A hypothetical explanatory model for meningococcal meningitis in the African meningitis belt.

Despite much progress in surveillance and biological research, no explanation exists to date for the epidemic pattern of meningitis in the African meningitis belt, which is required to mathematically model the impact of vaccine strategies or to predict epidemics. This paper presents a hypothetical explanatory model for epidemic meningococcal meningitis. Four incidence patterns are defined as model states, including endemic incidence during the rainy season, ubiquitous hyperendemicity during the dry season, occasional localized epidemics, and-at the regional level-regular epidemic waves spanning over communities or years. While the transition from endemic to hyperendemic situation in a community is caused by an increase in risk of meningitis given colonization by a virulent meningococcus (due to damage of the pharyngeal mucosa by dry climate), the transition from hyperendemic to epidemic situation involves increased pharyngeal colonization and transmission (possibly caused by viral respiratory infection epidemics). The described mechanisms are sufficient to explain the 10- to 100-fold incidence increase that both transitions usually imply. Epidemic waves occur if new meningococcal strains which escape pre-existing immunity, enter the population. Future research should include the impact of viral co-infection on bacterial colonization and invasion.