Influenza

Influenza

 

Influenza is an acute viral infection that occurs in outbreaks of varying severity throughout the winter.

Epidemiology

  • Incidence – >1/1,000
  • Age
    • Attack rates highest in young children
    • Mortality highest in elderly
  • Transmission – respiratory droplet, airborne route

Organism

  • Single-stranded RNA virus
  • Family – Orthomyxovindae
  • Three types based on antigenic differences – A, B, C
    • Only A undergoes antigenic shift
    • Antigenic shift is responsible for pandemics
  • Classified by hemagglutinin (HA)/neuramidase (NA) activity
    • Named with H and N components: H1-15 and N1-9

Risk Factors

  • Chronic pulmonary disease – especially asthma
  • Chronic renal, cardiovascular or metabolic diseases
  • Elderly
  • Immunocompromised state
  • Pregnancy

Clinical Presentation

  • Abrupt onset of fever, chills, myalgias and malaise
  • Severe retroorbital headache, sore throat
  • Cough and upper respiratory infection signs and symptoms occur 3-4 days after initial onset
  • Complications
    • Primary viral influenzal pneumonia
    • Secondary bacterial pneumonia
    • Myositis
    • Myocarditis, pericarditis
    • Guillain Barré, transverse myelitis, encephalitis
    • Reye syndrome

Treatment

  • Current antivirals given within 48 hours of symptoms may lessen the severity of the course
    • Neuraminidase inhibitors are used for treatment and prophylaxis against Influenza A and B
    • Most strains of Influenza A are considered resistant to treatment with amantadine sulfate (>95%)
  • Supportive

Prevention

  • Yearly vaccination
    • Inactivated – intramuscular
    • Live attenuated – intranasal
      • Contraindicated in settings of immunocompromised persons or households with persons who are immunocompromised
    • Antiretroviral prophylaxis with neuraminidase inhibitors when vaccination is contraindicated

Click here for CDC Seasonal Influenza Vaccination Resources for Health Professionals