Life Changing Forecasts: Helping to Manage Meningitis in the West African Sahel

Within the meningitis belt, which stretches from Ethiopia to Senegal as shown in red in Fig. 1, the occurrence of Neisseria meningitidis, often referred to as meningococcal meningitis, is high enough to be considered an epidemic in the developed world (Molesworth et al. 2003). Against this background, larger epidemics occur every 7–14 years (WHO 2012). The largest epidemic in recent history, from 1996 to 1997, affected 250,000 people, caused 25,000 deaths, and left 50,000 people disabled (WHO 2012).

Figure 1. Observed distribution of meningitis epidemics in Africa, compared to the location of the Kassena-Nankana District in northern Ghana. The red shaded region indicates the meningitis belt. The inset box in the lower left shows Ghana, with the Kassena-Nankana District highlighted in blue. 

The epidemics have a devastating impact on the region and its people. Untreated meningitis is fatal 50% of the time (WHO 2012). Even with treatment, the fatality rate can exceed 10%, and 10%–20% of survivors experience long-term after effects including brain damage and hearing loss. Meningitis can push a family into severe poverty, which is especially significant in a region where the annual per capita income ranges from US$500 to US$1500 (World Bank 2013).

Two types of vaccines are used for meningitis outbreaks. Until 2010, polysaccharide vaccines were used to manage epidemics (WHO 2012). These vaccines are only effective for 2 years, are not protective for young children, and do not confer herd immunity. So these polysaccharide vaccines were not used for preventive vaccination. Instead, vaccination campaigns were initiated reactively when the rate of disease increased within a public health district. If the number of confirmed cases of meningitis exceeds the epidemic threshold defined by the World Health Organization (WHO 2012), the country can request emergency vaccines from the International Coordinating Group (ICG) on Vaccine Provision.

A conjugate vaccine was introduced in 2010 in Burkina Faso and parts of Mali and Niger to address the limitations of the polysaccharide vaccine and allow preventive vaccination. This vaccine appears to be very effective: 2011 saw the lowest number of meningitis cases ever recorded (WHO 2012). However, this newer vaccine is only effective against the most common strain of meningitis. The virulence of other strains requires continued monitoring and reactive management with the polysaccharide vaccine.

So where does weather fit in?

The emergence and spread of meningococcal meningitis in the Sahel depends on a complex interplay of environmental, epidemiological, economic, and sociological factors. However, there are links to weather and climate that, if understood and operationalized, could be used to lessen the disease’s impact.

All reported meningitis epidemics in the Sahel have occurred during the dry season, which runs from December to May. In 1984, researchers first documented a correlation between low humidity and meningitis in the scientific literature (see Fig. 2). Higher humidity is associated with decreased meningitis transmission and epidemics stop with the onset of the monsoon (WHO 2012). 

Figure 2. A comparison of (bottom) mean maximum temperature (red line), (middle) absolute humidity (blue line), and (top) number of cases of meningitis (black line) in the Sahel. Figure was adapted from Greenwood et al. (1984).

Several studies have highlighted the correlation of dusty, dry conditions and meningitis. Airborne particulates have been linked to meningitis cases in the Sahel, including naturally occurring dust, dust borne by strong Harmattan winds, and particulates from smoke associated with cooking.

While it would be very helpful to use environmental factors to predict the onset of meningitis epidemics, our project shied away from that for two key reasons: First, it is extremely unlikely that environmental conditions alone can be used to predict an epidemic, because epidemics result from a combination of environmental, social, and biological factors. Many of these factors lack comprehensive data sources that can be used to inform predictive models. 

In contrast, we found substantial evidence that environmental conditions alone, in particular high relative humidity, can end an epidemic. This meant that predicting high relative humidity allowed us to immediately produce information that public health decision makers can use to manage reactive vaccination campaigns. Indeed, members of the ICG already avoid launching vaccination campaigns near the end of the dry season, since they believe the epidemic will end naturally with the start of the monsoon. 

The second reason for focusing on the end of the season rather than the beginning builds on existing practices in the public health community. This provides a clearer path for integrating new research findings into practice. Since there are limited supplies of vaccine, it makes sense to prioritize vaccines in dry areas where the epidemic is more likely to continue to spread. Forecasting higher humidity would allow public health officials to transfer vaccine supplies away from areas where environmental conditions will contribute to the end of epidemics.

The NHRC has a unique dataset of epidemiological and meteorological data collected for the same region and time period. The epidemiological data include total monthly counts of laboratory-confirmed meningococcal meningitis in Kassena-Nankana for the 11-yr period from 1998 to 2008. The meteorological data come from a local weather station operated by the Ghana Meteorological Agency.

We analyzed these data using generalized additive models, which have been widely used to study air pollution and public health (e.g., Schwartz 1994). We found that including weather dependence in our generalized additive model improves in-season prediction of monthly laboratory-confirmed meningitis cases by up to 40%. In particular, the maximum monthly temperature of the current month and the previous month’s relative humidity and carbon monoxide emissions due to fires showed the most influence on meningitis cases. This is consistent with the results of the survey of Kassena-Nankana residents, who indicated that meningitis is associated with hot conditions, and with studies that suggest exposure to smoke increases the risk of meningitis. 

Figure 3. Public health districts in Africa for which data were available between 2007 and 2009. Black dots highlight districts that crossed the epidemic threshold at least once in the 2-yr period. Note that not all countries are uniformly represented; this is because not all data from all parts of the country were available.

