Haemophilus species


  • Small aerobic Gram-negative coccobacillus [Fig] found mainly in the respiratory tract.
    • Fastidious organisms, factors X (hemin, haemophilius from the Latin "blood loving") and V (nicotinamide-adenine-dinucleotide) are required for growth on chocolate agar, the preferred media.
  • Haemophilus influenzae: two categories causing human colonization or infection.
    • Typeable strains: six types generally recognized (Types a-f).
      • Encapsulated, type B strain (Hib, with capsular antiphagocytosis and anticomplement virulence factors) historically accounts for most invasive infections and bacteremic pneumonia. Now rarely encountered due to childhood Hib vaccine success.
      • H. influenzae serotype a (Hia) is increasingly noted as a cause of severe, invasive infection. Emergence due to intrinsic capsular virulence factors of Hia and pressure from the success of Hib immunization.
        • Predominantly noted in U.S. and Canada. Incidence in Europe is low.
        • In one series, it accounted for ~30% of invasive H. influenzae infections in Manitoba.
      • Type f strain is also increasingly recognized as a cause of severe infection, though less common than Hia.
    • Non-typeable strains: less invasive and cause more otitis media, epiglottitis, acute exacerbations of chronic bronchitis (AECB), sinusitis and nonbacteremic pneumonia.
      • Increasingly recognized as a cause of URTIs, pneumonia and occasional bacteremia in all age groups.
      • High colonization rates in children, up to 70%.
      • Recent data suggest that non-typeable strains (along with Hia) account for more significant percentages of invasive Haemophilus disease[8].
  • Other species: all are part of the normal flora of human oral and pharyngeal cavities but may also be causes of bacterial respiratory tract infections and endocarditis, similar to H. influenzae.
  • H. ducreyi, agent of chancroid, see the separate module.

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Last updated: January 13, 2023