Avian influenza overview
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]; Associate Editor(s)-in-Chief: Yazan Daaboul, M.D., Synonyms and keywords:, Gerald Chi, M.D.
Overview
Historical Perspective
Avian influenza was first described by Perroncito in 1878 in northern Italy following an outbreak of contagious disease of poultry. In 1918, the avian-descended influenza A H1N1 caused the first major human influenza pandemic. The first avian influenza A H5N1 virus infection in humans was described in 1997 in Hong Kong, where 18 cases were documented (including 6 deaths). The first human-to-human transmission of avian influenza infection was described in 2003 during the outbreaks in Southeast and Central Asia.
Pathophysiology
Classification
To date, only influenza type A has been associated with avian influenza. Neither influenza B nor influenza C is associated with avian influenza. Classification of avian influenza may be based on either the pathogenicity of the virus (low pathogenicity vs. high pathogenicity) or the viral genetic subtypes (H5 vs. H7 vs. H9).
Cause
Avian influenza is caused by influenza A virus. Neither influenza B nor influenza C causes avian influenza. Influenza belongs to the Orthomyxoviridae family. Influenza is an enveloped, pleomorphic (spherical and filamentous forms) virus that contains a linear, segmented (8 segments), negative-sense, single-stranded RNA genome. The genome is composed of 8 segmented genes that encode 11 proteins.
Differential Diagnosis
Avian influenza should be differentiated from the following diseases or pathogens that cause upper or lower respiratory tract infection or flu-like illness, such as other influenza viruses, such as human or swine influenza, other viral, bacterial, fungal, and parasitic agents that are typically associated with nasopharyngeal and respiratory tract infections, and non-infectious causes, such as asthma, chronic obstructive pulmonary disease (COPD), drug adverse effects, and cardiac causes.
Epidemiology and Demographics
The World Health Organization (WHO) reports an incidence of 3 to 5 million cases of severe influenza disease annually, including 250,000 to 500,000 deaths every year. The case fatality rate per outbreak is highly variable and may range from less than 1 to more than 200 per 100,000 cases. Influenza may infect patients of all age groups, but elderly patients > 65 years, young children (especially patients < 2 years of age), and adolescents are at high risk of developing complications and death. There is no racial or gender predilection for avian influenza infection.
Risk Factors
The majority of cases of avian influenza infection in humans have resulted from contact with infected poultry, or contaminated environments. Human to human transmission is still rare and inefficient.