Droughts are fundamentally altering infectious disease transmission worldwide, according to a major synthesis of nearly 100 studies published in Trends in Ecology & Evolution. Researchers at the University of Colorado at Boulder warn that prolonged dry spells and climate shifts drive animals and disease-carrying insects into shrinking water sources, creating dangerous new transmission hotspots.
How Dwindling Water Sources Create Contagion Hotspots
While many infectious pathogens and vectors rely heavily on moisture to survive, dry conditions do not simply cause diseases to vanish. Instead, scientists have found that the ecological response to drought is profoundly non-linear. As rivers, ponds, and reservoirs dry up across Europe and the United States, wildlife and disease-carrying insects are forced into ever-smaller pockets of remaining water.
According to the analysis published by researchers, these crowded remaining habitats become intense transmission grounds. The phenomenon was observed during a severe drought in California, where shrinking ponds concentrated amphibians together, resulting in severe parasite infections and higher disease rates among frogs.
“Climate change isn’t simply making Earth warmer or drier. 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.”
Pieter Johnson, University of Colorado at Boulder
Over the past few decades, more than 77 percent of Earth’s land has experienced drier conditions, driven by rising global temperatures. In the western United States, the worst megadrought in 1,200 years has drained vital reservoirs like Lake Mead to only a third of their capacity, shifting the underlying dynamics of natural ecosystems and pathogen exposure.
Global Pathogen Shifts: Winners and Losers in a Parched World
Not every pathogen benefits from arid conditions, but those capable of adapting to harsh environments or infecting multiple hosts are gaining ground. Pathogens that can survive outside aquatic environments and successfully target terrestrial animals have an edge over single-species specialists.
In North America, mosquito-borne illnesses such as West Nile virus and St. Louis encephalitis have become more prevalent. This increase occurs partly because dwindling water levels deplete aquatic organisms like dragonfly larvae that normally prey on mosquito larvae. Meanwhile, fungal spores responsible for conditions like Valley fever spread more easily through the abundance of contaminated dust churned up during dry periods.
Conversely, certain organisms struggle significantly under prolonged dry spells. The chytrid fungus, a deadly pathogen partially responsible for global amphibian declines, tends to spread poorly when moisture is scarce. Scientists emphasize that while certain infections may disappear in specific localities, extreme transmission surges elsewhere will offset those local losses.
“Some infections will disappear in certain places, but under different conditions, transmission can become so intense that it offsets the loss of infection elsewhere.”
Pieter Johnson, University of Colorado at Boulder
Heightened Public Health Risks Across the UK and Europe
European nations face distinct water-related pressures as climate change strains public infrastructure and degrades water quality.
Across Britain, where exceptionally dry weather has parched landscapes and left waterways depleted, scientists are closely monitoring domestic disease risks. Researchers from the University of Colorado at Boulder note that leptospirosis—commonly known as Weil’s disease—represents a significant concern in the UK. Carried primarily in the urine of rats and livestock, the bacteria contaminate surface waters and can infect humans through skin abrasions or mucous membranes, causing symptoms ranging from mild flu-like aches to life-threatening organ failure.
Public health authorities are also tracking vector-borne threats that could establish themselves as regional temperatures and weather patterns continue to shift.
“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.”
Pieter Johnson, University of Colorado at Boulder
Although West Nile virus is not currently circulating in British mosquitoes, the UK Health Security Agency detected fragments of its genetic material in domestic mosquitoes for the first time in 2023.
