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Original Papers
Influenza
[dagger]. These authors contributed equally to this work.
INTRODUCTION
Influenza A virus is a highly infectious respiratory pathogen that can infect both humans and animals; it poses a public health threat every year. This virus is a member of the family Orthomyxoviridae and is further classified into subtypes based on characteristics of two surface glycoproteins: haemagglutinin (HA) and neuraminidase (NA). Eighteen HA (H1-H18) and 11 NA (N1-N11) subtypes have been identified that circulate in wild birds and bats [1, 2]. Of these subtypes, only H1N1, H2N2 and H3N2 have been known to establish stable lineages in humans. These subtypes have caused sustained epidemics in human populations since 1918 [3]. In addition, the H5, H6, H7, H9 and H10 subtypes have caused infections in humans since 1959 [4-7]. Sporadic infections resulting from these subtypes have occurred mainly as a result of direct viral transmission from infected birds to humans through direct and indirect contacts [8, 9]. Human-to-human transmission of these influenza viruses of avian origin has rarely occurred.
Investigation of the relationship between poultry exposure and avian influenza infections in human populations is important for understanding possible transmission of the disease at the poultry-human interface. Previous epidemiological and virological reports have proposed that individuals with intense occupational exposure, especially poultry-farm and live-market workers, may be at an increased risk for avian influenza infection because environmental exposure may promote the transmission of avian influenza viruses (AIVs) [10-12]. To date, it remains unclear whether subclinical infections with regionally predominating AIVs have occurred in these high-risk populations through direct or indirect poultry contact. In Taiwan, several subtypes of AIVs, including H5N2, H6N1 and H7N3, have been identified in domestic poultry [13-15]. During the past decade, the H5N2 virus, which has low pathogenicity, has become the predominant infectious agent in chickens. Outbreaks caused by this virus were reported in 2003-2004 and 2008-2014 [16]. The highly pathogenic avian influenza (HPAI) A(H5N2) virus was first isolated in Taiwan in 2012; since then, this virus has caused subsequent outbreaks in several poultry farms [17]. Avian influenza A(H6N1) virus is frequently isolated from Taiwanese layers and broilers. It usually presents as a low pathogenic virus and continuously circulates as an endemic enzootic agent in animals....