Those who have not looked into the matter have the idea that actual combat was the chief source of the destruction of horseflesh. But, as a matter of fact, that source is probably not to be credited with one-tenth of the full losses of the army in this respect. It is to be remembered that the exigencies of the service required much of the brutes in the line of hard pulling, exposure, and hunger, which conspired to use them up very rapidly; but the various diseases to which horses are subject largely swelled the death list.
John D Billings, 10 MA battery
Hardtack and Coffee, 1887
If the army was not near a river, watering the horses might require miles of riding and then result in brackish or muddy puddles.
George Peck, in his memoir, has an entertaining account of shooting glanders-infected* horses, and then managing to give away the ones that could recover with good care. *a contagious bacterial disease that created swelling on the jaws and discharge from the nostrils