We also performed an analysis of meningitis cases across the entire meningitis belt using 2 years of data for the districts shown in Fig. 3. We found that a relative humidity of 40% marked an inflection point for the probability of a district exceeding the epidemic threshold. Based on the 2 years of epidemiological data alone, the risk of a district experiencing an epidemic on any given week is only 2%. This represents background risk, an average risk that does not account for the meteorological influence on meningitis. If the relative humidity in the district is well below 40%, however, the risk of epidemic can increase up to a maximum of 25%. Conversely, districts with a relative humidity above 40% have a lower risk of exceeding the epidemic threshold.

It is interesting to note that relative humidity is a better predictor of epidemic risk than absolute humidity, water content, or other measures of the absolute amount of water. This is consistent with the hypothesis that drying out the nasopharnyx increases the susceptibility to meningitis, since drying depends on the relative, not absolute, humidity in the environment.

USING HUMIDITY FORECASTS TO MANAGE MENINGITIS.

Given the impact of meningitis in the region, the correlation between meningitis cases and the average relative humidity, and the predictability of subseasonal and meridional variations in humidity, our next step was to help public health decision makers use relative humidity predictions to inform their vaccination decisions. Current global models routinely predict relative humidity up to 14 days in advance; coupled with the observed 2-week lag between relative humidity and meningitis cases, this means it is possible to make a meningitis prediction as much as a month ahead of time, enough lead time to influence a vaccination campaign.

To assess the potential impact of the relative humidity forecasts, we also estimated how many vaccinations could have been saved had perfect relative humidity forecasts predicting the natural end of the epidemic been used to avoid launching vaccination campaigns. The value of these avoided vaccinations can be considered in terms of cost savings that can be reallocated toward treating meningitis, an opportunity to reallocate vaccines to more at-risk districts, or the ability to conserve vaccine for future epidemics. This methodology is imperfect, since it does not account for errors in the humidity prediction, including the negative impact of incorrectly anticipating high humidity and prematurely ending a vaccination campaign, but it does provide an upper bound for the value of the meteorology forecasts, which can be used to compare to the potential benefit of other interventions and thus a range of humidity outcomes that are indicators of forecast uncertainty.

Our historic analysis used disease data from Niger, Burkina Faso, Benin, Togo, and Chad from 2006 (Agier et al. 2013) until the conjugate vaccine was introduced in the region (roughly 2010–11). Adapting the approach used by Leake et al. (2002), we identified the districts that reached the epidemic threshold defined by WHO (WHO 2000). Then we identified the subset of those districts where the relative humidity would have naturally ended the epidemic within the next 3–6 weeks, according to the linearized regression model we developed. 

During our study period, 474 epidemics occurred. Of these, there were 18 instances where the risk of continuation of epidemic levels dropped below the background risk because of the actual onset of high relative humidity, as shown in Fig. 4. Given that the accumulated population living in these districts was 3 million people, this implies that roughly 2.6 million doses of vaccine (about 3 million × 0.85 coverage) could have been more effectively positioned elsewhere around the meningitis belt if accurate weather forecasts had been provided and heeded. At an average cost of US$0.45 per vaccine, this translates into a savings of nearly US$1 million over 4 years and five countries. This would be enough to cover the average medical expenses due to meningitis for 11,000 families. More importantly, those 3 million vaccines, properly deployed, are enough to prevent as many as 24,000 cases of meningitis and 2,400 fatalities.

Figure 4. A map showing avoidable vaccination campaign (red dot) between 2006 and 2011. In each of these places, a vaccination campaign was launched less than 6 weeks before the onset of high relative humidity would have ended the epidemic naturally. Given the population of these districts, this accounts for about 2.6 million vaccines at a cost of over US$1 million.

COMMUNITY-INSPIRED METEOROLOGY.

One outcome of this project is difficult to quantify: a subtle change in the way the U.S. scientists involved think about science and generate research questions and methods. Part of this came from interaction with the project sponsor, Google.org. The original driver for this project was a desire to improve meteorological capacity in Africa, premised on the idea that meteorological capacity was worthwhile in its own right and any investment in meteorology would automatically protect lives and livelihood. Google.org insisted on specific measurable impacts and steered us toward public health–oriented impacts. Simultaneous conversations with several African scientists emphasized the need to tie the research question to clear societal benefit; reserve project funds for education, training, capacity building; and develop plans for sustaining the solution after funding ends (Lamptey et al. 2009). Finally, the collaboration with public health officials introduced us to the idea that research projects can involve community members as partners in defining the project, collecting data, and applying the results (Israel et al. 1998).

CONCLUSIONS.

This project produced several original results: it clarified and quantified the long-observed relationship between relative humidity and meningitis; revealed and documented knowledge, attitudes, and practices related to meningitis in rural Ghana; and provided one of the first estimates of the household costs of meningitis. It also produced operational results, including a rule of thumb public health decision makers can use in allocating vaccine (if the average relative humidity exceeds 40% in a district for a few weeks, the epidemic will end naturally with no vaccine) and a decision-informing tool that leverages existing forecasts to predict future average relative humidity. The results also suggest several potential interventions that merit further investigation: use of moistened curtains to raise the humidity within a compound, improved education about early symptoms of meningitis so that people seek medical attention sooner, and use of cookstoves to reduce local and regional carbon monoxide. In fact, a follow-up project examines the social and economic factors around the adoption of cleaner-burning cookstoves and the change in local and regional air quality that would result from widespread use of these cookstoves.

This article has been abridged and edited specifically for the AMS Weather Band. Any omissions or errors may be attributed to AMS Staff. Copyright remains with the AMS.