Haemophilus influenzae

Haemophilus influenzae is a Gram-negative, non-motile, coccobacillary bacterium belonging to the family Pasteurellaceae. It is a facultatively anaerobic, capnophilic organism that grows optimally at temperatures between 35 and 37°C. First described in 1893 by Richard Pfeiffer during an influenza pandemic—hence its misleading species name—the organism was incorrectly identified as the causative agent of influenza. It possesses a thin peptidoglycan layer surrounded by an outer membrane containing lipopolysaccharide, and some strains are encapsulated with a polysaccharide capsule that enhances protection and colonization capacity.

H. influenzae is responsible for a wide range of clinical manifestations, from localized respiratory infections to severe invasive disease. The organism most commonly affects infants and children, presenting as pneumonia, meningitis, or bloodstream infections. Serotype B H. influenzae (Hib) was historically a major cause of meningitis in young children, frequently resulting in deafness and cognitive impairment. Treatment involves antibiotic therapy, though H. influenzae commonly exhibits resistance to penicillin-class antibiotics; amoxicillin/clavulanic acid may be used for mild cases. The introduction of the Hib vaccine in the 1980s has substantially altered the clinical epidemiology of this pathogen in developed countries.

H. influenzae infections occur worldwide, with the highest burden historically concentrated in infants and young children. Prior to vaccination, Hib was the predominant invasive strain and a leading cause of bacterial meningitis in children under five years of age. The six encapsulated serotypes (a through f) differ in their epidemiological prominence, with Hib being the most common in children, while types a, e, and f have been isolated infrequently and types d and c are rarely encountered. Unencapsulated strains exhibit greater genetic diversity and contribute to different clinical patterns. The implementation of routine childhood immunization with Hib conjugate vaccines has nearly eliminated Hib disease in developed nations, though surveillance remains important for monitoring vaccine impact and emerging resistance patterns.

Source-backed detail on the specific transmission routes of H. influenzae is not yet available in the provided source material.

Infants and young children represent the primary risk group for invasive H. influenzae disease, particularly before completion of the Hib vaccination series. Children under five years of age, especially those in the first two years of life, historically bore the greatest burden of Hib meningitis and other invasive infections. The impact of vaccination has substantially altered this risk profile in immunized populations.

Prevention of H. influenzae disease relies primarily on vaccination. The Hib conjugate vaccine, introduced in the 1980s, has proven highly effective in preventing Hib infections in the target pediatric population and has dramatically reduced the incidence of meningitis and other invasive diseases in countries with routine immunization programs. General infection control measures appropriate for respiratory pathogens likely contribute to reducing transmission, though specific public health guidance is not detailed in the available sources.

Surveillance for H. influenzae should distinguish between encapsulated serotypes (particularly Hib) and unencapsulated strains, as their epidemiological profiles and clinical significance differ substantially. The dramatic decline in Hib disease following vaccine introduction represents one of the most successful outcomes of conjugate vaccination programs, making Hib an important indicator of immunization program effectiveness. Monitoring for antibiotic resistance patterns is also warranted given the organism's documented resistance to penicillin-class antibiotics. Source-backed detail on current surveillance methodologies or case definitions is not yet available.

1. 1 Wikidata. Haemophilus influenzae.
2. 2 Wikipedia. Haemophilus influenzae - Wikipedia.

ICD-10

ICD-11

Total cases

39K

Peak month

2024-01

Coverage

3 reporting countries · 2000-01-01 → 2026-05-09