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Respiratory illnesses are responsible for more than half of all acute illnesses each year in the United States. The Orthomyxoviridae (influenza viruses) are a major determinant of morbidity and mortality caused by respiratory disease, and outbreaks of infection sometimes occur in worldwide epidemics. Influenza has been responsible for millions of deaths worldwide. Mutability and high frequency of genetic reassortment and resultant antigenic changes in the viral surface glycoproteins make influenza viruses formidable challenges for control efforts. Influenza type A is antigenically highly variable and is responsible for most cases of epidemic influenza. Influenza type B may exhibit antigenic changes and sometimes causes epidemics. Influenza type C is antigenically stable and causes only mild illness in immunocompetent individuals.

Three immunologic types of influenza viruses are known, designated A, B, and C. Whereas antigenic changes continually occur within the type A group of influenza viruses and to a lesser degree in the type B group, type C appears to be antigenically stable. Influenza A strains are also known for aquatic birds, chickens, ducks, pigs, horses, and seals. Some of the strains isolated from animals are antigenically similar to strains circulating in the human population.

The following descriptions are based on influenza virus type A, the best-characterized type (Table 39-1).

Table 39-1 Important Properties of Orthomyxovirusesa

Structure and Composition

Influenza virus particles are usually spherical and about 100 nm in diameter (80–120 nm), although virions may display great variation in size (Figure 39-1).

Figure 39-1

Influenza virus. A: Electron micrograph of influenza virus A/Hong Kong/1/68(H3N2). Note the pleomorphic shapes and glycoprotein projections covering particle surfaces (315,000×). (Courtesy of FA Murphy and EL Palmer.) B: Schematic view of influenza. Virus particles have segmented genomes consisting of seven or eight different RNA molecules, each coated by capsid proteins and forming helical nucleocapsids. Viral glycoproteins (hemagglutinin and neuraminidase) protrude as spikes through the lipid envelope. (Reproduced with permission from Willey JM, Sherwood LM, Woolverton CJ: Prescott, Harley, and Klein's Microbiology, 7th ed. McGraw Hill, 2008. © The McGraw-Hill Companies, Inc.)

The single-stranded, negative-sense RNA genomes of influenza A and B viruses occur as eight separate segments; influenza ...

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