Rains can help predict cholera, say scientists

Rains can help predict cholera, say scientists

New Delhi: Maybe the India Meteorological Department’s (IMD) brief will be enlarged beyond forecasting the weather to predicting cholera outbreaks. That’s because a team of scientists from India, Bangladesh and the US has found a strong link between the outbreak of the disease, rainfall anomalies and sea surface temperatures.

The study, conducted jointly by Kolkata-based National Institute of Cholera and Enteric Diseases (Niced), the University of Maryland and the International Centre for Diarrhoeal Disease Research, Dhaka, will appear in a forthcoming issue of the Proceedings of the National Academy of Sciences, a peer-reviewed journal published from the US.

It also linked the outbreak of the disease to the level of chlorophyll-a, a pigment found mainly in plants that influences the population growth of a class of fresh water and sea water-dwelling organisms called copepods, which host vibrio cholerae, the bacterium that causes cholera.

Cholera is transmitted to humans through food or water that is contaminated with the bacterium. The result is severe diarrhoea that may lead to dehydration, even death, if not treated promptly.

The research analysed cholera outbreaks in Kolkata over a nine-year period from September 1997 to December 2006 and concluded, among other things, that a 1 milligram/cu. m increase in chlorophyll-a increased cholera cases by 32.5%.

“With a predictive model in place, we can warn citizens, maybe tell them of the higher likelihood of cholera in a particular month, and this could probably make them more careful with their sanitary habits," said G. Balakrish Nair, director of Niced, who is one of the authors of the report.

In their report, the scientists have also formulated an equation that relates the number of cholera cases, sea surface temperatures, rainfall and chlorophyll-a levels in water sources in a region.

According to World Health Organization (WHO) data, India, which was the epicentre of six out of the last seven cholera pandemics in the last century, had an average 3,700 cholera cases a year between 1996 and 2005.

Though the mortality rate of cholera is low (less than 0.1%), it causes severe morbidity and affects the productivity of citizens, besides significantly adding to public health expenditure, said Nair. He added that actual figures of cholera cases in the country were grossly underestimated.

Though it’s known that cholera outbreaks typically follow seasonal cycles and that many international studies have shown links between climate variables and cholera cases, it’s the first time that such a study has been done for India, Nair said.

“Studies done elsewhere cannot be always applied here. In our own study, sea surface temperatures had a greater influence on cholera cases in Bangladesh than India," he pointed out.

The study used satellite-mapped data that measured algal blooms in the Bay of Bengal (that reflect the copepod population), sea surface temperatures and other climate variables—all of which are continuously mapped by IMD, the nodal weather agency, for the annual monsoon forecast.

Niced now plans to use the results of this study to find other climate variables that influence—and may potentially help predict cholera outbreaks—as part of a larger WHO-funded study. Already, the institute is in advanced stages of preparing a cholera vaccine, which Nair said would be ready in the next two years.

However, some doctors say such predictive models often lose their accuracy over time and it will be unwise to implement public health measures solely based on their results. “There are several climate models that can give a good explanation for past phenomena, but very often their predictions for the future go awry," said G.R. Pillai, a gastroenterologist and former adviser to the health ministry.

“Considering that cholera’s public health burden is over a century old, I am not sure that one such study will greatly accelerate state-level efforts to contain the disease."