Human H5N1 Influenza

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Abstract
As is consistent with previous studies of outbreaks of avian influenza A (H5N1) virus, the epidemiologic investigations reported by Kandun et al. in Indonesia and by Oner et al. in Turkey (Nov. 23 issue)1,2 show that H5N1 virus primarily infects young people (median age, 9 years). As of late November 2006, 258 cases of human H5N1 virus infection had been identified. More than half of the patients were under the age of 20 years (median age, 18.5 years), and 25% of them were under the age of 10 years. Although both studies report clusters within families and cite exposure to dead poultry as a common risk factor, it is unlikely that the intensity of exposure differed among household members. Rather, higher incidence rates in children may represent age-dependent differences in host susceptibility to H5N1 virus infection. Human infection is mediated by a receptor recognized by avian influenza (α2,3-linked sialic acid) that is expressed in the lower respiratory tract.3 In children this receptor may be expressed in the upper airway, increasing the risk of infection. Indeed, α2,3-linked sialic acids are homogeneously distributed in the human fetal lung, and the expression of the receptor appears to decrease with age.4