Causes of Meningitis

Most cases of meningitis are caused by microorganisms (infectious meningitis) such as;

  • viruses,
  • bacteria,
  • fungi, or
  • parasites,

These microorganisms spread into the blood and into the cerebrospinal fluid (CSF).
Non-infectious causes of meningitis, though not common,  include:

·         cancers, (neoplastic conditions e.g carcinomatous meningitis);
·         systemic lupus erythematosus; 
·         certain drugs;
·         physical injury (subarachinoid haemorrhages).

The most common cause of meningitis is viral or aseptic mengitis, which often runs its course within a few days.   It may also be associated with other clinical entities such as mumps, poliomyelitis and measles.

Bacterial meningitis is the second most frequent type and can be serious and life-threatening. Acute bacterial meningitis may be caused by the following  (in order of frequency):

Pneumococci – Streptococcus pneumoniae
Haemophilus influenza – Haemophilus incluenzae
Meningococci – Neisseria mengitidis
Coliforms  -   Escherichia coli
Salmonella – Salmonella typhi
Staphylococci – Staphylococcus aureus
Streptococci  -  Streptococcus pyogenes


Neisseria meningitidis ("meningococcus") and Streptococcus pneumoniae ("pneumococcus") are the most common pathogens in patients without immune deficiency.  Meningococcal disease is more common in children. Staphylococcus aureus may complicate neurosurgical operations, and Listeria monocytogenes is associated with poor nutritional state and alcoholism.

The incidence of Haemophilus influenzae (type B) has been much reduced by immunization in many countries. Mycobacterium tuberculosis (the causative agent of tuberculosis) rarely causes meningitis in Western countries but is common and feared in countries where tuberculosis is endemic.

These bacteria however are common in some age groups and the table below will help you make a better diagnosis.


Table 14.1: Causative and predisposing factors in meningitis (AMREF, 2007, Communicable Diseases Manual)

Causative Organisms
Age group most at risk
Predisposing condition
Pneumococci
Adults
Mastoiditis, otitis media, sinusitis, pneumonia, haed injury, pe\uerperium, pregnancy
H. Influenzae
Children
Respiratory tract infection, otitis media, mastoiditis
Meningococcus
Children and young adults
Over crowding
M. Tuberculosis
Children
Malnutrition, HIV
Salmonellae
< 2 years
Diarrhoea or septicaemia
Virus
Children
Epidemics of mumps, measles, polio, chckenpox and other viruses.



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