A major human pathogen, Neisseria meningitidis, which causes meningitis and other meningococcal diseases. The bacterium known as Neisseria meningitidis (see illustration), commonly termed meningococcus, is a pathogenic microorganism that can cause serious infections in humans. In particular, meningococcus is responsible for meningococcal meningitis (a bacterial form of meningitis, that is, an inflammation of the protective membranes covering the brain and spinal cord) and meningococcemia (meningococcal septicemia, or dissemination of meningococcal bacteria into the bloodstream). Meningococcus is a Gram-negative, aerobic, nonmotile diplococcus, characteristically occurring in smears from clinical material in pairs with flattening of the adjoining sides. It is fastidious in its growth requirements and is very susceptible to adverse physical and chemical conditions. See also: Bacteria; Bacteriology; Medical bacteriology; Meningitis; Pathogen
Twelve serogroups of N. meningitidis can be distinguished serologically: A, B, C, 29E, H, I, J, L, W135, X, Y, and Z. The most consequential serogroups, being responsible for most instances of meningococcal disease, are A, B, C, W135, X, and Y. Grouping is based on antigenic differences between capsular polysaccharides and is usually accomplished by agglutination reactions. Almost all strains isolated from blood or cerebrospinal fluid of cases can be serogrouped. In addition, within each serogroup, serotypes and subtypes can be identified on the basis of antigenic differences between the major outer-membrane proteins. Serotypes are identified by means of monoclonal antibodies. See also: Antibody; Antigen; Monoclonal antibodies; Serology
Humans are the only known natural host of meningococcus. Transmission occurs through droplets disseminated directly from person to person. The most frequent site of infection is, asymptomatically, the nasopharynx. Infection of the nasopharynx is uncommon in infants and young children, but is relatively frequent in adolescents and adults.
Most sporadic cases are caused by serogroups B and C. Sporadic disease caused by other serogroups is much less common. Almost all large epidemics and pandemics of meningococcal disease are caused by serogroup A. See also: Epidemiology
Pathogenesis and disease
Susceptibility to bloodstream invasion from the nasopharynx has been shown to correlate with the absence of bactericidal antibody. The most common clinical syndrome caused by meningococcus is meningitis, which is characterized by fever, headache, nausea, vomiting, and neck stiffness. It also can be fatal in severe cases (especially in infants and adults over 60). Disturbance of the state of consciousness quickly occurs, leading to stupor and coma. Many cases also have a typical skin rash. In 10–20% of cases caused by serogroups B and C (but less often with serogroup A), a much more fulminant disease occurs in which overwhelming meningococcemia develops. As a result of the rapid release of large quantities of endotoxin, profound shock and hemorrhagic phenomena predominate. Despite available treatment, death occurs within 6–24 h in most cases. See also: Endotoxin
Occasionally, pneumonia or a transient febrile illness with self-limited meningococcemia occurs. The bacteremia frequently clears without antibiotic treatment.
Chronic meningococcemia is a rare manifestation, appearing most often in adults. This illness is characterized by recurrent episodes of fever, arthritis, myalgia, and rash. Meningococci are present in the blood only during febrile episodes.
Because of the widespread occurrence of strains of meningococci resistant to sulfonamides, other types of antibiotics, including rifampin, penicillin, and ciprofloxacin, are used to treat all forms of meningococcal disease. See also: Antibiotic
In closed population groups and among household contacts of cases (especially children), there is a significantly increased risk of secondary cases. Antibiotics are recommended for close contacts of cases in order to interrupt transmission by eradicating the carrier state.
Vaccines are effective in preventing meningococcal diseases. Vaccines against the six major serogroups have been licensed and approved for use. See also: Vaccination