Epidemiology of Meningitis

Epidemiology of Meningitis Caused by Neisseria meningitidis,
Streptococcus pneumoniae and Haemophilus influenzae
The procedures described in this manual are not new; most have been used for many years. Even though they require an array of laboratory capabilities, these procedures were selected because of their utility, ease of performance, and ability to give reproducible results. The diversity of laboratory capabilities, the availability of materials and supplies, and their cost, were taken into account.

Bacterial menigitis, an infection of the membranes (meninges) and cerebrospinal fluid (CSF) surrounding the brain and spinal cord, is a major cause of death and disability world-wide.

Beyond the perinatal period, three organisms, transmitted from person to person through the exchange of respiratory secretions, are responsible for most cases of bacterial meningitis:
Neisseria meningitidis, Haemophilus influenzae, and Streptococcus pneumoniae.

The etiology of bacterial meningitis varies by age group and region of the world. Worldwide, without epidemics one million cases of bacterial meningitis are estimated to occur and 200,000 of these die annually.

Case-fatality rates vary with age at the time of illness and the species of bacterium causing infection, but typically range from 3 to 19% in developed countries. Higher case-fatality rates (37-60%) have been reported in developing countries.

Up to 54% of survivors are left with disability due to bacterial meningitis, including deafness, mental retardation, and neurological sequelae.

Two clinically overlapping syndromes – meningitis and bloodstream infection (meningococcaemia) - are caused by infection with N. meningitidis (meningococcal disease).

While the two syndromes may occur simultaneously, meningitis alone occurs most frequently. N. meningitidis is classified into serogroups based on the immunological reactivity of the capsular polysaccharide.

Although 13 serogroups have been identified, the three serogroups A, B and C account for over 90% of meningococcal disease.

Meningococcal disease differs from other leading causes of bacterial meningitis because of its potential to cause large-scale epidemics.

A region of sub-Saharan Africa extending from Ethiopia in the East to The Gambia in the West and containing fifteen countries and over 260 million people is known as the “meningitis belt” because of its high endemic rate of disease with superimposed, periodic, large epidemics caused by serogroup A, and to a lesser extent, serogroup C.

During epidemics, children and young adults are most commonly affected, with attack rates as high as 1,000/100,000 population, or 100 times the rate of sporadic disease. The highest rates of endemic or sporadic disease occur in children less than 2 years of age.

In developed countries, endemic disease is generally caused by serogroups B and C. Epidemics in developed countries are typically caused by serogroup C although epidemics due to serogroup B have also occurred in Brazil, Chile, Cuba, Norway and more recently in New Zealand.

Meningitis caused by H. influenzae occurs mostly in children under the age of 5 years, and most cases are caused by organisms with the type b polysaccharide capsule (H. influenzae type b, Hib).

While most children are colonized with a species of H. influenzae, only 2-15% harbour Hib. The organism is acquired through the respiratory route. It adheres to the upper respiratory tract epithelial cells and colonizes the nasopharynx. Following acquisition of Hib, illness results when the organism is able to penetrate the respiratory mucosa and enters the blood stream.

This is the result of a combination of factors, and subsequently the organism gains access to the CSF, where infection is established and inflammation occurs. An essential virulence factor which plays a major role in determining the invasive potential of an organism is the polysaccharide capsule of Hib.

Meningitis is the most severe form of Hib disease; in most countries, however more cases and deaths are due to pneumonia than to meningitis. Meningitis in individuals at the extremes of age infants, young children and the elderly is commonly caused by S. pneumoniae.

Younger adults with anatomic or functional asplenia, haemoglobinopathies, such as sickle cell disease, or who are otherwise immunocompromised, also have an increased susceptibility to S. pneumoniae infection. S. pneumoniae, like Hib, is acquired through the respiratory route.

Following the establishment of nasopharyngeal colonization, illness results once bacteria evade the mucosal defences, thus accessing the bloodstream, and eventually reaching the meninges and CSF.

As is the case with Hib, many more cases and deaths are due to Pneumococcal pneumonia, even though pneumococcal meningitis is the more severe presentation of pneumococcal disease.

The risk of secondary cases of meningococcal disease among close contacts (i.e. household members, day-care centre contacts, or anyone directly exposed to the patient’s oral secretions) is high.

Antimicrobial chemoprophylaxis with a short course of oral rifampin, a single oral dose of ciprofloxacin, or a single injection of ceftriaxone is effective in eradicating nasopharyngeal carriage of N. meningitidis.

Although very effective in preventing secondary cases, antimicrobial chemoprophylaxis is not an effective intervention for altering the course of an outbreak. In epidemics, mass chemoprophylaxis is not recommended.

Vaccines have an important role in the control and prevention of bacterial meningitis. Vaccines against N. meningitidis, H. influenzae, and S. pneumoniae are currently available, but the protection afforded by each vaccine is specific to each bacterium and restricted to some of the serogroups or serotypes of each bacterium.

For example, vaccines are currently available to prevent H. influenzae infections due to
serotype b (Hib) but not those infections due to other serotypes or unencapsulated organisms (i.e. nontypeable H. influenzae).

In addition to establishing a diagnosis, an important role for the laboratory, therefore, is to determine the bacteria and serogroups/serotypes that are causing meningitis in a community.

In industrialized countries, routine use of polysaccharide-protein Hib conjugate vaccines for immunization of infants has almost eliminated Hib meningitis and other forms of severe Hib disease. Several studies in developing countries have corroborated these finding.

Pneumococcal polysaccharide vaccines have been used to prevent disease in the elderly and in persons with chronic illnesses that may impair their natural immunity to pneumococcal disease.

Meningococcal polysaccharide vaccines are generally used in response to epidemics and for the prevention of disease in overseas travellers although other uses are currently under investigation.

In addition to the existing armamentarium of vaccines, new generation vaccines against meningococcal and pneumococcal disease are under development and evaluation.

These vaccines may provide a high degree of protection and broad coverage in all age groups. Until these vaccines become widely available, the current vaccines should be used appropriately and efficiently.

Use of any of these vaccines will require laboratory identification of the agents causing disease in addition to epidemiological information about the age and risk groups that are most affected.

Source: National Institutes of Health