TAILIEUCHUNG - Chapter 180. Influenza

Influenza is an acute respiratory illness caused by infection with influenza viruses. The illness affects the upper and/or lower respiratory tract and is often accompanied by systemic signs and symptoms such as fever, headache, myalgia, and weakness. Outbreaks of illness of variable extent and severity occur nearly every winter. Such outbreaks result in significant morbidity in the general population and in increased mortality rates among certain high-risk patients, mainly as a result of pulmonary complications. Etiologic Agent Influenza viruses are members of the Orthomyxoviridae family, of which influenza A, B, and C viruses constitute three separate genera. . | Chapter 180. Influenza Definition Influenza is an acute respiratory illness caused by infection with influenza viruses. The illness affects the upper and or lower respiratory tract and is often accompanied by systemic signs and symptoms such as fever headache myalgia and weakness. Outbreaks of illness of variable extent and severity occur nearly every winter. Such outbreaks result in significant morbidity in the general population and in increased mortality rates among certain high-risk patients mainly as a result of pulmonary complications. Etiologic Agent Influenza viruses are members of the Orthomyxoviridae family of which influenza A B and C viruses constitute three separate genera. The designation of influenza viruses as type A B or C is based on antigenic characteristics of the nucleoprotein NP and matrix M protein antigens. Influenza A viruses are further subdivided subtyped on the basis of the surface hemagglutinin H and neuraminidase N antigens see below individual strains are designated according to the site of origin isolate number year of isolation and subtype for example influenza A Hiroshima 52 2005 H3N2 . Influenza A has 16 distinct H subtypes and 9 distinct N subtypes of which only H1 H2 H3 N1 and N2 have been associated with epidemics of disease in humans. Influenza B and C viruses are similarly designated but H and N antigens from these viruses do not receive subtype designations since intratypic variations in influenza B antigens are less extensive than those in influenza A viruses and may not occur with influenza C virus. Influenza A and B viruses are major human pathogens and the most extensively studied of the Orthomyxoviridae. Type A and type B viruses are morphologically similar. The virions are irregularly shaped spherical particles measure 80-120 nm in diameter and have a lipid envelope from the surface of which the H and N glycoproteins project Fig. 180-1 . The hemagglutinin is the site by which the virus binds to sialic acid cell .

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