Updated: 7/29/2021

Influenza Viruses

Review Topic
  • Snapshot
    • A 26-year-old woman presents to the emergency room for 3 days of high fever, chills, muscle aches, a severe headache, and sore throat. She works in a nursing home but has not had her flu shot yet this year. She has a past medical history of systemic lupus erythematosus and hypothyroidism. A rapid strep test is negative, and a rapid antigen detection test for flu is positive. She is started on oseltamivir for this infection.
  • Introduction
    • Classification
      • influenza virus
        • an enveloped, segmented, negative-sense, single-stranded RNA virus with a helical capsid
        • an orthomyxovirus
        • transmission via respiratory droplets
    • Epidemiology
      • incidence
        • annual epidemics, often occurring in the winter
      • demographics
        • affects both adults and children, though with higher frequency in children
      • risk factors
        • advanced or young age
        • pregnancy
        • immunosuppression
        • morbid obesity
        • pulmonary disease
    • Etiology
      • influenza A and B are causes of seasonal epidemics
      • can originate from animal strains
        • avian flu A
        • swine flu A (H1N1)
    • Pathogenesis
      • surface protein hemagglutinin (H) and neuraminidase (N)
      • hemagglutinin binds to sialic acid and allows for viral entry into cells
        • target of the influenza vaccine
      • neuraminidase allows for progeny virion release from cells
    • Genetics
      • influenza viruses have 8 segments in the genome
      • genetic/antigenic shift
        • reassortment of viral genome segments, such as between human and swine strains, can cause pandemics
      • genetic/antigenic drift
        • random mutations in the H or N gene can cause epidemics
    • Prevention
      • annual flu vaccine for those 6 months or older
        • contains multiple killed viral strains that are thought to be likely to appear during flu season
          • intramuscular
        • live-attenuated vaccine
          • intranasal
    • Prognosis
      • may lead to severe bacterial superinfections
        • most commonly S. aureus, S. pneumoniae, and H. influenzae
      • may be fatal
  • Presentation
    • Symptoms
      • systemic symptoms predominate
        • high fevers and chills
        • myalgias and malaise
        • headaches
        • nausea and vomiting
      • may have cough or sore throat
    • Physical exam
      • may have cervical lymphadenopathy
  • Imaging
    • Chest radiography
      • indication
        • if pneumonia is suspected
      • findings
        • consolidation or infiltrate
        • ground-glass opacities
  • Studies
    • Labs
      • rapid antigen detection tests (RADTS)
        • most commonly used
      • detection of RNA on reverse transcriptase polymerase chain reaction
    • Making the diagnosis
      • based on clinical presentation and confirmed with laboratory studies
  • Differential
    • Upper respiratory infection
      • distinguishing factor
        • upper respiratory symptoms (e.g., cough, congestion, etc.) often predominates rather than systemic symptoms (e.g., high fevers, chills, malaise, etc.)
  • Treatment
    • Management approach
      • mainstay of treatment is supportive care
      • avoid aspirin in children
        • can cause Reye disease, leading to hepatorenal failure
    • Conservative
      • supportive care
        • indication
          • all patients
        • modalities
          • hydration
          • antipyretics
    • Medical
      • oseltamivir/zanamivir
        • indication
          • patients with severe disease or at high risk of severe disease
  • Complications
    • Secondary bacterial infection
    • Myositis or myocarditis
    • Central nervous system involvement

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