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Meningitis, an infection of the subarachnoid space and leptomeninges caused by a variety of pathogenic organisms, continues to be an important source of mortality and morbidity. Despite the introduction of new vaccines that prevent the most severe causes, bacterial, or purulent, meningitis remains the most important form in the United States in terms of incidence, sequelae, and ultimate loss of productive life. Aseptic meningitis, usually caused by viruses, especially enteroviruses (see Chapter 306) is more common; however, significant sequelae are uncommon and the disease is usually self-limited. Granulomatous meningitis, caused either by M tuberculosis or fungi, is a major cause of neurologic injury and death in the developing world (See Chapter 269).

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Epidemiology

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The first month after birth represents the period of highest attack rate for meningitis, with likely pathogens including group B streptococci (Streptococcus agalactiae), Escherichia coli, other gram-negative enteric organisms, and less commonly, Listeria monocytogenes (Table 231-1). Beyond the neonatal period, the most important pathogens are Streptococcus pneumoniae and Neisseria meningitidis. Formerly, Haemophilus influenzae type b (Hib) was the most common pathogen causing meningitis in infants and children, but the incidence has been reduced substantially by immunization with conjugate vaccines in developed countries.1,2 Recent studies of conjugate pneumococcal vaccine, introduced in the United States in 2000, demonstrate that it is effective in preventing pneumococcal meningitis.3 Similarly, the meningococcal polysaccharide vaccine introduced in the early 1970s has been effective in reducing meningococcal meningitis. Thus, the incidence of infection and prevailing predominant causative organisms varies depending upon the immunization status of the population.

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Table 231-1. Bacterial Causes of Meningitis
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Bacterial meningitis is reported with increased frequency among African Americans, Native Americans, and individuals in rural areas. It is unclear whether environmental or genetic factors are responsible for enhanced susceptibility. Seasonal patterns of disease have been noted to occur, with meningitis caused by N meningitidis and S pneumoniae peaking in the winter months, Hib showing a biphasic distribution with peaks in the early winter and spring, and L monocytogenes occurring most frequently in the summer months. In the “meningitis belt” of sub-Saharan Africa, the shift from the wet to the dry season is associated with an increase in meningitis. These patterns are likely due to the modes for acquiring the organisms, with N meningitidis, S pneumoniae, and Hib spread by the respiratory route, in months with increased ...

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