Climate and Cholera: an increasingly important link

A study by the coordinator of the Research Group on Climate at the Barcelona Science Park, University of Barcelona, Dr Xavier Rodó, and other researchers at the University of Michigan and the International Center for Diarrhoeal Disease Research in Bangladesh, provides evidence not only that climatic variation associated with the El Niño-Southern Oscillation (ENSO) affects the appearance of cholera epidemics, as shown by a previous publication in Science by the same group, but also that the relationship has become stronger in the past few years, in parallel with the intensification of ENSO caused by climatic change. The study is to be published in Proceedings of the National Academy of Science (PNAS) on 17 September.

The relation between infectious diseases and climate has been a subject of controversy in the scientific community in recent years. According to Rodó, this study “is one of the first demonstrations of the possible effects of climatic change on the incidence of infectious diseases”. In addition, the study has led to the development of a new statistical tool that has enabled researchers to show that the correlation between cholera and climate is three times as strong as that shown by earlier statistical studies.

The study was performed in cooperation with Mercedes Pascual at the University of Michigan and George Fuchs and Anwaral S. G. Faruque at the International Center for Diarrhoeal Disease Research in Bangladesh.

In the earlier study, published in Science (8 September, 2000), the authors found evidence that ENSO influenced the cholera cycles using climate and epidemic data from the area of Bangladesh for the last two decades. In contrast, in the paper to be published on 17 September, the authors have enlarged the earlier study with new statistical tests also based on data for the period 1893-1940. Their aim was to see whether the influence of climate on the cholera cycles had become more intense in recent years.

The statistical study revealed a very high correlation between the effects of ENSO and the appearance of cholera epidemics in recent times, and a weaker or absent correlation in the historic data. This shows that the incidence of cholera is associated with the climatic change induced by ENSO. After warm periods, the incidence of the disease increases, while it decreases markedly after cold ENSO episodes.

This correlation is also revealed by a predictive model that was developed during the study, which could be used to plan health measures in the areas affected.

The authors emphasise that these results probably reflect the changes caused by the joint action of ENSO and planetary warming. However, this correlation between the variation shown by ENSO and climatic change has yet to be demonstrated, and it remains a source of controversy among scientists. Nevertheless, it has been shown that since the end of the 1970’s, in parallel with planetary warming, the warm periods induced by ENSO have been progressively more intense. In South-East Asia, the regional rise in regional temperature as a result of climatic change and ENSO can lead to the proliferation of bacteria that inhabit salty waters and thrive at high temperatures. These findings, together with the fact that people tend to have more contact with water during hot periods, may explain the higher correlation between ENSO and cholera in recent decades.

*Cholera is an acute infectious disease of the small intestine, caused by the bacterium Vibrio cholerae and characterized by profuse watery diarrhoea, vomiting and leg cramps. It is transmitted via contaminated food and water, and is present in more than 75 countries.