An El Nino event may have helped create the environment conducive for spread of cholera by driving climate extremes such as unusual temperatures and rainfall, according to a new study that analysed strains circulating during the early 20th-century pandemic in India.

Researchers found that the environment could have helped in creating and spreading a new strain of the bacterium Vibrio cholerae, first discovered in 1884 by German microbiologist Robert Koch, that caused the water-borne disease affecting the small intestine.

The role of environment in spreading cholera has been debated for the last two centuries, according to the team of researchers, including those at the Barcelona Institute for Global Health, Spain.

The cholera pandemic during 1899-1923 was the sixth since the first in 1817, and documented to have been especially lethal in India, Arabia and northern Africa. In 1900, an unprecedented 700,000 deaths were reported from provinces across India, which was then under the British rule.

In this study, published in the journal PLoS Neglected Tropical Diseases, the researchers analysed historical data of prevailing climate conditions and cholera deaths across various regions in India.

Their analysis revealed that the patterns in cholera deaths coincided with the unusual seasonal temperatures and rainfall driven by the El Nino event of 1904-1907.

The researchers correlated the timing of these unusual weather events with the development of a new invasive strain, suggested by a "very steep rise in the case fatality (that) was apparent from the harbour population in the capital Calcutta." Additionally, the team also analysed past climate conditions and cholera data for the ongoing 'El Tor' pandemic, which began in 1961.

The authors found that the climate conditions in the past bear similarities to strong El Nino events that have been linked with cholera strain changes -- new strains replacing predominant ones -- during the ongoing seventh pandemic. Each year, there are about 13-40 lakh cases and 21,000 to 1.4 lakh deaths around the world due to the bacterial disease, according to the World Health Organization.

"Our results support a role of climate acting as a major driver of the 1904-07 anomalous cholera episode, which would have facilitated the establishment of the novel strain," the authors wrote.

They further examined the possibilities of new cholera strains emerging aided by a warmer climate, using models.

The authors found that climate extremes, driven by global warming, could boost the chances of new strains of cholera to emerge through the end of the current century.

"Increased climate variability and extremes under global warming provide windows of opportunity for emerging pathogens," they wrote.

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