What is the role of colonization in 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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Answer

To become infected with Haemophilus, individuals must first acquire a state of nasopharyngeal colonization, a fairly common event of early life. Acquisition of Haemophilus probably occurs by inhalation of aerosolized respiratory droplets, although nose-finger-finger-nose routes may play a role in person-to-person transmission.

Haemophilus colonization occurs mostly with unencapsulated strains; encapsulated strains are only rarely detected. In North America, 2-5% of young children are colonized with Hib, the most important cause of Haemophilus infections. Hib colonization is much less frequently found in adults and in children younger than 1 year.

Rates of carriage are even lower in immunized populations. However, rates of carriage are much higher among household contacts of an index case. Twenty to 25% of all those exposed to the index case become colonized. Among children younger than 6 years who are exposed, carriage rates are as high as 50%. Interestingly, the rate of nasopharyngeal colonization is lower in household contacts of a child with Hib epiglottitis than those of a child with Hib meningitis.

Carriage is generally asymptomatic and may occur despite circulating antibodies or effective eradication of meningitis. It may persist for weeks to months. Over time, colonization rates decline, dropping to below 1% by adulthood.

Hib infection in a colonized individual may be either invasive or noninvasive. Epiglottitis is an example of noninvasive infection that occurs in the upper airways of susceptible individuals. Invasive infection requires that Haemophilus organisms from the nasopharyngeal colony become locally invasive and enter the bloodstream, resulting in bacteremia. The mechanisms of this invasiveness are not as yet understood, but both bacterial and host factors are likely involved (see below).


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