Haemophilus influenzae is a pleomorphic gram-negative bacterium that is either encapsulated (typeable) or unencapsulated (nontypeable). Six antigenically distinct capsulated strains (types a-f) can cause invasive disease among persons of all age groups. In contrast, nontypeable strains only occasionally cause bacteremia and invasive infections in adults, particularly the elderly and those with underlying cardiopulmonary and/or immunocompromised conditions.
Humans are the only known hosts of H. influenzae. Person-to-person transmission occurs by direct contact with secretions or with airborne droplet particles. Prior to the advent of effective vaccines, H. influenzae type b (Hib) was responsible for >95% of all invasive H. influenzae disease and was the leading cause of bacterial meningitis in the United States in children younger than 5 years of age. From 1989 to 2000, there was a strong correlation between the introduction and implementation of Hib-conjugated vaccines in infants and young children and the striking (99%) decline in the incidence of Hib invasive disease among children younger than 5.
Clinical manifestations of invasive diseases associated with H. influenzae reflect the organ systems affected by the infection; for example:
- Meningitis can be preceded by an upper respiratory tract infection and presents with fever, nuchal rigidity, and/or altered mental function.
- Invasive disease can also present initially as a localized infection, such as cellulitis, with erythema, induration, pain, and tenderness and progress to bacteremia and fulminant sepsis and hypotension.
- Sequelae of bacteremia and disseminated invasive disease include septic arthritis, osteomyelitis, and/or pericarditis.
- One of the most significant presentations of invasive H. influenzae disease is epiglottitis, which requires prompt diagnosis and management because of its fulminant onset and high rate of complications.
Pediatric Patient Considerations:
Epiglottitis usually occurs in a child aged 2-7 years.
Immunocompromised Patient Considerations:
Hib is the second most common organism associated with infections in post-splenectomy patients. Patients with underlying diseases that cause functional asplenia, eg, sickle cell anemia, are also at increased risk of invasive H. influenzae infection.