The African meningitis belt, spanning 26 countries, experiences a significant burden of meningococcal meningitis in part driven by regional climatic factors. Outbreaks occur during the October-March dry season, characterised by low humidity, high temperature, and increased dust levels driven by the movement of the Harmattan winds. Whilst many studies have examined the relationship between meningococcal meningitis outbreaks and climate, few studies have examined this relationship on a continental level. We developed a Bayesian spatiotemporal model to estimate monthly meningitis risk at a district (admin2) level across Africa using climatic variables.
For all district-months between 2003-2022 we generated a Boolean variable to identify districts affected by meningitis outbreaks, defined as a reported incidence of suspected cases of 10 per 100,000 in a week. Covariates included rainfall, humidity, windspeed, MenAfriVac campaign status, population density, aerosol optical depth (AOD), temperature and land cover type. We used a Bayesian Integrated Nested Laplace Approximation (INLA) model to account for spatial and temporal heterogeneity. Covariate inclusion was decided using the Watanabe Akaike Information Criterion (WAIC), the Deviance Information Criterion (DIC), as well as considering variable epidemiological relevance. Our final model included linear effects for humidity, AOD, wind strength and direction, an interaction term between wind direction and AOD, and a non-linear temperature effect.
Zonal wind strength had a statistically significant positive relationship with outbreaks whilst humidity had a negative association. Although individually, AOD and binary wind direction did not have a statistically significant effect on epidemic incidence, when considering these effects together, the interaction had a highly positive association. When zonal wind moved westwards, higher AOD levels increased the likelihood of outbreak occurrence. Furthermore, whilst the log odds of meningitis outbreak occurrence remained low at cooler temperatures, the effect peaked between 32- 35 °C before beginning to plateau.
The non-linear association between meningitis incidence and temperature is suggestive of an optimal temperature for bacterial transmission. Additionally, the significance of the AOD and wind direction interaction highlights the role of the Harmattan winds in exacerbating meningitis risk, rather than outbreaks solely being driven by higher dust levels.