Content area
Full Text
1. Introduction
Avian influenza refers to the infection of birds with avian influenza type A viruses [1]. These viruses occur naturally among wild aquatic birds worldwide and can infect over 100 domestic sources of poultry as well as other birds and animal species [2,3,4,5,6,7]. Avian influenza viruses do not normally infect humans, but human infections may occur after contact with infected birds or their secretions or excretions, or through limited human-to-human transmission [8,9,10,11,12]. Given the significant global improvements in laboratory characterization and surveillance, additional novel avian viruses are likely to be identified. Following the appearance of the H5N1virus in 1997, ongoing surveillance efforts have already improved not only the detection of the H7N9 (in 2013), H10N8 (in 2013) and H5N6 subtypes (in 2014), which have all caused severe infections, but also the detection of other subtypes such as H6N1, H7N2, H7N3, H7N7, H9N2 and H10N7, which have resulted in mild infections in a limited number of humans [1,13,14].
Each new virus may have a distinct potential for animal-to-human transmission or to cause mild, severe or even fatal human illness. On the basis of the molecular characteristics of the viruses and their ability to result in disease and mortality in chickens in a laboratory setting, avian influenza A viruses have been classified into the following two categories: low pathogenic avian influenza (LPAI) A viruses and highly pathogenic avian influenza (HPAI A viruses [15]. The majority of those isolated have been LPAI A viruses, although HPAI A viruses have occasionally been detected. Notably, the case fatality rate (CFR) among human cases of avian influenza has ranged from 36%-60% overall, which is alarmingly high compared with all previous outbreaks of human cases of seasonal influenza in the United States, for which the CFR has ranged from 0.04%-1.0% [1,16,17]. This high level of illness severity and high mortality rate was unexpected and increased disease burden, resulting in concern among clinicians and public health officials; however, the risk factors that are most highly associated with the deaths from avian influenza were not clear.
On the basis of laboratory-confirmed deaths and the number of survivors, we examined human HPAI and LPAI infections in terms of the overall population, pediatric and clustered cases, with the aim of identifying the high-risk factors that...