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Influenza is usually a mild illness of the upper respiratory tract. Involvement of the lower respiratory tract, however, can lead to 4 types of clinical syndromes; physical signs of lower respiratory tract involvement without roentgenographic evidence of pneumonia, influenza complicated by bacterial pneumonia, primary influenza virus pneumonia, and combined influenzal and bacterial pneumonias.
Influenza virus infections are most severe in patients with certain preexisting conditions such as rheumatic heart disease, bronchopulmonary disease, impaired renal function, and diabetes mellitus. Infections can be more severe in elderly patients, pregnant females, and immunocompromised patients.
Influenza virus type B generally produces less severe disease than type A. Outbreaks of influenza type B virus are usually more localized than type A. Both infections occur in the United States between November and March.
Influenza A is susceptible to antiviral activity of amantadine while influenza B is not inhibited by this drug.
Diagnosis of recent infection by influenza virus type B when isolation of the organism by culture is unsuccessful
The presence of IgM class antibody or a 4-fold or greater rise in titer in paired (acute and convalescent) sera indicates recent infection.
The presence of IgG class antibody alone generally indicates past exposure.
Influenza virus infections occur exclusively from November through March.
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