Neisseria meningitidis

Neisseria meningitidis, also simply known as meningococcus is a gram-negative bacterium best known for its role in meningitis. It only infects humans, there is no animal reservoir. It is the only form of bacterial meningitis to cause epidemics.

Clinical Presentations
Meningitis is the most well publicised condition. Whilst a non-specific illness initially, this can rapidly progress through fever, headache and neck stiffness to coma and death. The mortality is approximate 10% of cases. Suspicion of meningitis is a medical emergency and immediate medical assessment is recommended.

Septicaemia ("blood poisoning") has received much less public attention, but has been linked to infant deaths. Whilst there may be an absence of the classical meningitis symptoms, the presence of a non-blanchable purpuric rash is easily ignored by those not aware of its significance. Septicaemia carries an approximate 50% mortality rate over a few hours from initial onset. Anyone developing a rash that does not turn white ("non-blanching") if pressed with a glass is encouraged to attend a hospital casualty department as soon as possible.

Waterhouse-Friderichsen syndrome a massive, usually bilateral, hemorrhage into the adrenal glands caused by fulminant infection.

UK policy is that any General Practitioner doctor seeing a suspected case of meningococcus meningitis or septicaemia should give intravenous antibiotics (benzylpenicillin) whilst hospital admission is sought. The possible reduction in subsequent microbiological confirmation of infection, due to starting treatment before testing, is offset by the reduced mortality.

Not all cases of a purpura-like rash are due to septicaemia, but the other causes also need prompt investigation (eg ITP, a platelet disorder).

Strains
There are many strains of meningococcus, clinically the most important are A, B, C, Y and W135:
 * A - occurs most often in sub-sahara Africa and vaccination is recommended prior to travel with the Men A&C vaccine.
 * B - is the most lethal form, comprising 40% of UK cases. The changing nature of the B group has prevented formation of a general B vaccine in the UK. However there has been developed the vaccine MeNZB against a specific strain of group B meningococcus, currently being used to control an epidemic in New Zealand.
 * C - caused approximately 60% of UK cases before the introduction of successful vaccination programme for infants. Previously the unconjugated C component of Men A&C was ineffective in those under 2 years. The development of a conjugated form (Men C conj) was needed to provoke infant immunity.
 * W135 - is particularly a problem for those undergoing annual pilgrimage to Mecca. It is a requirement of Saudi Arabia that all those intending to go on Hajj have a certificate of Men W135 vaccination.
 * Y - In the last decade serogroup Y has emerged as a cause of disease in Northern America

Headline text
Those with impaired immunity may be at particular risk of meningococcus, e.g. those with nephrotic syndrome or splenectomy. In asplenia (removed or non-functioning spleen), vaccination is performed according to protocols.

Vaccines
There are 13 different types of Meningococcus bacteria. These are classified according to the antigenic structure of their polysaccharide capsule. Five serogroups, A, B, C, Y and W135 are responsible for virtually all cases of the disease in humans. There is currently no effective vaccine for Meningococcus B.
 * Polysaccharide vaccines
 * Conjugate vaccines