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The following text is from an archived Red Book® edition and may not reflect current recommendations or information. To view the current edition, click here.
Section 3. Summaries of Infectious Diseases
Influenza
Clinical Manifestations
Etiology
Epidemiology
Diagnostic Tests
Treatment
Isolation of the Hospitalized Patient
Control Measures
CLINICAL MANIFESTATIONS: Influenza classically is characterized by sudden onset of fever, often with chills or rigors, headache, malaise, diffuse myalgia, and a nonproductive cough. Subsequently, the respiratory tract signs of sore throat, nasal congestion, rhinitis, and cough become more prominent. Conjunctival injection, abdominal pain, nausea, and vomiting can occur. In some children, influenza can appear as an upper respiratory tract infection or as a febrile illness with few respiratory tract signs. In young infants, influenza can produce a sepsis-like picture and occasionally can cause croup, bronchiolitis, or pneumonia. Acute myositis characterized by calf tenderness and refusal to walk may develop after several days of influenza illness, particularly with type B infection. Reye syndrome has been associated with influenza infection, primarily with influenza B. Influenza can alter the metabolism of certain medications, especially theophylline, potentially resulting in the development of toxic effects from high serum concentrations.
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ETIOLOGY: Influenza viruses are orthomyxoviruses of 3 antigenic types (A, B, and C). Epidemic disease is caused by influenza virus types A and B. Influenza A viruses are subclassified by 2 surface antigens, hemagglutinin (HA) and neuraminidase (NA). Viruses bearing 3 immunologically distinct hemagglutinin subtypes (H1, H2, and H3) and 2 neuraminidase subtypes (N1 and N2) have been recognized as causing global human epidemics. Specific antibodies to these various antigens, especially hemagglutinin, are important determinants of immunity. Major changes leading to the emergence of a new hemagglutinin, such as H1 to H2, or emergence of a new hemagglutinin or new neuraminidase, are called antigenic shifts; minor antigenic variations within the same subtypes are called antigenic drifts. Antigenic shift has occurred only with influenza A, usually at irregular intervals of
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