Long known as a powerful disruptor of global weather, El Niño also shifts the geography of cholera in Africa.

When associate professor of Epidemiology Justin Lessler and colleagues mapped annual cholera incidence in Africa from 2000 to 2014, they found something unexpected: While the number of cholera cases across the continent was roughly the same in El Niño and non-El Niño years—about 210,000 cases—there was a big difference in where the disease occurred.

During El Niño years, East Africa experienced about 50,000 more cases of cholera, while Southern, Central and West Africa saw decreases. Interestingly, while most increases in incidence were seen in areas with increased rainfall, some areas with decreased rainfall also saw increases.

“We weren’t able to pick out a single clear association between rainfall and cholera incidence that applies across the continent,” Lessler says. “The epidemiology of cholera is extremely local.”

Knowing when and where cholera incidence is likely to increase can save lives—especially when El Niño forecast information is available up to a year in advance. “We have a vaccine that can be used preemptively,” Lessler says. “If we can get resources and training on the ground ahead of time when risk is highest, we can potentially eliminate mortality from cholera.”