Haemophilus influenzae

H. influenzae is a small non-motile gram negative bacillus most commonly found in the upper respiratory tract. Its shape on Gram stain can vary from coccobacilli to filamentous rods. The name Haemophilus translates as ‘blood-loving’ and is derived from the organism’s growth requirement for both hemin and NAD (also known as X and V factor) that are acquired from red blood cells. Colonization of the upper respiratory tract with H. influenzae occurs in early childhood. 

Pathogenic Haemophilus influenzae strains are frequently mucoid & encapsulated, Encapsulated strains express one of six antigenically distinct capsular polysaccharides that are named types a, b, c, d, e, or f.

Before 1985, Haemophilus influenzae type b (Hib) was the leading cause of bacterial meningitis. It also caused other invasive diseases in children aged <5 years including epiglottitis, pneumonia, septic arthritis, cellulitis, pericarditis, and bacteremia. Fifteen to 30 percent of survivors had permanent hearing impairment or severe neurologic sequelae. Approximately 4% of cases were fatal. Serious invasive infections in young children due to serotype b have been largely eliminated due to use of conjugate Hib vaccine. Hib disease is uncommon in adults and in children aged >5 years. 

Encapsulated H. influenzae strains, other than type b, can cause invasive disease similar to Hib disease. Persons who are immunocompromised are considered at increased risk for invasive Hib disease including:

• Functional or anatomic asplenia
• HIV infection
• Immunoglobulin deficiency including immunoglobulin G2 subclass deficiency
• Early component complement deficiency
• Hematopoietic stem cell transplant
• Chemotherapy or radiation therapy for malignancy 

Non-encapsulated strains are not typeable. Unlike type b strains, which enter the blood stream, nontypeable strains cause disease by local invasion of mucosal surfaces. Hence, these strains are most frequently associated with conjunctivitis, otitis media, exacerbations of COPD, community-acquired pneumonia, and sinusitis. They can cause invasive disease. Neonatal and maternal sepsis occur less commonly but have an overall mortality rate of 50%. The causative nontypeable strain of these infections (biotype IV) is also associated with tubo-ovarian abscess and salpingitis.

Blood and body fluids can be tested for Haemophilus influenza. A Gram stain reveals pleomorphic gram-negative coccobacilli. Cultures of blood and body fluids confirm the presence of the organism. Its growth is potentiated on chocolate agar and BVCCA, which contains antibiotics (bacitracin, vancomycin, and clindamycin). Real time PCR can be performed for capsule serotyping. 

Cerebrospinal fluid can be tested by a multiplex meningitis/encephalitis pathogen panel, which includes detection of Haemophilus influenza. One example is the FilmArray Meningitis/Encephalitis Panel by BioFire Diagnostics. 

References

Van Eldere J, et al. Non-typeable Haemophilus influenzae, an under-recognised pathogen, The Lancet Infect Dis, 2014;14(12):1281-1292.

Khattak ZE, Anjum F. Haemophilus influenzae Infection. [Updated 2023 Apr 27]. In: StatPearls [Internet].