What is Haemophilus influenzae (H. influenzae)?

From the Guidelines

Haemophilus influenzae is a gram-negative bacterium that can cause various infections, particularly in children and immunocompromised individuals, with encapsulated strains expressing one of six antigenically distinct capsular polysaccharides and non-typeable strains commonly causing mucosal infections. The bacterium can be classified into two main categories: encapsulated (typeable) and unencapsulated (nontypeable) strains, with six distinct capsular polysaccharides (types a, b, c, d, e, or f) in encapsulated strains 1. Some key points about Haemophilus influenzae include:

• It is a species of bacteria that colonizes the upper respiratory tract of humans and is transmitted person-to-person by inhalation of respiratory droplets or by direct contact with respiratory tract secretions 1.
• Encapsulated strains, particularly type b, can cause invasive disease such as meningitis, epiglottitis, pneumonia, septic arthritis, cellulitis, purulent pericarditis, and bacteremia, while non-typeable strains more commonly cause mucosal infections like otitis media, conjunctivitis, and sinusitis 1.
• Hib vaccination has dramatically reduced invasive disease, but non-typeable strains remain common causes of respiratory infections.
• Certain individuals, such as those with functional or anatomic asplenia, HIV infection, immunoglobulin deficiency, early component complement deficiency, or recipients of hematopoietic stem cell transplant or chemotherapy/radiation therapy for malignant neoplasms, are at increased risk for invasive Hib disease 1.
• The Advisory Committee on Immunization Practices (ACIP) recommends routine vaccination with a licensed conjugate Hib vaccine for infants and certain persons at increased risk for Hib disease, as well as antimicrobial chemoprophylaxis of contacts of persons with Hib disease 1.

From the Research

Definition and Characteristics of Haemophilus influenzae

• Haemophilus influenzae is a small, nonmotile, non-spore-forming bacterium, and a strict parasite of humans found principally in the upper respiratory tract 2.
• The production of capsule is of major significance to clinicians since it is an important virulence factor, with six antigenically distinct capsular types, designated a-f 2.
• Haemophilus influenzae can produce factors that inhibit the ciliary activity of human epithelial cells in vitro, including lipopolysaccharide and a heat-stable glycopeptide 2.

Types of Haemophilus influenzae Infections

• Type b strains are distinguished by the production of capsular polysaccharide and account for greater than 95 percent of systemic infections in children 2.
• Invasive infections, such as meningitis, septic arthritis, epiglottitis, and cellulitis, are usually caused by type b strains and occur in young children 2, 3.
• Less serious but numerically more common infections, such as otitis media and sinusitis, are usually caused by unencapsulated strains 2.
• Non-typeable Haemophilus influenzae (NTHi) commonly colonizes the upper respiratory tract and causes recurrent infections of the adjacent organs 4.

Transmission and Prevention

• Spread from one individual to another occurs by airborne droplets or by direct contagion with secretions 2.
• Household contacts of individuals who develop invasive Haemophilus influenzae type b disease are at higher risk of developing secondary Hib infection themselves 5.
• Rifampicin is highly effective in eradicating pharyngeal carriage of Hib and reducing the risk of invasive Hib disease among household and pre-school contacts 5.
• Vaccination is an effective way to prevent Haemophilus influenzae type b disease, and chemoprophylaxis should be offered to all room contacts in pre-school or primary school settings where more than one case occurs 5.

Treatment and Antibiotic Resistance

• Antibiotic therapy is only one facet of the management of the child with Haemophilus influenzae infection, and critical attention must also be given to supportive therapy 2.
• Cephalosporins are often chosen for treatment of adults with pneumonia when Haemophilus influenzae is documented 2.
• There is a rapid development of antibiotic resistance in NTHi strains, and therefore it is important to select appropriate antibiotics for treatment 4.
• Penicillin-binding protein 3 mutation is an important emerging mechanism of antibiotic resistance of NTHi 4.