Morphology and culture. Meningococci are Gram-negative, coffee-bean-shaped cocci that are frequently pleomorphic and have a diameter of 1 im (Fig. 4.16b). They are nonmotile and feature a polysaccharide capsule.
Growing meningococci in cultures requires mediums containing blood. A concentration of 5-10% CO2 encourages proliferation.
Antigen structure. Serogroups A, B, C, D, etc. (a total of 12) are differentiated based on the capsule chemistry. Epidemics are caused mainly by strains of serogroup A, sometimes by B strains as well and, more rarely, by group C strains. Serogroups are divided into serovars based on differences in the outer membrane protein antigens.
Pathogenesis and clinical picture. Meningococci are parasites of the nasopharynx. These microorganisms are carried by 5-10% of the population. If virulent meningococci colonize the nasopharyngeal mucosa of a host lacking the antibodies, pathogen invasion of the mucosa by means of "parasite-directed endocytosis" becomes possible (see p. 12). The CNS is doubtless the preferred compartment for secondary infections, although hematoge-nously disseminated pathogens can also infect the lungs, the endocardium, or major joints.
Onset of the meningitis is usually sudden, after an incubation period of two to three days, with severe headache, fever, neck stiffness, and severe malaise. Severe hemorrhagic sepsis sometimes develops (Waterhouse-Frie-drichsen syndrome).
Diagnosis requires detection of the pathogen in cerebrospinal fluid or blood by means of microscopy and culturing techniques. For success in culturing, the material must be used to inoculate blood agar without delay. Identification of the pathogen is based on identification of metabolic properties. The slide agglutination test is used to determine the serogroup.
Latex agglutination or coagglutination (p. 217) can be used for direct antigen detection in cerebrospinal fluid.
Therapy. The antibiotic of choice is penicillin G. Very good results have also been obtained with third-generation cephalosporins, e.g., cefotaxime or ceftriaxone. It is important to start treatment as quickly as possible to prevent delayed damage.
The advantage of cephalosporins is that they are also effective against other meningitis pathogens due to their broad spectrum of action (with the exception of Listeria monocytogenes).
Epidemiology and prevention. Meningococcal infections are more frequent in the winter and spring months. Transmission of meningococci is by droplet infection. Humans are the only pathogen reservoir. Sources of infection include both carriers and infected persons with manifest disease. In developed countries, meningitis occurs sporadically or in the form of minor epidemics in more or less isolated collectives (work camps, recruiting camps, school camping facilities). The incidence level is approximately 12 cases per 100 000 inhabitants per year. In parts of the developing world (African meningitis belt) the level is higher. Lethality runs to 85% if the disease is left untreated, but is reduced to less than 1 % if treatment is begun early enough. Prophylactic antibiosis is indicated for those in close contact with diseased persons (e.g., in the same family). Prophylactic measures also include treatment of
carriers to eliminate this reservoir, whereby minocylin or rifampicin must be used instead of penicillin G. Prophylactic immunization can be achieved with a vaccine made from the purified capsule polysaccharides A, C, Y, and W-135. There is no serogroup B vaccine, since the capsule in serogroup B consists of polyneuraminic acid, which the immune system does not recognize as a foreign substance.
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