Influenza, Seasonal

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)

Diagnosis

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

Imaging

• 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.

Treatment

• 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

Outcome

Complications

• 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