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EL NIÃ'O: CAUSAL FACTOR OF CHOLERA IN BANGLADESH

September 06, 2000

A study by a climatologist of the Climate Research Group at the Barcelona Science Park and member of the Department of Ecology at Barcelona University, Xavier Rod'³, together with researchers at various universities in the United States and Great Britain provides evidence that the cholera epidemics in Bangladesh are favoured by meteorological phenomena deriving from El Ni'±o. The study, which will be published on 8 September in Science, concludes that the dynamics of cholera in the area studied is determined by the phenomenon known as El Ni'±o and also by other factors inherent to the disease itself, such as the number of individuals with a previous history of infection or the health conditions in the area, which are exacerbated by the outbreak of the epidemic. The evidence is a step forward in the development of a predictive model for cholera epidemics which would permit the application of preventive measures in the affected areas.

The relation between cholera in Bangladesh and El Ni'±o has been the subject of controversy in the scientific community in recent years. According to Rod'³, this report is "a step forward, which clearly points in this direction." The research is the result of a joint study by Xavier Rod'³, with Mercedes Pascual and Rita Colwell, both at the Center of Marine Biotechnology at the University of Maryland, and Stephen P. Ellner at Cornell University, together with Menno J. Bouma at the London School of Hygiene and Tropical Medicine at the University of London.

Relation between Cholera and El Ni'±o
The research is based on data on the incidence of cholera in Bangladesh between January 1980 and March 1998, and a satellite data base for the same period, which includes ocean and atmospheric parameters such as sea-surface temperature in the equatorial Pacific, which is an indicator of the appearance of El Ni'±o. A statistical study has shown that these two series share characteristics: the frequency observed in the El Ni'±o series can be identified in the cholera series. "The cholera series shows a bimodal pattern each year: a small peak in spring and a larger peak at the end of autumn and beginning of winter. Superimposed upon this variation there is another, inter-annual rhythm with a period of about 3.7 years, which coincides with the period observed in the El Ni'±o series". This coincidence between the characteristic frequencies of the two series led the researchers to hypothesise that climatic variation might affect the dynamics of the disease.

Development of a predictive model
Once the relation between the cholera series and the El Ni'±o series had been recognised, the researchers set about developing a mathematical model to predict future cholera epidemics on the basis of outbreaks observed in the period of study. The model was also required to confirm the relation between the two phenomena. "In the data series we observed changing pluri-annual variations in the base levels of the epidemic, and so we developed a non-linear model in order to reflect the non-linearity", adds Rod'³. The model, which allows predictions of cholera outbreaks two months in advance, includes not only data from the cholera and El Ni'±o series, but also factors inherent to the disease itself.

Studying regional climatic variables: the link between cholera and El Ni'±o

In the Indian Ocean and South East Asia El Ni'±o does not have such obvious effects as the hurricanes, fires or torrential rains that it causes in other areas, but it does induce changes in the thermal equilibrium which are observable four or five months after the heating of the Pacific. These alterations lead to variations in the temperature, cloud cover and convective currents that may increase temperature in the region.

Studying these regional climatic variables to confirm that they act as a link between the Pacific anomalies and the cholera epidemics was the aim of the last stage of the research. "This stage centred on tracing the connections between the temperature anomalies in the Pacific, firstly with those of the regional climate in the Indian subcontinent, and secondly with the outbreak of epidemics in Bangladesh." The analysis consisted of studying the correlation between these parameters by concentrating on three regional climatic parameters measured from satellites which gave an indirect measurement of the regional precipitation and temperature. These were humidity in the upper troposphere (which allows monitoring of the tropical atmospheric circulation), total cloud cover and absorbed solar radiation.

With this analysis the authors show that the same changes in atmospheric circulation that originate in Southern Asia as a result of the heating up of the Pacific also cause inter-annual variation in the cholera epidemics in Bangladesh. The results of this study "point towards a regional connection mediated by changes in the regional energy equilibrium. One expression of this could be increases in the surface temperature that could act as environmental variables that determine the outbreaks of epidemics", concludes Rod'³.

The rise in temperature leads to an increase in the temperature of the waters of the rivers and estuaries in the region, which provides favourable conditions for the growth of the bacterium responsible for the disease, Vibrio cholerae. Similarly, the rise in temperature during the periods affected by El Ni'±o may also be linked to an appreciable increase in the thaw in certain areas of the Himalayas, which would affect the precipitation, the monsoons and the levels of the rivers. This in turn would influence the interactions between humans and the water, and thus the interaction with pathogens, and also the health conditions and the susceptibility to the disease.




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