Haemophilus influenzae

• Risk Factors:
  • Unvaccinated children (missed Hib conjugate series).
  • Asplenia (SCD, post-splenectomy) -> impaired clearance of encapsulated organisms.
  • Complement deficiency, hypogammaglobulinemia, elderly, chronic lung disease (COPD).

Microbiology

• Nontypeable (non-encapsulated) strains are less invasive and cause mucosal infections. (e.g., otitis media, sinusitis, pneumonia).
• The type b strain (encapsulated) can cause more severe infections (e.g., meningitis, epiglottitis), but the incidence of these infections has decreased with routine vaccination.
• The Hib vaccine only has effect on type b strain.
• Growth Requirements:
  • Grows on Chocolate Agar (lysed RBCs).
  • Requires Factor V (NAD+) and Factor X (Hematin).
  • Satellite Phenomenon: Grows on blood agar (complete RBC) only when surrounding colonies of S. aureus (which lyses RBCs to release Factor X and secretes Factor V). t

Virulence factors

• IgA protease
• Capsular polysaccharides (polyribosylribitol phosphate)
  • Contained in Hib vaccine, conjugated to a protein (e.g., diphtheria toxoid)
• Infections are usually caused by unencapsulated strains (vaccine protects against encapsulated H. influenzae type B)

Clinical Features

• Encapsulated (Hib):
  • Epiglottitis: High fever, 3 Ds (Dysphagia, Drooling, Distress), tripod position, muffled “hot potato” voice, inspiratory stridor.
  • Meningitis: Fever, headache, nuchal rigidity, altered mental status, bulging fontanelle (infants).
  • Cellulitis: Rapidly progressive, violaceous/bluish discoloration on cheeks/periorbital region in young infants.
  • Septic Arthritis: Monoarticular (knee/hip), fever, inability to weight-bear.
• Nontypeable (NTHi):
  • Acute Otitis Media (AOM): Otalgia, bulging TM, fever (often concurrent w/ conjunctivitis: otitis-conjunctivitis syndrome).
  • Sinusitis / Bronchitis / COPD Exacerbations: Purulent sputum, dyspnea, wheezing.