meningococcus

introduction

the meningococcus (Neisseria meningitidis) is an obligate aerobe, bean-shaped diplococcus, fastidious, oxidase +ve Gram -ve coccoid
only infect humans and have never been isolated from animals because the bacterium cannot get iron other than from human sources (transferrin and lactoferrin).
exists as normal flora (nonpathogenic) in the nasopharynx of up to 5-15% of adults.
in some communities it may be a commensal in over 95% of the community, yet less than 1% develop invasive disease
is spread through the exchange of saliva and other respiratory secretions during activities like coughing, kissing, and chewing on toys, spread appears more likely in favorable overcrowded conditions such as night clubs which on occasion may be the source of a small epidemic.
invasive disease is a medical life-threatening emergency which may present as either:
- meningococcal septicaemia - 50% mortality rate over a few hours from initial onset but there is also a chronic form
- meningitis

subtypes are classified according to the antigenic structure of their polysaccharide capsule.
of the twelve groups of N. meningitidis identified, six of these (A, B, C, W135, X, and Y) are able to cause epidemics.

newly acquired infection of a virulent subtype (eg. contact with a carrier or patient with invasive disease)
immunocompromised eg. splenectomy, nephrotic syndrome patients