Meningococcal Meningitis

Meningitis is defined as an inflammation of the meninges, or the membranes surrounding the brain. It is typically caused by a bacterial or viral pathogen. The bacterium Neisseria meningitidis (a Gram negative diplococcus), commonly referred to as meningococcus, is a common and dangerous cause of meningitis in humans. Numerous strains, or serogroups, of meningococcus have been identified, defined by the different sugar structures of their capsules. Outbreaks of meningococcal meningitis in most of the world are found among groups living in close quarters such as college students in dormitories, army recruits, and pilgrims. In sub-Saharan Africa, the so-called “meningitis belt” experiences thousands of fatalities from meningococcal meningitis each year, with larger epidemics every 5-12 years. However, the organism is more commonly found colonizing the nasopharynx in asymptomatic human carriers.

Meningitis has also been reported in chimpanzees, gorillas, and orangutans; however no documented case thus far involves Neisseria species. Chimpanzees have reported to be infected with Streptococcus pneumoniae and Klebsiella pneumoniae, there is a report of a single traumatic case causing meningitis from Proteus mirabilis in gorilla, and an amoeba caused meningitis in an orangutan. Notably, however, meningococcal infection has been documented as infecting chimpanzee urethra, indicating that the species is capable of infecting other great apes. This particular case did not match the known human meningococcal types.

The record to date makes clear that meningococcal meningitis is relatively frequent in humans, but rare if not absent in other nonhuman hominids. Differences in human social structure may enable transmission of infection between individuals. There may also be differences in human molecular biology that allow the pathogen to access the central nervous system. In support of the latter idea, it is interesting to note that the related species Neisseria gonorrheae, causing the sexually transmitted and uniquely human infection gonorrhea, has been demonstrated to bind to certain human proteins that inhibit the immune response. Specifically, these are human complement C4b-binding protein (C4bp) and complement factor H (fH), which inhibit the complement cascade of the immune system. No similar investigation of Neisseria meningitidis has been published. Further, several strains of meningococcus contain the sugar sialic acid in their capsule; human genes related to this molecule are highly enriched for unique changes. These or other candidate molecules may underlie the species specificity of meningitis caused by Neisseria meningitidis.