Updated: 10/15/2018

Influenza Viruses

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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
    • 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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