Meningitis epidemics driven by the wind

Apr 5 2005

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, IRD and University of Paris-VII researcher 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, researchers from the IRD and partner institutes 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.

The Harmattan winds are the dominant feature of the winter climate of Sahelo-Sudanian Africa. The research team therefore sought to assess the influence of these winds on the epidemics. In order to do this, they used wind speed and atmospheric humidity data to develop synthetic atmospheric indices essential for building models of annual climate cycles. These indices were then correlated with epidemiological datasets coming from WHO (World Health Organization) weekly reports on Mali, between 1994 and 2002.

For each of these nine years, a winter maximum for the Harmattan was established. This represents the week when the wind index was highest. On average, this maximum corresponds to the sixth week of the year, between 7 and 15 February, when the Intertropical Front is situated at its southern latitude. Statistical analysis shows that changes in the wind speed index in the course of these seasons are concomitant with the rise in the number of people infected with meningitis, the start of the epidemic coinciding with the Harmattan winter maximum. The end of the epidemics is usually signalled by a recession in the sixteenth week, with the onset of the rainy season, which removes the conditions favourable for meningococcus transmission. The same analysis was then conducted on the atmospheric humidity index. It reinforced the message of these results, the lowest atmospheric humidity and the epidemic onset also corresponded to the sixth week of the year, 7 to 15 February.

Thanks to this preliminary research, combining climate science and health sciences, it is possible to think in terms of setting up epidemiological monitoring systems in order to have early warning of meningitis epidemics in this region of West Africa and attempt to limit their impact. However, these results do not provide a way of defining a possible link between the intensity of winter and the size of the epidemic. Moreover, the climate model used is founded on a period of only nine years, and possibly large variations of climate or epidemics over a larger time-scale cannot be taken into account. This model will consequently have to be tested over longer periods and on a finer spatial scale, in order to study these variations in more detail and assess their impact on the progression of the disease within Mali.

The setting-up of an Environmental Research Observatory (ORE AMMA-Catch), devoted to collecting measurements of climatic and health parameters in West Africa, should contribute to extending and continuing this research in a public health perspective.