Severe drought conditions spreading across the world are reshaping disease transmission patterns in unexpected ways, according to a new scientific review published last week in Trends in Ecology and Evolution. The research, which examined nearly 100 studies on infectious diseases across animal species, identified a counterintuitive phenomenon: while some water-dependent illnesses decline during droughts, others flourish. Diseases including cholera, West Nile virus, and dengue have shown increased transmission during periods of water scarcity, contradicting the assumption that water-borne pathogens require abundant water supplies.
The concentration of animal populations around remaining water sources during droughts creates ideal conditions for disease spread. When natural water bodies shrink, animals congregate at remaining watering holes, increasing contact and exposure to parasites and pathogens. Drought can also alter water chemistry, making conditions warmer and more concentrated, which favors certain dangerous organisms. Additionally, the clustering of animals around limited water sources means higher concentrations of feces and other contamination, facilitating parasite transmission. Birds infected with West Nile virus, for instance, come into closer proximity with mosquitoes that then spread the disease to human populations.
Historical examples demonstrate the public health consequences of drought-driven disease spread. A 2014 superdrought in California contributed to the state’s second-highest recorded cases of West Nile virus, while droughts in the 2000s led to thousands of excess valley fever cases. In 2017, Somalia experienced nearly 80,000 cholera cases linked to drought conditions, resulting in over 1,000 deaths. Brazil continues to see elevated dengue risk in the months following drought periods.
Climate change is intensifying these dynamics by creating more frequent and prolonged droughts globally. A United Nations report indicates the world has become 77 percent drier over the past three decades. Scientists note that understanding how drought affects disease transmission will help predict which pathogens pose the greatest future threats to human health. Weather patterns characterized by alternating dry and wet periods, increasingly common as the planet warms, can further exacerbate outbreaks when droughts break and precipitation returns.
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