Box 92 Bacterial meningitis causal organisms

Neonates Gram negative bacilli Streptococci (usually group B) Listeria monocytogenes

Children Meningococci Pneumococci

Adults Pneumococci Meningococci Staphylococci Listeria monocytogenes Streptococci

Mycobacterium tuberculosis can affect children and adults

Table 9.1 Bacteria linked to underlying causes of meningitis

Organism

Pneumococci

Staphylococci

Gram negative bacilli

Pneumococci

Pneumococci

Gram negative bacilli

Staphylococci

Pneumococci

Gram negative bacilli

Staphylococcus epidermidis

Listeria

Streptococci

Listeria

Pneumococci

Haemophilus

Meningococci

Pseudomonas

Cause

Diabetes mellitus

Alcohol

Sickle cell disease Skull fracture

Dural fistula/CSF leak

CNS shunt Pregnancy/childbirth

Cell mediated immune defect Humoral immune defect

Neutropenia organisms with particular virulence properties, which allow access to the CNS, cause most cases.

The epidemiology of meningitis is complex and varies with the age of the patient and geography. About 70% of cases are seen in children. The introduction of childhood vaccination against Haemophilus influenzae type b has led to a dramatic fall in the incidence of this pathogen, which previously was the most common cause of meningitis in children less than five years old. Now, with the exception of the neonatal period and the elderly, Neisseria meningitidis and Streptococcus pneumoniae account for about 70% of all cases of bacterial meningitis. Neonatal meningitis may be caused by any organism but the most frequently encountered pathogens are gram negative bacilli, particularly Escherichia coli, other enteric bacilli, Pseudomonas spp., Listeria monocytogenes, and group B streptococci. Neonatal meningitis is not discussed further here. In the elderly there is an increased risk of meningitis caused by gram negative bacilli, staphylococci, and L. monocytogenes.

The meningococcus, of which there are a number of serogroups, remains the only significant cause of epidemics.69-71 In sub-Saharan Africa there are annual epidemics of meningitis due to serogroup A. Serogroup B tends to cause epidemics spread over several years and these have been reported in Europe, Latin America, and New Zealand. Recent years have seen an increased incidence in adolescents and young adults (particularly on college campuses) of meningococcal disease caused by a particularly virulent strain (serotype 2a) of serogroup C organisms. The United Kingdom was the first country to use meningococcal serogroup C conjugate vaccines and their introduction has led to a decline in serogroup C disease in adolescents.72

S. pneumoniae meningitis is associated with pneumonia, acute and chronic otitis media, alcoholism, diabetes mellitus, splenectomy, hypogammaglobulinaemia, head trauma, and CSF rhinorrhoea or otorrhoea resulting from dural leaks.

Gram negative bacilli tend to occur in association with head injuries, following neurosurgery, in patients who have compromised immune systems, and in the elderly. One disturbing factor is the increase in the number of cases that have been acquired nosocomially.73

Staphylococcal infection may complicate neurosurgical procedures and head trauma and also affects those with immunosuppression or endocarditis. Anaerobic organisms alone or as part of polymicrobial infections cause 1-2% of cases that complicate paranasal sinus and ear infections, skull fractures, neurosurgery, and immunosuppression.74 Tuberculosis remains a significant cause of meningitis in the United Kingdom and United States.75 Many other organisms may also cause meningitis, commonly of a more chronic nature, but which may rapidly develop into an acute emergency in those with compromised immune systems.

Advances in the investigation and management of bacterial meningitis over the past 40 years or so, including the continuing development of antibiotics, the introduction of CT and MRI, the application of improved techniques of bacterial culture, and now the widespread use of molecular methods incorporating PCR, have permitted treatment to be given on a more rational basis to a wider range of patients. It is disappointing, therefore, that mortality and morbidity from the main forms of bacterial meningitis have failed to diminish by any substantial degree over the same length of time. Fortunately, in recent years new understanding of the pathophysiology of the changes that occur in bacterial meningitis has become available,76,77 from which it may be possible to develop new strategies for treatment.

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