Haemophilus

ORGANISM:

• Genus: Haemophilus
• Species: influenzae

GENERAL CONCEPTS:

• Haemophilus influenzae is responsible for producing a variety of infections including meningitis and respiratory infections.
• Six serological types (a,b,c,d,e,f) based on the antigenic structure of the capsular polysaccharides are recognized. Nonencapsulated strains are (by definition) nontypable.
• Other species of Haemophilus include: H. parainfluenzae (pneumonia, endocarditis), H. ducreyi (venereal chancre) and H. aegypticus (conjunctivitis).

DISTINCTIVE PROPERTIES:

• The genus Haemophilus is composed of Gram-negative coccobacilli.
• These organisms are fastidious and require factors X (hemin) and/or V (NAD).
• Haemophilus possess LPS in the cell wall but produce no apparent extracellular toxins.

PATHOGENESIS:

• The organisms colonize the nasopharynx and are spread by direct contact. Haemophilus are capable of penetrating the epithelium to produce a bacteremia that may lead to localization of the organisms in many organs. The capsule is Haemophilus' major virulence determinant yet unencapsulated strains produce ear, sinus and respiratory infections.
• H. influenzae type b is the most common cause of bacterial meningitis in children aged 6 months-2 years. It is uncommon in adults because of protective antibody.
• Cellulitis, conjunctivitis, epiglottitis and arthritis may also result from Haemophilus infection.
• For pneumonia in adult men, unencapsulated H. influenzae is second only to the pneumococcus (S. pneumoniae). Those affected are usually chronic smokers, alcoholics or elderly.

EPIDEMIOLOGY:

Spread of Haemophilus is human to human. Day care centers are common sites for transmission from healthy, unaffected adults to susceptible infants.

DIAGNOSIS:

• Clinical: A Gram stain of cerebrospinal fluid may reveal the organisms. One can also detect capsular material directly.
• Laboratory: The organisms are cultured on chocolate agar because it contains both factors X and V. Incubation in 10% CO₂ is required.

CONTROL:

• Sanitary: Avoidance of carriers is not always possible.
• Immunological: A vaccine against type b is available. Unfortunately, the vaccine is nonimmunogenic in infants where it is needed the most.
• Chemotherapeutic: Third generation cephalosporins are probably the drugs of choice because of their ability to cross the blood-brain barrier and their bactericidal activity.