Worldwide, diarrheal diseases kill more than 1,300 children each day, according to the United Nations Children’s Fund (UNICEF). The bulk of these deaths occur in children under age 2 in southern Asia and sub-Saharan Africa.
Scientists have long associated outbreaks of diarrheal diseases with weather patterns such as the El Niño–Southern Oscillation (ENSO). ENSO oscillates in a 3- to 7-year cycle between El Niño (associated with warmer ocean temperatures) and La Niña (associated with cooler ocean temperatures), which affect local weather patterns, including temperatures and precipitation.
“El Niño events have been linked to diarrhea outbreaks in Peru, Bangladesh, China, and Japan, but studies of the effects of ENSO on diarrheal disease in Africa have been limited to cholera,” researchers wrote in a recent study focused on diarrhea-related deaths of children in Botswana. In addition to cholera, researchers found Rotavirus, Norovirus, Salmonella, and other pathogens to be agents of diarrhea in the region studied.
“Under-5 diarrhea incidence data [are] hard to come by. Existing studies have just taken advantage of where data [are] available,” noted Alexandra Heaney, a postdoctoral scholar at the University of California, Berkeley, and the first author on the study. (Heaney was a doctoral student in environmental health sciences at Columbia University’s Mailman School of Public Health when the study was conducted.)
Team members studied 2007–2017 data from Botswana’s Chobe District, which has an approximate population of 25,000 people. They analyzed a monthly time series of sea surface temperature anomalies, weekly diarrhea incidence reports collected from 10 health care facilities, and environmental data—including the height of the Chobe River, daily rainfall and minimum temperature data, and satellite-derived regional rainfall data.
Studying the Chobe River was crucial to researchers, as many diarrheal diseases, including cholera and Giardia, are waterborne illnesses. Treated water from the Chobe River is the main water source for eight of the district’s nine villages.
Links Between La Niña and Childhood Diarrhea
“La Niña conditions are associated with cooler temperatures, increased rainfall, and higher flooding in the Chobe region during rainy season,” Heaney and her colleagues wrote. Researchers looked at ENSO data lagged up to 5 months. Lagged La Niña conditions were “associated with higher than average incidence of under-5 diarrhea in the early rainy season,” the researchers noted.
The connection held even after a Rotavirus vaccine was introduced in July 2012. “It makes sense that the vaccine didn’t modify the relationship [between ENSO and diarrhea outbreaks] much, because Rotavirus is a dry-season pathogen and we saw our strongest ENSO diarrhea effects in the wet season,” Heaney said.
“This is an interesting and important study, though looking only at the statistical significance of the results might give the impression that they are more rock-solid than they actually are. It is always easy to be misled when looking at the impact of only a few events in time series analysis,” Justin Lessler, an associate professor at the Johns Hopkins School of Public Health who has studied connections between El Niño and cholera in Africa, wrote in an email to Eos. “The problem is made somewhat worse by the number of different pathogens that cause childhood diarrhea. However, even with these limitations, the work is compelling and is a type of analysis that I would like to see more of,” he added.
Getting a Handle on the Problem
“I think the main takeaway from this study is that freely available climate information (ENSO indices) can be used to predict under-5 diarrhea up to 5 months in advance in a developing region with limited resources for prospective planning,” Heaney noted. “In locations where resources are scarce, planning…can make a huge difference in preventing morbidity and mortality from a diarrhea outbreak.”
Although there are water treatment plants that process drinking water from the Chobe River before it is distributed for consumption, additional work is needed to understand how these treatment processes need to be altered to remove more diarrhea-causing pathogens from the water.
“Unfortunately, we don’t know what is happening to the water between the river and people’s homes. We assume that the water treatment plant is not sufficiently cleaning the water, but we would need to acquire information about the treatment plant, which was not available to us, and, ideally, measure the quality of water coming out of the treatment plant and the water in people’s homes,” Heaney said.
The study was published in Nature Communications in December 2019.
—Rachel Crowell (@writesRCrowell), Science Writer