Cholera is one of the most rapidly fatal diseases known and in its severest form it can cause death within a few hours, caused by a bacterium, the bacillus Vibrio cholerae. In 2004, 2 345 deaths and 101 383 cases of cholera, including 95 000 for the African continent, were reported to the World Health Organisation. Changes in the global climate have contributed to the spread of cholera; these stem from increases in the frequency of torrential rain, floods and periods of drought. A wide range of factors influence climatic conditions. Some of these depend on the region of the world, whereas others act on a global scale. Therefore, interactions between the climate and emergence of cholera must be studied region by region. To date, few studies have been undertaken in Africa, despite this being the very continent that gives the most cause for concern.
Changes in the global climate have contributed to the spread of cholera.
A recent study by researchers from the Laboratoire de Génétique et Evolution des Maladies Infectieuses (GEMI) mixed research unit IRD/CNRS, was the first to demonstrate the correlations between the outbreak of cholera epidemics and climate data in the five West African countries of Togo, Ivory Coast, Ghana, Benin and Nigeria. Scientists established an epidemiological database in each of these countries based on cases recorded by the World Health Organisation (WHO) over a 20-year period, between 1975 and 1995. Following a comparison of these figures with parameters affecting local and global climate variations, the factors found to be particularly involved are the volume of rainfall and the Indian Oscillation Index (IOI). The IOI is an indicator of the global climate variability, constructed from variations in atmospheric pressure in the Indian Ocean.
Rainfall and the IOI affect the aquatic environment in which Vibrio
cholera develop, where the cholera bacillus lives in contact with elements of the zooplankton. These tiny aquatic crustaceans, known as copepods, comprise the main reservoir of the bacterium and feed upon microscopic algae called phytoplankton. Therefore, they collect in areas where the density of phytoplankton is highest. Understanding this relationship provides a way of monitoring areas rich in phytoplankton using remote sensing, allowing scientists to identify potential reservoirs for Vibrio
cholera from space.
Cholera outbreaks appear irregular, and in order to predict epidemic patterns GEMI researchers compared the frequencies of outbreaks with a range of climatic and environmental parameters which took into account the random variation of frequencies of emerging epidemics. Analysis of rainfall patterns between 1975 and 1996 reveal a cycle with a duration of 3 to 5 years in the appearance of the disease, for the whole area covered by the study. The IOI and the amount of annual rainfall were two climatic variables that appear to be strongly correlated with the emergence of cholera.
In the future, the results of the project should contribute to the creation of an early warning system for the prediction of cholera epidemics, that takes climatic factors into account. This would allow preventative action to be taken, including the filtering of drinking water and the planning of care provision by supplying medical and rehydration kits. This approach could also be used for understanding and preventing other climate-sensitive diseases, such as malaria and dengue fever.