Child receives an oral vaccine.
A Yemeni child receives a cholera vaccination during a house-to-house immunization campaign in April. Credit: picture alliance/Getty Images

Increased rainfall in East Africa caused by a particularly strong El Niño and subsequent weather conditions may have helped usher in one of the worst cholera epidemics in modern history in war-torn Yemen.

“In Yemen, there is a catastrophic crisis in terms of human health and the availability of fresh water,” said Shlomit Paz, head of the Department of Geography and Environmental Studies at the University of Haifa in Israel and author of a research note published in July in Environmental Research.

Yemen has been embroiled in a civil war since 2015, and the cholera epidemic has been ongoing since 2016–2017. Cholera bacteria thrive in untreated water, and the Yemeni outbreak may be due to the breakdown of basic infrastructure (including water sanitation facilities) in the country. Estimates of more than 1.2 million infections make the Yemeni cholera epidemic “the largest in epidemiologically recorded history.”

Early Outbreaks in East Africa

Cholera cases first started to pick up in Somalia and surrounding countries in 2016, coinciding with particularly heavy rainfall due to extreme El Niño conditions.

Paz said it’s possible the cholera bacteria that started the epidemic in Yemen may have migrated out of East Africa through human hosts, but they also may have been carried by small insects. Chironomids, small midges that spend their early life stages in water, are known to carry the disease and can transmit bacteria between water reservoirs, she said.

Strong winds blowing from the Horn of Africa across the Gulf of Aden in July, August, and September 2016 could have carried the midges northeastward to Yemen, Paz said. There, they may have infected water supplies that helped start the epidemic.

Colin Stine, a professor of epidemiology at the University of Maryland who was not involved in Paz’s paper, isn’t sure that midges would have been able to carry sufficient loads of cholera bacteria to cause the epidemic in Yemen. He agreed that the Yemeni cholera epidemic likely originated in East Africa but thinks it came from a human source rather than from midges.

“It is very clear that climate change has a lot of effect on vector- and food-borne diseases.”

Stine said that just one human can carry cholera bacteria in amounts orders of magnitude higher than many midges could.

“The weakness of the paper [is] that it doesn’t have those kinds of calculations in it,” Stine said.

Stine said it’s hard to disprove the El Niño theory, but he pointed to a similar idea that circulated around a 2010 cholera epidemic in Haiti. Some researchers initially believed that weather conditions prompted by La Niña had a hand in causing the outbreak, but it was later found to have originated in a United Nations peacekeeping camp in the Caribbean country.

Regardless, Paz said that health authorities would gain a lot from monitoring weather conditions, especially because the warming climate will create ideal conditions for the rapid spread of disease.

“It is very clear that climate change has a lot of effect on vector- and food-borne diseases,” Paz said. “In order to be prepared for such an epidemic, I think that first of all there is a need for constant monitoring and forecasting—collaboration between scientists and doctors.”

Stine agreed that there should be more dialogue between environmental scientists and doctors tracking the spread of disease, though he’s not sure how important this will be for monitoring the spread of cholera.

—Joshua Rapp Learn, Freelance Journalist


Learn, J. R. (2019), El Niño may be a culprit behind the cholera epidemic in Yemen, Eos, 100, Published on 03 September 2019.

Text © 2019. The authors. CC BY-NC-ND 3.0
Except where otherwise noted, images are subject to copyright. Any reuse without express permission from the copyright owner is prohibited.