This evaluation is the first attempt to quantify and characterize localised meningitis epidemics in the meningitis belt at the health centre level. The advantage of our definition in comparison to existing district level approaches [18
] is that it allows attributing an epidemic event precisely to specific communities and the exact calendar weeks, while the analysis is based on national case reporting systems which collect data routinely in virtually all sanitary districts of the meningitis belt and are available for several past years.
By applying a definition of ≥ 75 cases per 100,000 inhabitants during at least two weeks and ≥ 5 cases during at least one of these weeks to these health centre level data, we could describe several characteristics of meningitis epidemiology in general, and localised epidemics in particular. First, we observed great heterogeneity in weekly and annual incidence among health centres of the same district, especially during epidemic years. This confirms that, to fully understand the dynamic of meningitis epidemics, health centre level data should be analysed.
Secondly, in most years, localised epidemics concerned small communities (< 5,000 inhabitants in several instances), small portions of the district population (as low as 1%), restricted geographic areas (individual health centre zones) and short periods (as short as two weeks). These elements together point to the fact that epidemics at the community level are events that are distinct from ubiquitous hyper-endemic meningitis incidence during the dry season, which is most likely driven by climatic factors [19
], and distinct also from epidemic waves across larger regions, which occur every 10 years or so [21
]. While no strong evidence exists on what makes these localised epidemics occur, specific spatially restricted factors other than climate or meningococcal strains are likely responsible, possibly including micro-epidemics of viral infections [13
Third, the importance of the epidemic at the district level correlated with the number of localised epidemics in the district, but not with the duration or the level of incidence rates during the localised epidemics. If epidemics at the district level are considered the smallest form of epidemic waves, our results lend support to the concept that epidemic waves are the sum of individual localised epidemics and their importance depend on the number of localised epidemics that occur simultaneously. In consequence, the occurrence of epidemic waves would be triggered by a greater than usual geographic expansion of local factors that drive the epidemic, rather than the quality or intensity of a distinct cause of epidemic waves.
Fourth, localised epidemics with appropriate laboratory evaluation appeared to be caused by meningococcus and not by pneumococcus, although the latter was occasionally identified. Despite the fact that pneumococci are responsible for almost half of bacterial meningitis cases in the meningitis belt [22
] and cause meningitis with pronounced seasonality, no localised epidemic so far has been reported due to pneumococci, which suggests that the focal epidemic pattern is limited to meningococci. Serogroups W135 or X were found only sporadically in Burkina Faso during the observed years, such that we cannot evaluate whether these serogroups cause localised epidemics, as well. An analysis of the recent years with high pneumococcal contribution to seasonal meningitis and the NmX epidemic in Burkina Faso during 2010 [5
] should evaluate these hypotheses. Because only a relatively small proportion of suspected cases actually are meningococcal meningitis cases, as some studies have shown [4
], epidemiological analyses of these surveillance data can yield evidence with only limited precision. If the explosive character of localised epidemics was typical for meningococci, the proposed definition could help overcome this limitation of routine surveillance data and allow analyses that are specific for meningococcus.
Lastly, localised epidemics occurred in 38% of the 26 evaluated district years, while in the total evaluated population (1.9 million), at least two epidemics were identified during each year, except 2004. That year showed moderate meningitis incidence in Burkina Faso and it is likely that localised epidemics occurred elsewhere in the country. We hypothesize that in a somewhat larger population than that evaluated here (e. g., three sanitary regions), several localised epidemics occur every year, even outside epidemic waves. The disappearance of these localised epidemics could be used for quantifying the impact that meningococcal serogroup A conjugate vaccine (MenAfriVac®) will have on epidemic meningitis. The introduction of MenAfriVac® in the meningitis belt is expected to substantially change the meningitis epidemiology and a key issue will be to evaluate the impact of the vaccine on the frequency and extend of epidemics. It is possible that the presented definition of epidemics at the health centre level will allow quantifying the long-term impact in a sensitive and economic way, especially in areas where no widespread laboratory surveillance is conducted.
In three instances, an epidemic was declared at district level although the proposed definition LE75 did not identify any localised epidemic. Detailed analyses showed that typically a few health centres had WIR above 75 per 100,000 but only during one week or with < 5 cases per week. The proposed definition appears to have a reduced sensitivity in these instances, but for the sake of robustness and specificity of the definition, as needed for risk factor research, we preferred to maintain the requirement of two consecutive weeks with high WIR.
While the proposed definition of localised epidemics at the health centre level (LE75) did not improve timeliness of epidemic declaration at the health centre level compared to the current practice of district level analysis, our study showed that if RT@HC was used, epidemics could be identified geographically precisely and in one quarter of instances at least one week earlier than by district level analysis. This may be important for reactive mass vaccine campaigns, which will remain necessary after MenAfriVac®
introduction, at least for occasional serogroup W135 epidemics [2
]. Some countries, such as Mali and Togo, do organize reactive vaccine campaigns based on health centre level incidence data and therefore hold vaccine stocks at district level. Many factors influence the effectiveness and costs of a reactive vaccination campaign (speed of data transmission, campaign logistics, and target population) and the usefulness of health centre level analysis of surveillance data for informing vaccination strategies need to be evaluated in more detail on larger data sets.
Our study has several limitations. We used data from a surveillance system without routine quality control and some biases may arise. For example, heterogeneity between health centre level incidences may result from differences in reporting practices. Incidences during the rainy season were comparable in health centres across all districts, but differences in practices may arise specifically during the meningitis season. We included only a small sample of districts spanning five years and there may be geographic variations in the nature of localised epidemics. However, our study included two years outside epidemic waves (2004-5) and three years of an epidemic wave (2006-8), and therefore could be representative of the longer-term variations in epidemiology in Burkina Faso. Data on past vaccination campaigns and coverage at the health centre level were not available. Population immunity likely is an important factor of epidemic occurrence and heterogeneous vaccination coverage across the district may explain why some outbreaks remain localised and why outbreaks may occur several years in a row despite vaccination campaigns.
We excluded cases reported from reference medical centres as they could not be assigned to a specific health care centre population. This certainly led to an underestimation of incidence rates, especially in health centres close to reference centres. Similarly, patients infected in other health centres, districts or regions may have been included in a health centre case report due to consultation preference or travel. However, we believe that our definition of localised epidemics is sufficiently robust not to be biased by this, except in the case of major population movements.
To validate the approach, similar analyses should be conducted on a wider geographical area and include more recent years that were characterized by low serogroup A incidence and high serogroup X or pneumococcal incidence, use spatial epidemiological methods and include more systematic laboratory information on the aetiology of meningitis epidemics and vaccination. Also, it will be interesting to evaluate at which interval localised epidemics occur in individual communities.