Shrinking water bodies can turn drought into disease hotspots or dead ends
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.
Drought can transform water bodies into hotspots for diseases, or conversely, lead to a decline in aquatic pathogens. A recent study published in Trends in Ecology and Evolution provides a framework for understanding how drought impacts parasite traits and strategies. The research, led by Pieter Johnson from the University of Colorado Boulder, synthesizes various mechanisms by which water-associated diseases behave during dry spells.
Approximately 65% of the 437 parasites known to infect humans depend on water. Aquatic parasites can negatively impact important food sources like fish and crabs, and may harm tourism and recreational activities. While some droughts, such as a prolonged dry spell in Kenya during the early 2000s, have resulted in fewer waterborne diseases due to snail transmission of schistosomiasis, droughts often increase disease prevalence caused by water-dependent parasites.
The authors identify three mechanisms that determine whether a specific parasite will thrive or die out during drought: changes in host and parasite distribution, increased contact among hosts and parasites, and the creation of "desiccation traps." Concentrating animals and parasites around remaining water sources increases the likelihood of host-parasite encounters. However, if a water body dries up before transmission can occur, the parasite can't spread, leading to a reduction in overall infection rates.
Drought also alters water quality, including temperature, salinity, dissolved oxygen levels, and nutrient concentrations. These changes can either benefit or harm parasites, depending on the species. For example, saltier water can increase Perkins' disease in oysters but reduce copepod infections in fish. Warmer, drier conditions can weaken host immunity, making them more susceptible to infections like chytrid fungus in frogs.
Drought-induced habitat changes, such as the shrinking of water bodies and increased mosquito breeding grounds, can amplify the spread of vector-borne diseases like malaria. While the effects of drought on disease prevalence vary across different pathogens, the study suggests that droughts generally amplify parasites if they facilitate host gatherings and increase overlap between hosts and parasites, provided that both remain viable in remaining water sources.
Conversely, drought may decrease aquatic infections if it eliminates too many hosts, parasites, or water bodies.
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