Influenza Viruses

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