Climate Conditions Contributed to the Cholera Pandemics of the 19th and 20th Centuries

The combination of pathogen evolution and climate anomalies, such as those during El Niño events, significantly contributed to the spread of cholera. The research, led by the Barcelona Institute for Global Health (ISGlobal), a centre supported by “la Caixa” Foundation, highlights the synergistic role of climate and pathogen changes in the emergence and spread of infectious diseases.

During the 19th century, cholera spread across the world from its original reservoir in the Ganges delta, in India. Six subsequent pandemics have killed millions of people on every continent. The current (seventh) pandemic started in South Asia in 1961, reached Africa in 1971 and the Americas in 1991. Since 2022, the number of cholera cases has risen sharply in many countries, particularly in places with poor socioeconomic conditions.

How the different cholera pandemics originated and spread has been a matter of debate. Scientists have long suspected genetic changes in the bacterium. For example, during the seventh pandemic, the dominant strain (“Classical”) was gradually replaced by another strain (“El Tor”), against which people had less immunity. But Xavier Rodó, ICREA researcher at ISGlobal, and his colleagues suspected that climate conditions could also play a role by facilitating the regional spread of the emerging strain, once it reaches a new area.

Going back in history

Using historical records, Rodó and his team examined the links between climate patterns and cholera deaths across different provinces of former British India during the sixth cholera pandemic (1899-1923). They compared these findings with the more recent seventh pandemic and the brief emergence of a new cholera strain in Bangladesh. They used advanced statistical methods to study how outbreaks in different regions were connected and how they related to changes in temperature and rainfall, particularly during events like El Niño, which causes significant climate fluctuations and extreme weather conditions.

One of the most striking findings was the synchronisation of cholera outbreaks in the Bengal region, which is a sign of a new strain of cholera for which the population lacked immunity. This was accompanied by an El Niño event from 1904 to 1907. It appeared that abnormal weather conditions, similar to those seen in the late 20th century during cholera expansions into Africa and South America, played a crucial role in helping the new strain outcompete the previous one. In particular, the unusual rainfall during 1904-05, which was among the most extreme on record, seemed to provide the perfect conditions for the new cholera strain to thrive and spread.

“Our findings indicate that genetic shifts in the dominant cholera strain, combined with unusual climate conditions, led to the emergence and spread of the sixth pandemic”, says Rodó. "However, climate facilitation does not always lead to a new strain replacement unless the new strains are already shedding in the environment and can take over. We have seen how even larger climate anomalies in the recent past, have not always been accompanied by new strains replacing former ones. These results highlight the nonlinear dynamics of such relationships and the need for a joint action of both factors in the same direction", he explains. The findings also underscore the role of environmental conditions in generating opportunities for the spread of new variants or new pathogens.

“We need to better understand and predict the dynamics of climate-sensitive pathogens, such as those transmitted by water or vectors,” adds Rodó. This understanding is particularly important to prevent and mitigate future pandemics as we face increasing climate variability and extremes due to global warming.

"Variation in climate conditions or the evolutionary change of a pathogen can be important drivers of major epidemics. But these two drivers are typically considered separately in studies seeking to explain the emergence of unusually large outbreaks. Here, we present indirect evidence that the two can act together to synergistically underlie the establishment and widespread transmission of a new strain", says Mercedes Pascual, senior author of the study and researcher at the New York University.

Reference

Rodó X, Bouma MJ, Rodriguez Arias MA, et al. Strain variation and anomalous climate synergistically influence cholera pandemics. Plos NTD. 2024.