had been playing catch with the heads of dead chickens, according to published reports. Those deaths marked the first time the virus has killed humans outside of East Asia, WHO officials told the Associated Press.
The Swedish study appears in the Jan. 9 issue of the Archives of Internal Medicine.
"Our results are consistent with H5N1 among humans being a lot more common than has previously been recognized," said study author Dr. Anna Thorson, of the Division of International Health at the Karolinska Institute, in Stockholm. "In addition, they suggest that the symptoms of H5N1 in humans most often are relatively mild, and that close, direct contact with sick or dead poultry is needed for transmission to humans."
In its study, Thorson's team collected data on 45,478 people in the FilaBavi area of Vietnam, which has been hard hit by the H5N1 flu virus, from April 1 to June 30, 2004. The researchers asked people about exposure to poultry and flu-like illness during the preceding months; individuals with a history of disease and/or exposure were interviewed in person.
"We found a dose-response relationship between flu-like illness and degree of contact with sick or dead poultry," Thorson said. "In layman's language, this would read as the closer the contact with sick or dead poultry, the higher the risk for flu-like illness."
Thorson's group found that simply having sick or dead poultry in the house did not significantly increase the risk of flu-like illness; those people only showed a 14 percent higher risk of flu-like illness compared to someone without poultry. However, having direct contact with sick or dead poultry raised that increased risk to 73 percent, Thorson said.
The researchers found that between 650 and 750 of the human cases with flu-like illness could be attributed to direct contact with sick or dead poultry. "Taking into account the type of symptoms reported from the human cases and the reported ongoing H5N1 epidemic in poultry in the district, transmission of highly pathogenic avian flu to humans is the most likely cause of the 650 to 750 cases," Thorson said.
Without blood tests, it's not possible to know for sure if the flu-like illness reported is actually bird flu, Thorson explained. "However, if transmission from poultry to humans is more common than what has earlier been anticipated, as suggested in our study, it may imply an increased risk of viral re-assortment through infected individuals serving as mixing vessels for highly pathogenic avian influenza and human influenza," Thorson noted.
One expert is not sure what the finding means in terms of evaluating the actual danger of avian flu.
"There certainly is considerable concern that relatively asymptomatic infections with H5 or other avian viruses may be occurring in situations where there is a high level of exposure to sick birds," said Dr. John Treanor, a professor of medicine, microbiology and immunology at the University of Rochester. "After all, only a tiny fraction of all of the exposed persons goes on to develop recognized H5 influenza. It would seem like the attack rates should be much higher."
"One possibility might be that the virus is actually very poorly infectious, and it is only the unfortunate few who are exposed to extremely high levels that become infected," Treanor said. "The other possibility is that for each recognized case there are many others that are not recognized."
Another expert sees the finding as proof that the fear of an avian flu outbreak among people has been exaggerated.
"It's exactly what I suspected all along," said Dr. Len Horovitz, a pulmonary specialist at Lenox Hill Hospital in New York City. "Basically this illness exists as a very mild cold or flu in thousands, maybe hundreds of thousands, of people in China, and it's never reported because they're not sick enough to go to the doctor."
Horovitz thinks that since this flu has been around since 1997, if it were going to become a deadly pandemic it would have done so already. "It's just like any other flu," he said. "It kills some vulnerable people, and gives other people a mild illness that goes unreported."
According to the WHO and local governments, six countries have had 146 confirmed cases of the deadly H5N1 strain of bird flu, with 76 fatalities.
More information
The U.S. Centers for Diseases Control and Prevention can tell you more about bird flu.
