Avian flu transmission to humans may be higher than thought

Close contact with sick or dead poultry seems required

A new study suggests that there is an association between direct contact with dead or sick poultry and flu-like illness in humans and that the transmission is probably more common than expected, according to a new study in the January 9 issue of Archives of Internal Medicine, one of the JAMA/Archives journals.

Anna Thorson, M.D., Ph.D., from the Karolinska Institutet, Stockholm, Sweden, and colleagues analyzed data from household interviews conducted in FilaBavi, a Vietnamese demographic surveillance site in Bavi district, northwest Vietnam, with confirmed outbreaks of highly pathogenic avian influenza (HPAI) in poultry, subtype H5N1. The researchers included 45,478 randomly selected people in the district to answer screening questions about exposure to poultry and flu-like illnesses (defined as a combination of cough and fever). The study was performed from April 1 to June 30, 2004.

As background information, the authors explain that in Vietnam, an epidemic of HPAI in poultry, subtype H5N1, has been ongoing since 2003, despite efforts by the government to eliminate the diseased birds from flocks. "In addition to being an important source of income, poultry is kept by many rural households for subsistence farming. The current epidemic in poultry is thus not only a public health problem but also an economic drawback for the many Vietnamese who live in rural areas." The researchers note the Vietnam is the country hardest hit by the ongoing H5N1 epidemic, with 87 confirmed human cases of HPAI (with 38 deaths) and 1,838 verified outbreaks in poultry in July 2005.

In this study, the researchers found "a total of 8,149 individuals (17.9 percent) reported flu-like illness, 38,373 persons (84.4 percent) lived in households keeping poultry, and 11,755 (25.9 percent) resided in households reporting sick or dead poultry." The researchers report that having poultry in the household was not a risk factor for developing a flu-like illness, but having direct contact with sick or dead poultry produced the highest risk for flu-like illness. "Low socioeconomic status, female sex, and young or old age were also risk factors for the disease," they note. The researchers estimate that between 650 and 750 cases could be attributed to direct contact with sick or dead poultry.

"Our results from a large epidemiological population-based study in an area with an ongoing epidemic of HPAI in poultry are consistent with a higher incidence of HPAI among humans than has been recognized previously. The results suggest that the symptoms most often are relatively mild and that close contact is needed for transmission to humans," the authors conclude.

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(Arch Intern Med. 2006;166: 119-123. Available pre-embargo to the media at www.jamamedia.org.)

Editor's Note: This study was financially supported by the Swedish Institute for Infectious Disease Control. The FilaBavi demographic surveillance site is financed by the Department for Research Cooperation within the Swedish International Development Agency (Sida/SAREC, Stockholm). The financiers have had no role in the research process.


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