A meningitis epidemic hits West Africa every year, affecting 25 000 to 200 000 people. It has long been known that the timing of this epidemic and its spatial distribution within the “Meningitis Belt”, situated between 10° and 15° North, are closely linked to climatic conditions. Involved in the AMMA programme ( 1 ), IRD and University of Paris-VII researchers ( 2) produced the first quantified description of this relationship using statistical methods. They linked up the annual changes in atmospheric humidity and wind speed to epidemiological data collected over several years in this region. The number of cases of meningitis thus appears in synchrony with seasonal climate variations, epidemic onset coinciding with the time when the winter winds are strongest. These results should be useful for the development of epidemiological early-warning systems in this region, in order to prevent epidemics and try and limit their effect.
The Sahelo-Sudanian band of Africa is an endemic area for meningococcal meningitis (MCM). The disease, an infection of the meninges by the bacteria Neisseria meningitidis, is highly contagious and affects 25 000 to 200 000 people per year, particularly children. Outbreaks occur every year between February and May. Their geographical distribution is contained within what is called the Meningitis Belt circumscribed between 10° and 15° of latitude North. Several factors contribute to epidemic onset, in particular loss of immunity among communities in the face of the bacterial agent, owing to the renewal of generations and the increase in the number of individuals who have never been in contact with it, and also because of climatic conditions. Sahelo-Sudanian Africa is under a climate of alternating dry season in winter and a monsoon season in summer. This alternation is linked to the shift in latitude of the Intertropical Front which corresponds to the convergence zone between the northern winds, called the Harmattan, and the monsoon winds coming from the South. In winter, Sahelo-Sudanian Africa receives the influence of the Harmattan winds. These warm, dry winds, are dust-loaded and cause damage to the mucous membranes of the respiratory system. Conditions are propitious for the transmission of the MCM bacteria to the blood and hence for triggering meningitis epidemics.
Through their involvement in the AMMA programme (1), researchers from the IRD and partner institutes (2) have just for the first time quantified this link between the epidemics cycle and climate variations. Statistical methods brought out evidence of a coincidence between the seasonal variability of the number of cases of meningitis recorded in Mali and the climate dynamics on the scale of this zone.
Helene Deval | EurekAlert!
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