What is the pathophysiology of Haemophilus influenzae type b (Hib) meningitis?

Updated: Jul 09, 2018
  • Author: Prateek Lohia, MD, MHA; Chief Editor: Niranjan N Singh, MBBS, MD, DM, FAHS, FAANEM  more...
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Haemophilus species are small oxidase-positive pleomorphic gram-negative aerobic or facultative anaerobic coccobacilli. Humans are the only known host for Haemophilus influenzae.

Haemophilus strains may be encapsulated or unencapsulated. Encapsulated strains (also known as typeable) are surrounded by a polysaccharide capsule that plays an important role in the determination of virulence of the organism. The outer membrane lipo-oligosaccharides also contribute to the degree of virulence. Encapsulated strains are divided, on the basis of capsular antigens, into 6 serotypes designated a through f. Unencapsulated strains lack the polysaccharide capsule and are designated untypable strains.

Transmissibility of H influenzae infection and the capacity of this organism to cause purulent meningitis were first demonstrated by Wollstein in 1911. She also drew attention to the marked tendency for Haemophilus meningitis to occur in infants and young children. In 1931, Pittman demonstrated that H influenzae type b (Hib) accounted for almost all cases of meningitis.

In an important series of studies published in 1933, Fothergill and Wright enlarged the epidemiologic understanding of Hib meningitis, the protective role of passively transmitted maternal antibodies, and the inadequacy of host immune response from infancy to age 3 years. They demonstrated that this maternally conferred protection largely dissipates by age 4 months.

Between 46 and 60% of all serious Hib-related diseases present as meningitis. Hib’s medical importance has included the role that it has played in the experimental and pathological study of infectious diseases in a wide variety of organ systems. The bacterium has provided particularly valuable information concerning the understanding of the pathophysiology of meningitis.

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