Meningitis

Abstract

Bacterial meningitis is an inflammation of the meninges, including the pia, arachnoid, and subarachnoid space, that occurs in response to infection with bacteria and/or bacterial products. Bacterial meningitis is a significant cause of mortality and morbidity worldwide, with considerable variation in incidence depending on age and geographic location of the patient and the causative agent. Young children are at highest risk for mortality and morbidity, especially those from lower socioeconomic strata in countries with poor medical infrastructure and those infected with Neisseria meningitidis (the meningococcus) or Streptococcus pneumoniae (the pneumococcus). Additional risk factors for poor prognosis after infection include the severity/stage of illness on presentation, exposure to an antibiotic-resistant organism, and the fact that medical professionals lack understanding of mechanisms underlying the pathological features of meningitis. When bacterial meningitis is suspected, immediate action is imperative to establish a definitive diagnosis, and antimicrobial treatment must be initiated immediately as a precautionary measure, because the mortality rate for untreated bacterial meningitis approaches 100 %; even with optimal treatment, mortality and morbidity remain high. Neurological sequelae are relatively common in meningitis survivors, especially if the agent of disease is a pneumococcal microorganism. Most pathogenic microbes could potentially cause meningitis in the human brain; however, only two pathogens, N. meningitidis and S. pneumoniae, account for most cases of acute bacterial meningitis, when patients in all age groups are considered. In contrast, in very young children and neonates, most cases are caused by group B streptococcus, Escherichia coli, and Listeria monocytogenes. In developing countries, Haemophilus influenza type b and Salmonella species are still major causes of meningitis in infants and young children. Salmonella meningitis has a particularly dismal prognosis. Meningitis is, in the majority of the cases, a consequence of a preceding bacteremia with encapsulated strains. Although the reasons for this association are incompletely understood, bacterial agents that cause meningitis tend to express surface structures mimicking structures and epitopes on human cells and a capsule with antiphagocytic properties that protect them from phagocytosis and normal immune surveillance. Thus, the absence of opsonic or bactericidal antibodies is considered a major risk factor for meningitis. In this regard, age-related incidence of meningococcal and pneumococcal disease is inversely related to prevalence of serum bactericidal activity. Successful identification of microbial epitopes that induce opsonic or bactericidal antibodies and successful vaccination of infants and children using antigenic agents based on these epitopes has changed the epidemiology of bacterial meningitis, particularly due to reduced incidence of Haemophilus influenzae type b-induced meningitis more so in industrialized countries. However, antigenic epitopes suitable for this preventive approach have not been identified in all organisms that cause meningitis with significant frequency today.