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.