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Haemophilus influenzae

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Topic updated on 03/14/17 2:51pm

Introduction
  • Classification
    • Bacteria
      • other Gram-negative
        • Haemophilus
          • H. influenzae
  • Pathogenesis
    • transmission
      • respiratory droplets
    • predisposing factors for infection 
      • asplenia
        • H. influenza is one of the "SHiN" encapsulated bacteria that poses particular risk to asplenic patients (see Encapsulated Bacteria )
    • reservoir
      • nasopharynx
        • spreads nasopharynx -> lymphatics -> meninges in case of meningitis
    • molecular biology
      • encapsulated
        • capsular type B causes most severe disease
          • type B contains ribose in place of hexose   
          •  type B capsule is polyribosyl-ribitol-phosphate
      • non-encapsulated strains exist ("nontypable")
        • cause otitis media, sinusitis, and bronchitis
        • vaccine does not confer immunity to nontypable strains
      • produces IgA protease
        • aids colonization
Diseases
  • Haemophilus causes "EMOP"
    • epiglottitis
      • "cherry red" in children
      • formerly the most common cause
    • meningitis
      • in unvaccinated children 3 months to 2 years
      • formerly the most common cause in 1-5 year old children (until 1990)
    • otitis media
    • pneumonia (adults most commonly)
    • bronchitis
  • Does not cause the flu!
    • influenza virus does
Laboratory
  • Coccobacillary rods
  • Fastidious
  • Culture on chocolate agar 
    • requires factors V (NAD+) and X (hematin) for growth 
  • Satellite phenomenon
    • grows near S. aureus on blood agar
      • S. aureus supplies factor V
      • S. aureus lyses RBCs, releasing factor X
Treatment
  • Drugs
    • ceftriaxone for meningitis
    • rifampin prophylaxis for close contacts
  • Vaccine
    • contains type B capsular polysaccharide (polyribosyl-ribitol-phosphate - PRP) conjugated to protein (often diphtheria toxoid)
      • conjugation improves immune system recognition of polysaccharide and promotes class switching
      • confers immunity only to type B
    • given between 2 and 18 months of age
    • T-cell dependent


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