Risk factors for human infection with avian influenza virus, also known as H5N1, have been identified in three new studies from Asia, published in the June 15th issue of The Journal of Infectious Diseases, now available online. The factors include direct or indirect contact with sick or dead poultry and visiting a "wet" poultry market, where live birds are bought and slaughtered (China); keeping many chickens or fighting cocks in a household's yard (Thailand); and bathing or swimming in household ponds that are often shared with ducks (Cambodia).
The differences among these cultural risk factors support the view that controlling the spread of the infection from birds to humans--and ultimately preventing the development of human-to-human transmission--will require measures tailored to the varying ways in which humans can be exposed to infected birds. Cultural risk factors for H5N1 acquisition may also be important when considering the possibility of reassortment or recombination with currently circulating influenza virus strains—e.g., H1N1 (swine).
In the Chinese study, performed by Hongjie Yu, MD, and coworkers in China and the United States, 28 patients diagnosed with H5N1 influenza in 2005 to 2008 answered a questionnaire about potential exposures to the virus, and the results were compared with those from 134 randomly selected matched controls. In general, patients in urban areas had visited wet poultry markets, and patients from rural areas had either been directly exposed to sick or dead poultry or indirectly so (getting within a meter of poultry, poultry products, or poultry feces, without physical contact). All three factors--wet markets, direct exposure, and indirect exposure-- raised the risk of infection independently of the others.
The investigators commented that the Chinese public needs to be better educated about these risks, and specifically that poultry should be raised outside the home and wet markets may need to be subjected to such control measures as selling only vaccinated birds, segregating bird species, and centralizing poultry slaughtering sites.
The Thai study, by Thanawat Tiensin, MD, and colleagues in The Netherlands, Thailand, and Bangladesh, examined human and animal risk factors associated with avian influenza clusters in 824 subdistricts in Thailand during 2004-2007, compared with control subdistricts from other study periods. Statistically significant variables included the density of backyard flocks of chickens or fighting cocks, the presence of quail or free-grazing duck flocks, and the presence of a poultry slaughterhouse; wet poultry markets were not examined because there are very few in Thailand.
In addition, an association was demonstrated between subdistricts with H5N1-infected poultry flocks and evidence of prior and concomitant H5N1 infection of wild birds in the same districts.
The risk of H5N1 infection was also high in subdistricts with small to medium poultry flocks on family-run commercial farms; in contrast, the risk was low in subdistricts with large-scale commercial farms.
The authors concluded that controlling the spread of H5N1 in Thailand will require targeted public education campaigns and surveillance of various aspects of commercial poultry production, such as transportation, equipment maintenance, and waste disposal, as well as slaughterhouse practices.
In 2006, Sirenda Vong, MD, and other investigators from Cambodia interviewed and took blood samples from 674 residents of two Cambodian villages that had recently experienced outbreaks of H5N1 influenza in humans and poultry. Serologic tests, performed in the United States, showed 7 (1%) villagers with antibodies to H5N1, none of whom reported severe illness. All 7 were aged 18 years or less, and 6 were male. Compared with matched antibody-negative control subjects, the asymptomatic infected subjects were younger and more likely to have bathed or swum in household ponds. The investigators noted that such ponds are common in Cambodia and usually serve as a water source for backyard animals and gardening. “Ducks,” they noted, “can have access to these ponds and may deposit large amounts of feces where children commonly bathe and play.”
Although further studies of H5N1 transmission are needed, the investigators recommended control efforts that include “strict separation between human and backyard poultry areas in Cambodian households, restriction of access to potentially contaminated water, and limiting of interactions between poultry and children.”
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