By analyzing nationwide records of temperature and diarrhea incidence in Peru, researchers have determined that a temperature increase of just 1 degree Celsius is associated with a 3.8% higher rate of childhood clinic visits for diarrhea, and that these increases are not mitigated by better water access.

The study, published Feb. 26 in Environmental Health, suggests that temperature increases due to climate change will amplify what is already a significant cause of child mortality in Peru and in the developing world at large.  

"This adds to a body of work that really shows that climate change is expected to increase the burden of diarrheal diseases, and more broadly that climate change has potential health impacts," said senior author Karen Levy, an associate professor of environmental and occupational health sciences at the University of Washington.

The study also found that moderate to severe El Niño events increase the incidence of childhood diarrhea. This poses an additional challenge for managing this illness in the wake of climate change, as El Niño events, marked by a warming of central and eastern Pacific Ocean surface waters, are expected to increase in severity in the coming years. 

Diarrheal diseases are one of the leading causes of child mortality worldwide. According to the U.S. Centers for Disease Control and Prevention, one in nine deaths in children under 5 is a result of diarrhea. One of the challenges of managing these diseases is that they are not caused by any single bacterium or virus but are instead driven by many pathogens, spread through poor sanitation conditions and contaminated water supply.

"Because of COVID, we're worried about going outside," Levy said. "Well, some people have to be worried about that every day because of the threat of infectious diseases in their communities."

While the impacts of climate change on human health are already a major topic of study among climate scientists and public health specialists, Levy explained that childhood diarrhea is a particularly important case study for these impacts.

"We've actually done a really good job of addressing diarrheal diseases over the past couple of decades, and rates have gone down substantially," Levy said, referring to a steady decline in diarrhea incidence since 2005. "Climate change then threatens to jeopardize that progress that we've made."

Unfortunately, this progress has already begun to unravel. In 2016, Levy and her team conducted a comprehensive literature review of the effects of meteorological conditions on childhood diarrhea and other waterborne diseases, finding that there is abundant evidence that childhood diarrhea incidence is increased by rising temperatures.

While the trends have been observed, the drivers for them are less understood.

"We don't actually know exactly what the mechanisms are, but we have some that are hypothesized," Levy said. "There are some things about the bug itself, like replication is increased under higher temperatures, which can lead to faster food spoilage. There's also some evidence that the virulence of the pathogen may be greater under higher temperatures."

And while childhood diarrhea is often studied on regional scales, few large-scale analyses have taken a whole country into account. This is the problem Levy and her team sought to solve with this study.

They tackled it by compiling a massive dataset, a task Levy described as "not trivial." They used data on diarrhea incidence from the Peruvian Ministry of Health for all 195 Peruvian provinces from 2005 to 2015. They cross-referenced this with weather data from the Peruvian Meteorological Institute, as well as data on water access and socioeconomic status.

When looking at trends in temperature, they found that when temperature increased by 1 degree Celsius over the three weeks before reported diarrhea cases, clinic visits increased by 3.8%. They also found that moderate to severe El Niño events resulted in more clinic visits.

Notably, they found that water access did not mitigate the temperature-dependent effects on diarrhea rates, even though in general, better water access results in less diarrhea.   

And while the percentages involved may appear low to some, the impacts of these trends are heavily magnified when considering the baseline frequency of childhood diarrhea in Peru. In 2005, at the start of the study period, the researchers report that there were 28.8 annual clinic visits for every 100 children under five years old. 

"When people are experiencing high rates of disease at baseline, then increasing that rate by any amount is problematic," Levy said. "The health impacts of climate change should be viewed with that lens."  

The researchers are next interested in using this information to create projections about how climate change will continue to impact childhood diarrhea cases, as well as seeing their work implemented in political and infrastructural action to address these issues.

And regardless of the impact of climate change or water access on childhood diarrhea, the researchers maintain that developing better medical infrastructure and water access for developing nations has no substantial downsides.

"It's a no-regrets strategy. If we put money and if we put resources into reducing diarrheal diseases, and climate change doesn't end up having the effect we anticipated, it will still be a win to reduce the disease burden," Levy said. "There's still this major burden of diarrheal diseases globally, and kids are dying from it, and they shouldn't be."

The study, "Meteorological factors and childhood diarrhea in Peru, 2005–2015: a time series analysis of historic associations, with implications for climate change," published Feb. 26 in Environmental Health, was authored by Miranda J. Delahoy, Benjamin Lopman, Thomas Clasen and Kyle Steenland, Emory University; César Cárcamo, Vanessa Vasquez and Gustavo F. Gonzales, Universidad Peruana Cayetano Heredia; Adrian Huerta, Waldo Lavado and Yury Escajadillo, National Meteorology and Hydrology Service of Peru; Luís Ordoñez, Ministry of Health of Peru; and Karen Levy, Emory University and University of Washington.