In 2015-16, one of the strongest El Niño events in recorded history played havoc with the world’s weather. Among its impacts, heavy rainfall brought flooding to much of East Africa, affecting millions of people.
In the wake of the floods came a cholera epidemic, with outbreaks in Tanzania, Kenya, Somalia and Ethiopia. In Tanzania, for example, more than 24,000 people contracted the bacterial disease and 378 died.
Now a new study, published in the Proceedings of the National Academy of Sciences, has mapped how El Niño affects outbreaks of cholera in Africa. The research suggests that El Niño events shift the risk of cholera towards East Africa, resulting in an average of 50,000 extra cases in the region during El Niño years.
Knowing what impact El Niño has on the spread of disease means countries can be better prepared for events in the future, the researchers say.
Cholera is a diarrhoeal disease caused by consuming food or water contaminated with the bacterium Vibrio cholerae. While most people that become infected will have no – or only mild – symptoms, cholera can cause severe dehydration and death, if not treated.
In countries lacking access to adequate clean water and sanitation, outbreaks of cholera can spread rapidly. Globally, the World Health Organisation estimates there are 1.3-4 million cases of cholera each year, killing 21,000-143,000 people.
El Niño is a global weather phenomenon that originates in the Pacific Ocean. Every five years or so, a change in the winds in the equatorial Pacific causes a shift to warmer than normal ocean temperatures, affecting rainfall patterns worldwide. As the new study describes, this includes Africa.
As the map below shows, El Niño events tend to cause in a shift towards increases in rainfall in East Africa (green shading) and decreases across the Sahel and southern Africa (yellow).
And this, in turn, affects where cholera outbreaks occur, the researchers say.
Shifting disease burden
Using a statistical model, the researchers mapped over 17,000 observed and reported cases of cholera incidence between 2000 and 2014, in more than 3,000 locations.
They found that El Niño events don’t affect the total number of cholera cases across the whole of Africa, but they do shift where most of those cases occur.
You can see this in the map below. It shows where there are increases (red) and decreases (blue) of cholera cases during El Niño events. In general, the burden of cholera increases in East Africa during El Niño years and decreases in Madagascar and portions of southern, Central, and West Africa.
For example, between 2000 and 2014, East Africa experienced almost 50,000 more cholera cases during El Niño years, while southern Africa saw around 32,000 fewer.
Overall, around 45% of people in sub-Saharan Africa live in areas where outbreaks of cholera are affected – for better or worse – during El Niño years.
These areas largely overlap with where El Niño affects rainfall, the researchers say, which you can see if you compare the earlier map of rainfall changes with the one above.
However, the relationship between rainfall and cholera is quite complicated.
In drier areas, such as East Africa and the Sahel, less rainfall can trigger cholera outbreaks as people without access to clean water are forced to use unsafe drinking water sources.
“We suspect that increased rainfall in drier areas could be associated with increased cholera because the rainfall causes flooding or overrunning of sewer systems, which can contaminate drinking water.”
For areas that tend to be wetter – in Central and West Africa, for example – drier conditions might reduce the risk of flooding, and hence the chances of cholera outbreaks. And wetter conditions can also cause a decrease in cholera cases, explains Moore:
“In naturally wetter areas, increased rainfall may have a limited impact due to the higher volume of surface water in the region. Increased rainfall can also dilute [the concentration of cholera bacteria] in contaminated sources, reducing the cholera risk.”
Forecasting cholera risk
While the relationship between El Niño and cholera in Africa is far from straightforward, the findings “provide hope that we may be able to provide early cholera-risk forecasts,” the paper says.
Knowing an El Niño event is coming could give public health officials time to help lower the risk of an outbreak, explains Moore:
“Prevention efforts such as proactive vaccination campaigns could be organised in high risk areas. In addition, public health infrastructure, such as cholera treatment centres could be improved prior to the onset of the El Niño.”
This could become even more important in the future if extreme El Niño events become more frequent, as recent research suggests they might. However, this also depends on the wider impacts of climate change on Africa, says Moore:
“If climate change leads to extreme El Niño events occurring more frequently, then we may see a shift towards more frequent cholera outbreaks in East Africa; although this relationship may be altered by other climate trends such as the significant drying in the Horn of Africa over the last several decades.”
Understanding how the impacts of El Niño and climate change will interact is a challenge, notes Prof Mat Collins, Joint Met Office Chair in Climate Change at the University of Exeter, who wasn’t involved in the study. However, that shouldn’t prevent successful cholera risk forecasts from one season to the next, he tells Carbon Brief:
“I can see the potential for improving predictions on seasonal time scales, in which El Niño-Southern Oscillation forecasts are now quite accurate.”
Moore et al. (2017) El Niño and the shifting geography of cholera in Africa, Proceedings of the National Academy of Sciences, doi:10.1073/pnas.1617218114