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This work is licensed under a Creative Commons Attribution 4.0 License.


Water is a requirement for all organisms, including equids. Dehydration-caused mortality of feral horses (Equus ferus caballus) is often cited as a cause of concern and as justification for management of feral horses, yet a paucity of information exists on the matter. We conducted a systematic review from September 1, 2020 through January 15, 2021 of available news reports of feral horse and burro (E. asinus) dehydration mortalities and public interventions to save horses using a public search engine with a priori defined search term combinations and additional snowball sampling. We found 15 uniquely reported mortality incidents representing 744 horse in the United States and Australia that occurred between 1976 and 2019; no similar reports for burros were found. Mortalities occurred during hotter and drier than normal conditions with occurrences escalating through the summer and fall. The number of horses per dehydration mortality event ranged from 1–191 with a mean of 50 horses. Mortalities occurred on a wide range of land jurisdictions including private lands, tribal lands, national forests, national parks, and Bureau of Land Management lands. Increasing feral horse populations in western North America and Australia, coupled with the drought forecasts over the next century, simply cannot be ignored. This study represents the first global and longitudinal assessment of feral horse dehydration mortalities.