Haemophilus influenzae
Introduction
Haemophilus influenzae is a Gram-negative bacterium that was first identified during the 1892 influenza pandemic. Despite its name, it does not cause influenza, which is a viral infection. The bacterium is a common cause of localized respiratory tract and systemic diseases, particularly in children. It is also a significant cause of morbidity and mortality worldwide.
Classification and Structure
Haemophilus influenzae is a member of the Pasteurellaceae family. It is a small, pleomorphic, non-motile bacterium that can exist in both encapsulated (typeable) and unencapsulated (nontypeable) forms. The encapsulated strains are classified into six serotypes (a-f) based on their distinct polysaccharide capsules. The capsule is an important virulence factor, protecting the bacterium from phagocytosis by host immune cells.
Pathogenesis and Clinical Manifestations
Haemophilus influenzae primarily colonizes the human nasopharynx. From there, it can invade the bloodstream and disseminate to various body sites, leading to a range of clinical manifestations. The most severe forms of disease are caused by H. influenzae type b (Hib), which include meningitis, epiglottitis, pneumonia, arthritis, and cellulitis. Nontypeable strains are associated with respiratory tract infections, such as otitis media, sinusitis, and exacerbations of chronic obstructive pulmonary disease (COPD).
Epidemiology
Haemophilus influenzae is found worldwide and affects individuals of all ages, but children under 5 years of age and the elderly are particularly susceptible. Prior to the introduction of the Hib vaccine, H. influenzae type b was the leading cause of bacterial meningitis in children. The incidence of Hib disease has dramatically decreased in countries with routine Hib vaccination, but nontypeable H. influenzae continues to be a major cause of respiratory tract infections.
Diagnosis and Treatment
Diagnosis of Haemophilus influenzae infection is based on isolation of the bacterium from a normally sterile site (e.g., blood, cerebrospinal fluid). Identification and serotyping can be performed using biochemical tests, antigen detection, and molecular methods. Treatment typically involves antibiotics, with the choice of agent depending on the site of infection and local antibiotic resistance patterns. Vaccination against Hib is an effective preventive measure.
Prevention and Control
Prevention of Haemophilus influenzae disease is primarily achieved through vaccination. The Hib vaccine, which targets the type b polysaccharide capsule, has been highly effective in reducing the incidence of Hib disease. However, there is currently no vaccine for nontypeable H. influenzae. Control measures also include appropriate antibiotic use to prevent the emergence of antibiotic resistance.
Research and Future Directions
Research on Haemophilus influenzae continues to focus on understanding its pathogenesis, improving diagnostic methods, and developing new treatments and vaccines. Future directions include the development of a vaccine against nontypeable H. influenzae and strategies to combat antibiotic resistance.