Scraping horses
Found this interesting for anyone curious about U.S. Army Great War-era veterinary and farrier services for transport, cavalry, and artillery horses.
22 January 1919, U.S. Army of Occupation in Montabaur, Rhineland, Germany (official caption):
Horses from 1-7th Field Artillery [part of the 1st Infantry Division at the time] being led to “Dipping Vat” constructed by 1st Engineers for the Veterinary Dept. The animals take a plunge in a bath composed of sulfur, lime, carbolic acid, and creosote. The bath is kept at a temperature of 100 degrees fahr. After the plunge, the animals are “scraped.” This is the method of treating these animals for the mange [probably rain rot] and cooties. Horses are bathed at a rate of one a minute.
U.S. Army Signal Corps Photo 111-SC-51250 by SGT J.A. Marshall, via NARA
“Ready to Plunge.” 111-SC-51252 by SGT J.A. Marshall, via NARA
“Scraping Horses.” 111-SC-51251 by SGT J.A. Marshall, via NARA
laststandonzombieisland 
The chemical mix sounds terrible, but is pretty much what you use for rain rot today. Rain rot is a fungal infection/infestation of the skin that occurs when a horse is wet for a period of time and can’t get dry, kind of like athletes foot for the body. Today used a product with coal tar, sulphur and some modern antifungal and antibiotic ingredients on the quarter horse/ welsh cross that is kind of chronic with rain rot. Didn’t have the 1st Engineers to build a dipping vat, though.