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Definitions and Epidemiology
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Meningitis is defined as an inflammation of the leptomeninges by any cause. Bacteria cause meningitis by invading and replicating in the subarachnoid space and cause significant morbidity and mortality. Viral infections may also cause meningitis, mostly commonly enteroviruses, but few children with viral meningitis suffer any long-term sequelae. Therefore the focus of this chapter will be on bacterial meningitis. Figure 16-1 gives the age, organism, and specific rates of bacterial meningitis in the United States prior to the introduction of currently used conjugate vaccines (note the y-axis is a log scale). As can be seen, the greatest risk period for bacterial meningitis is in the first 6 months of life.
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Overall, there has been a remarkable decline in the rate of bacterial meningitis in the developed world over the last 2 decades with the introduction of the Haemophilus influenzae type b conjugate vaccines, the Streptococcus pneumoniae conjugate vaccines, and greater use of meningococcal vaccines. Haemophilus influenzae type b was once the leading cause of bacterial meningitis in children but has been virtually eliminated in countries utilizing the conjugate vaccine. In the first 2 months of life, Enterobacteriaceae (eg, E. coli, Klebsiella species), group B streptococci, and occasionally Listeria monocytogenes, Salmonella species, or enterococci will cause bacterial meningitis. Infections due to S pneumoniae occur with increasing in frequency over the second month to become the most likely cause of bacterial meningitis, and continue to increase in frequency until 4 or 5 months of age when they begin to decline. Neisseria meningitidis is the most common cause of bacterial meningitis by 1 year of age. S pneumoniae remains the second most common cause after 1 year of age, and all other pathogens trail behind considerably. These two pathogens occur more commonly in the winter months, presumably in association with common respiratory viruses that disrupt mucosal barriers, thereby allowing these colonizing pathogens to move from the nasopharynx to the bloodstream more easily. Research is ongoing to develop vaccines that will be effective against a greater number of pneumococcal serotypes and improved meningococcal vaccines that may work for younger children and against group B strains. Table 16-1 reviews microbial causes of meningitis.
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