Although a functioning health surveillance system should detect any unusual increase in the number of meningitis cases, meningitis epidemics are often first reported by hospitals, community leaders or the media.
In any instance, a rapid assessment is necessary.
It is important to choose a sufficiently large population for the assessment of weekly attack rates, at least 30 000 to 50 000 since disease rates in smaller populations can fluctuate widely even with a small number of cases.On the other hand, if only very large (> 1 000 000) populations are observed, low overall attack rates may obscure high rates within smaller populations in local areas. The most appropriate denominators are administrative areas with population ranging between 30 000 and 100 000.
The decision to call for an emergency response to a meningococcal meningitis outbreak is determined by:
- the seriousness of its health impact on the population at risk; and
- the ability of local health services to respond.
These two factors should be given priority during the assessment.
The rapid assessment consists of confirming a meningitis outbreak and estimating its geographical distribution, assessing the health impact, and the local response capacity and immediate needs.
Confirming a meningitis outbreak and estimating its geographical distribution
To confirm the existence of a meningitis outbreak and estimate its geographical distribution, establish an initial case definition, undertake case-findings, and collect appropriate specimens for laboratory analysis and confirmation.
Initial case definition
The initial case definition is best determined in advance, as part of emergency preparedness. Simple, viable case definitions should be determined for infants, older children, and adults.
The standard case definition of bacterial meningitis1 is as follows:
1 This case definition allows the detection of meningococcal septicaemia.
· Suspected case:2 Sudden onset of elevated temperature (>38.5°C rectal or 38.0°C axillary) with stiff neck or petechial or purpural rash or both.
2 Often the only diagnosis that can be made in dispensaries (peripheral level of health care).
In patients under one year of age, a suspected case of meningitis occurs when fever is accompanied by a bulging fontanelle.
· Probable case:3 Suspected case as defined above with turbid cerebrospinal fluid (CSF) (with or without positive Gram stain) or ongoing epidemic.
3 Diagnosed in health centres where lumbar puncture and CSF examination are feasible (intermediate level).
· Confirmed case:4 Suspected or probable case as defined above and either positive cerebrospinal fluid (CSF) antigen detection or positive culture.
4 Diagnosed in well-equipped hospitals (provincial or central level).
Case-finding is best undertaken through hospitals and other health facilities in the affected area. A rapid survey of households is probably not useful as, even in serious epidemics, the attack rate may not exceed 5 per 1000.
By reviewing hospital records for the same period during previous years, it may be possible to determine whether there is a significant increase in cases. Look at their geographical distribution, and at the speed at which new cases are being reported.
Collection of specimens
In areas where meningitis is hyperendemic or periodically epidemic, clinical recognition is usually reliable. However, every effort should be made to obtain CSF from cases. This is essential to:
- confirm the diagnosis and define the serogroup to determine whether vaccination is a useful strategy; and
- determine antimicrobial sensitivity for treatment and possible prophylaxis.
If routine bacteriological capability is available, CSF specimens can be plated and incubated on site (e.g. in an equipped provincial hospital).
Although diagnosis in the field can be undertaken by examining a smear of CSF, the results may be unreliable. In such cases, CSF specimens should be transported under sterile conditions for analysis at a laboratory equipped with commercially available antigen detection kits.
The following considerations are important in specimen collection:
· While it is preferable to obtain CSF specimens before antibiotic therapy has begun, treatment should not be delayed. Rather, it should be noted on the form accompanying the specimen that antibiotics have already been administered.
· If adequate laboratory capacity is not available, CSF specimens can be inoculated into transport-isolation media on site, and then transported to an equipped laboratory (screw-top tubes are less likely to become contaminated in field conditions than plates).
· If transport media are not available, then CSF should be collected in a clean and sterile container for transport to a suitably equipped laboratory.
To verify the laboratory diagnosis and confirm the organism serogroup and antibiotic sensitivities, it is advisable to ship specimens to WHO collaborating centres for urgent analysis.
Assessing the impact on health
To assess the impact on health of a meningitis outbreak, collect information on sample cases, analyse the information gathered, and draw initial conclusions.
Collecting information on a sample of cases
Time and resources permitting, information on age, sex, occupation, residence, and date of onset is helpful in identifying groups at greatest risk from:
- the spread of the disease (e.g. overcrowded squatter settlements where the potential risk of rapid transmission is great); and
- mortality (e.g. identify populations with poor access to health facilities and those with poorly equipped health facilities where a higher mortality risk might be expected).
Analysing the information
Time: When did cases occur? Is the number increasing?
· Draw a simple graph to show the number of cases reported per day for the epidemic so far.
· If the meningitis outbreak has affected a wide area, construct simple graphs for the different areas affected.
Place: Where have meningitis cases occurred? Is the outbreak spreading? Are there accessible health facilities in affected areas?
· Map cases geographically if possible by date of onset.
· Use maps that identify settlements, health facilities, and major transport routes. If these are not available, sketch a rough map including this information. This helps identify at-risk areas and their relation to road or rail links and existing health facilities that are important for organizing a rapid response.
Person: Which groups and communities are at greatest risk? How many cases are there so far, or could there be in the future?
· Estimate the number of hospital admissions and clinic attendances for affected areas and for specific facilities.
Drawing initial conclusions on the outbreak
To draw initial conclusions about the outbreak, you should obtain answers to the following questions:
· Is there an outbreak of acute meningococcal meningitis?
· How many cases and deaths so far?
· What is the geographical distribution of the cases?
· What is the size of the population at risk?
· Is the outbreak spreading? Where?
· What do preliminary laboratory results show?
Assessing local response capacity and immediate needs
Local response capacity and immediate needs should be assessed to determine the type and quantity of external support required.
Local epidemiological surveillance
· Are more extensive field investigations needed?
· If the outbreak has affected a large population or has occurred in an area inaccessible to the capital or both, is there at least one available person with training in epidemiology to maintain and supervise outbreak surveillance?
· Will she or he have available an appropriate vehicle to visit the area affected?
· Is outside help needed?
Response capacity of local health services
· What is the case-fatality ratio?
· What steps have local health officials taken to organize epidemic response? Is there a plan of action, standardized reporting procedures, and trained staff?
· What linkages have been established with key community leaders (e.g. to improve case detection and allay panic)?
· Are health facilities accessible to affected populations? Are temporary centres needed? Where?
· Is there at least one qualified physician in the affected area experienced in the clinical management of meningitis?
· At district-level facilities, is there at least one nurse or health worker with experience in the care of severely ill meningitis patients?
· Are health facilities equipped and do they have sufficient staff for projected patient load?
· What is the local cold chain capacity? Are there trained vaccinators, jet injectors, vehicles, stocks of syringes and vaccines?
· Is there access to vehicles for local distribution and supply of emergency drugs?
· What stocks of drugs (e.g. oily and oral chloramphenicol, crystalline benzylpenicillin, and supportive drugs) are available?
Determine immediate needs
When deciding on the need for emergency response the following questions should be considered.
· Is there an outbreak of meningococcal meningitis that has or could lead to a large number of cases?
· If so, are outside resources needed to contain it?
If the answer to both questions is “yes”, then an emergency response is needed.