From the American West to Europe, droughts are drying up rivers, fueling destructive fires and reshaping ecosystems. They are also changing the spread of infectious diseases, leading to surges in illnesses like cholera and West Nile virus around the world.
In a review paper published August 12 in Trends in Ecology & Evolution , CU Boulder ecologist, Pieter Johnson , and his collaborator, Tara Stewart Merrill at the Cary Institute of Ecosystem Studies, reveal how drier conditions could make humans and wildlife more vulnerable to certain infectious diseases, even though many parasites and disease-carrying organisms depend heavily on water to survive.
“Climate change isn't simply making Earth warmer or drier,” said Johnson, distinguished professor in the Department of Ecology and Evolutionary Biology. “It is making droughts more variable and less predictable. That unpredictability is becoming one of the defining challenges for understanding and managing disease in natural ecosystems.”
Over the past few decades, more than 77% of Earth’s land experienced drier conditions , according to a United Nations report. In the western United States, the worst megadrought in 1,200 years has drained crucial reservoirs like Lake Mead to only a third of their capacity. Rising temperatures driven by climate change are making droughts more frequent, more intense and more widespread.
While many infectious diseases are transmitted in water or through hosts like mosquitoes that rely on water to reproduce, drought doesn’t simply make those diseases disappear. For example, previous studies have found that periodic droughts in Florida increased the spread of St. Louis encephalitis virus, which is transmitted to people through infected mosquitoes.
Johnson first noticed these shifts in disease patterns while studying waterborne parasites in California that cause infected frogs to develop additional limbs, which in turn make them more vulnerable to predators. When a severe drought hit the state in 2014, Johnson and his team noticed that shrinking ponds forced amphibians to concentrate in remaining water bodies, resulting in more severe infections among frogs and higher infection rates.
“That got us curious about why the loss of water sometimes kills off pathogens, but in other cases actually amplifies them. It seemed that the effects of drought on disease patterns are not linear,” Johnson said.
To better understand how drought affects disease transmission, Johnson and Stewart Merrill synthesized nearly 100 previous studies and identified the main mechanisms through which infectious diseases may respond to drought.
One key pathway is that as drought dries up rivers and ponds, animals have to congregate around the remaining watering holes and habitats. These places can become hotspots for parasite transmission, as happened with the frogs in California.
Droughts can also make certain parasites and disease carriers that are adapted to harsh conditions more successful. In North America, mosquito-borne diseases like West Nile virus and St. Louis encephalitis virus have become more prevalent, in part because dwindling water levels have reduced the aquatic organisms like dragonfly larvae that prey on mosquito larvae. Valley fever, a fungal disease, infects people and other animals through contaminated dust, which is more abundant in drier times.
Not every pathogen benefits from drought, Johnson said. Chytrid fungus, a deadly pathogen partially responsible for global amphibian declines, tends to spread poorly under dry conditions.
“We are expecting to see shifts in the types of parasites and diseases that predominate in a drying world. Some infections will disappear in certain places, but under different conditions, transmission can become so intense that it offsets the loss of infection elsewhere," he added.
To predict which pathogens might become winners or losers as drought intensifies, Johnson identified characteristics that may determine their success. Pathogens that can infect a variety of hosts, like avian flu, are likely to fare better than specialists that infect only a single species, such as measles. Those that can survive outside aquatic environments and make their way to terrestrial animals will have an edge over those confined to aquatic species.
“As climate change brings more frequent and extreme droughts, we want to anticipate how disease transmission will change so we can better forecast outbreaks and prepare for them," Johnson said.
Trends in Ecology & Evolution
Plagued by drought: how water scarcity reshapes host–parasite interactions