Selenium is an essential nutrient that has a relatively narrow window between ingested amounts that result in deficiencies and those that cause toxicoses. Historically, occurrences of livestock disease that mimic clinical presentation of chronic selenium poisoning were recorded in the 13th century (Martin, 1973). Marco Polo wrote of such cases in western China in 1295. In 1560, Father Simon Pedro described human cases of presumably chronic selenosis in Columbia (Benavides and Mojica, 1965). The first documented record of selenium poisoning in livestock was reported in 1860 by US Army surgeon (Martin, 1973). T.W. Madison described a fatal disease of horses that grazed near Fort Randall, South Dakota. It also has been speculated that the horse illness that slowed General Custer's Cavalry relief may have been due to selenium, but chronic selenosis generally takes weeks to develop.
Hall, J. 2007. Selenium In: Veterinary Toxicology 453-460.