Drought might seem like bad news for diseases that depend on water.
After all, if lakes, ponds and wetlands shrink or disappear, parasites that need water should have fewer places to survive and spread. But scientists say the reality is much more complicated.
In some situations, drought reduces disease. In others, it can unexpectedly create ideal conditions for infections to spread.
A new paper published in Trends in Ecology and Evolution offers a framework for explaining this “drought-disease paradox.” Researchers examined how drought changes the relationships among parasites, their hosts and the environments they share.
The issue matters for human health. Of 437 parasites known to infect people, nearly 65% depend on water in some way. Aquatic diseases can also affect wildlife, kill important food sources such as fish and crabs, and damage tourism and recreation.
Sometimes drought helps stop disease. In Kenya, for example, a prolonged drought in the early 2000s killed snails that transmit schistosomiasis, a parasitic disease that affects humans. With fewer snails available to carry the parasite, human infections dropped significantly.
But drought can produce the opposite result. During major droughts in the Horn of Africa in the early 2020s, water shortages forced some communities to rely on unsafe water sources, contributing to serious cholera outbreaks.
The researchers identified several reasons why drought can have such different effects.
One is that shrinking water supplies can push animals together. When only a few ponds or watering holes remain, wildlife may gather around them in large numbers. This creates more opportunities for parasites to move between animals. Smaller water bodies can also make it easier for aquatic parasites to find their hosts.
But if a pond completely dries out before a parasite can complete its life cycle, the opposite happens. The researchers call these situations “desiccation traps.” Parasites become stranded and die before they can spread, causing infection rates to fall.
Drought also changes the quality of the remaining water. As water evaporates, temperature and salt levels may rise, oxygen can fall, and pollutants and nutrients can become more concentrated. Some parasites benefit from these conditions, while others cannot tolerate them.
At the same time, harsh environmental conditions can weaken animals. Warmer, drier ponds, for example, may reduce frogs’ immune defenses, making them more vulnerable to infections such as chytrid fungus.
Drought can also favor species that are particularly good at surviving dry conditions. Mosquitoes are one important example. Shrinking water bodies can leave behind shallow pools and patches of standing water that provide excellent mosquito breeding habitat. More mosquitoes can then increase opportunities for mosquito-borne diseases to spread.
The researchers say the key question is whether enough hosts, parasites and suitable water remain during a drought. If drought brings hosts and parasites closer together without killing them, disease may increase. If water loss destroys the habitats or species that parasites need, infections may decline.
As climate change increases drought risks in many regions, understanding this difference could become increasingly important. The new framework may help scientists and public health officials predict which water-related diseases are likely to become more dangerous—and which may disappear—as the world becomes hotter and drier.
