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Type b Haemophilus influenzae can cause meningitis, epiglottitis, bacteremia, and cellulitis. Nontypable H influenzae can cause otitias media, sinusitis, tracheobronchitis, and pneumonia. Other Haemophilus species and the syndromes they cause include H parainfluenzae (pneumonia and endocarditis), H ducreyi (genital chancre), and H aegyptius (conjunctivitis or Brazilian purpuric fever).
Haemophilus species are Gram-negative coccobacilli similar in ultrastructural features to other pathogenic bacilli. Haemophilus influenzae requires hemin (factor X) and NAD+ (factor V) for growth. Other Haemophilus species require only NAD+ and therefore grow on blood agar. Typable H influenzae isolates are classified on the basis of seven antigenically distinct capsular polysaccharides; isolates lacking these polysaccharides are called nontypable.
Type b H influenzae colonizes the nasopharynx, and may penetrate the epithelium and capillary endothelium to cause bacteremia. Meningitis may result from direct spread via lymphatic drainage or from hematogenous spread. Nontypable H influenzae colonizes the nasopharynx and, to a lesser extent, the trachea and bronchi and may infect mucosa damaged by viral disease or cigarette smoking. Lipooligosaccharide is largely responsible for inflammation; exotoxins do not play a role.
Serum antibody to the capsule (in the case of typable H influenzae) or to somatic antigens is bactericidal and promotes phagocytosis.
Haemophilus influenzae colonizes healthy children and adults (although the rate of colonization is far greater for nontypable than for type b H influenzae) and is spread by direct contact, secretions, and/or aerosol. Haemophilus ducreyi is spread by venereal contact. There is no animal reservoir for these organisms.
Respiratory secretions and cerebrospinal fluid must be cultured on chocolate agar. Blood cultures are positive in meningitis. Capsular antigen may be detected in cerebrospinal fluid for early identification if Gram stain is unsuccessful. Haemophilus ducreyi grows on Mueller-Hinton agar with 5 percent sheep blood in a CO2 enriched atmosphere.
Recommended treatment includes ampicillin for strains of H influenzae that do not make β-lactamase and a third generation cephalosporin or chloramphenicol for strains that do. Ampicillin or amoxicillin together with a substance, such as clavulanic acid, that blocks the activity of β-lactamase is also effective, but does not reliably treat meningitis. Tetracyclines remain effective in treating sinusitis or respiratory infection proven to be due to nontypable H influenzae. Use of polyribosyl ribitol phosphate (PRP) vaccine and, more recently, protein-conjugated PRP has vastly reduced the frequency of infection due to type b H influenzae.
