This article has been reviewed according to Science X's editorial process and policies. Editors have highlighted the following attributes while ensuring the content's credibility: Prolonged dry spells or droughts can sometimes reduce aquatic diseases, many of which affect people. Yet other times, water scarcity can counterintuitively lead to an uptick in diseases that depend on water.
A new Cary-coauthored paper published in Trends in Ecology and Evolution helps untangle this "drought-disease paradox." The paper establishes a framework for understanding parasite traits and strategies likely to be favored during drought. "We've created a general theory for how water-associated disease works during drought," explained co-author Tara Stewart Merrill, an aquatic disease ecologist at Cary Institute of Ecosystem Studies, "by synthesizing the many ways drought can influence the survival and interactions of different hosts and parasites. Understanding these mechanisms can help us determine which parasites are likely to become emerging problems in an increasingly drought-prone world." Of 437 parasites known to infect people, nearly 65% depend on water.
Aquatic parasites can also kill important food sources such as fish and crab and can harm tourism and recreational activities. Nobody wants to swim in a lake clogged with dead fish. Sometimes drought can work in our favor—for example, in Kenya, a prolonged drought in the early 2000s killed snails that transmit schistosomiasis and led to significantly fewer cases in humans.
"But in many cases, we actually see an unexpected amplifying effect, where all of a sudden, drought results in more disease from parasites and pathogens that are tied to water bodies," said Stewart Merrill. For example, the Horn of Africa suffered major cholera outbreaks in the early 2020s when drought-induced water shortages forced people to rely on unsafe water sources. "There has been a tremendous amount of excellent work on individual host-parasite systems, but most studies understandably focus on one disease or one ecosystem at a time," said the paper's lead author, Pieter Johnson of the University of Colorado Boulder.
"Our goal was to step back and ask whether there were common ecological mechanisms that could explain these seemingly contradictory patterns across many different systems." The idea for the study grew out of Johnson's observations while studying amphibians and their parasites in ponds across California. During multiple droughts between 2012 and 2025, many of the ponds dried up, and some diseases became less common, but others became more severe. "Those observations made me realize that drought was changing much more than water levels," said Johnson.
"It was fundamentally altering the ecological interactions that determine how diseases spread, and that realization ultimately inspired this paper." The new paper brings together scientific research and theory to identify three mechanisms that can help determine whether a specific parasite will thrive or die out in a drought. Changing how animals and parasites are spread out over time and space is one mechanism by which droughts can help or hurt parasites. Having less water on the landscape often means that animals and parasites concentrate in and around the water that persists, increasing contacts between hosts, between species and between hosts and parasites.
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