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Key Features

Essentials of Diagnosis

  • Fever, cough, pharyngitis, malaise, congestion

  • Pneumonia

  • Encephalitis

  • Seasonal: late fall through mid-spring

  • Detection of virus, viral antigens, or nucleic acid in respiratory secretions

General Considerations

  • Infection rates in children are greater than in adults because they lack immunologic experience with influenza viruses

  • Epidemics occur in fall and winter

  • Transmission occurs via airborne respiratory secretions

  • Incubation period is 2–7 days

  • Three main types of influenza viruses (A/H1N1, A/H3N2, B) cause most human epidemics

Clinical Findings

Symptoms and Signs

  • produces a characteristic syndrome of sudden onset of high fever, severe myalgia, headache, and chills

  • Coryza, pharyngitis, and cough are also present

  • Fever, diarrhea, vomiting, and abdominal pain are common in young children

  • Sepsis-like illness and apnea may develop in infants.

  • Chest examination is usually unremarkable

  • Unusual clinical findings include

    • Croup (most severe with type A influenza)

    • Exacerbation of asthma

    • Myositis (especially calf muscles)

    • Myocarditis

    • Parotitis

    • Encephalopathy (distinct from Reye syndrome)

    • Nephritis

    • Transient maculopapular rash

  • Acute illness lasts 2–5 days

Differential Diagnosis

  • All other respiratory viruses

  • Mycoplasma pneumoniae or Chlamydia pneumoniae (longer incubation period, prolonged illness)

  • Streptococcal pharyngitis (pharyngeal exudate or petechiae, adenitis, no cough)

  • Bacterial sepsis (petechial or purpuric rash may occur)

  • Toxic shock syndrome (rash, hypotension)

  • Rickettsial infections (rash, different season, insect exposure)


Laboratory Findings

  • Leukocyte count is normal to low, with variable shift

  • Virus can be found in respiratory secretions by

    • Direct fluorescent antibody staining of nasopharyngeal epithelial cells

    • ELISA

    • Optic immunoassay (OIA)

    • Polymerase chain reaction (PCR)

      • Has the highest sensitivity and specificity, close to 100%

      • Becoming the preferred test


  • Chest radiograph is nonspecific

    • May show hyperaeration, peribronchial thickening, diffuse interstitial infiltrates, or bronchopneumonia in severe cases

    • Hilar nodes are not enlarged

    • Pleural effusion is rare in uncomplicated influenza.


  • Supportive

  • Antivirals are beneficial in immunocompetent patients if given within 48 hours after symptom onset

  • Treatment duration is 5 days and the doses are twice those used for prophylaxis

  • Peramivir, a neuraminidase inhibitor that is available for intravenous administration, is approved for treatment



  • Lower respiratory tract symptoms are most common in children younger than age 5 years

  • Hospitalization rates are highest in children younger than 2 years

  • Influenza can cause croup in these children

  • Secondary bacterial infections (classically staphylococcal) of the middle ear, sinuses, or lungs are common

  • Influenza can also cause viral or postviral encephalitis, with cerebral symptoms much more prominent than those of the accompanying respiratory infection

  • Although the myositis is usually mild and resolves promptly, severe rhabdomyolysis and renal failure have been reported

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