A poison is a substance which in small quantities is capable of impairing health and destroying life. Animals in the feral state would appear to be largely endowed with an instinct which teaches them to avoid poisonous plants and other deleterious substances. The fox, lynx, and all the members of the feline tribe are suspicious to a degree in all that concerns their safety, and by the highly-developed sense of smell and taste they readily detect poison when introduced into flesh; though it be the "kill" or carcass they have reserved for a future meal.
Ages of domestication would appear to have so blunted these senses in horses that they will voluntarily take in their food many medicines which we are accustomed to regard as extremely nauseous. (See Methods of Administration.) It occasionally happens, therefore, that horses are poisoned, either by accident or personal malice, by the consumption of some toxic agent to which they have access in the stable or pasture.
Sudden and serious illness, with symptoms rapidly increasing in severity and without obvious reason, in animals apparently in good health up to the moment of seizure, is inconsistent with the majority of well-defined diseases, and affords sufficient cause to suspect some extraordinary or poisonous influence at work. Taylor, Stevenson, and others, famous by their special study of poisons, warn us that, though indicating a direction in which to make enquiry and search for the cause, such acute illnesses are not inconsistent with certain rare, but nevertheless well-known, causes for sudden and painful disease and death. The rupture of some large blood-vessel or abdominal organ, as the stomach, may lead to symptoms very similar to irritant poisoning.
Suspicions of poisoning may be justly entertained, and investigation pursued, although it might be unwise to express them, and at the same time calculated to defeat the object in view, especially where foul play has been practised. Wilful poisoning is happily infrequent in the present day, and its rarity tends rather to disarm suspicion. Moreover, sudden deaths among horses without previous "complaint" are quite common in large studs.