Self-medication in horses
Date
2008
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Abstract
Horses are known herbivore generalists that must rely upon available plant forage for dietary needs. Diet selection in rangeland herbivores has been shown to be based upon post-ingestive physiological consequences. The basic premise of post-ingestive physiological consequences is the ability of an animal to associate the taste of a particular food with its possible hedonic shift. A negative hedonic shift results in the animal to avoid the taste in future encounters, while a positive hedonic shift results in the animal to seek the taste in future encounters. Thus, taste determines the palatability of plant forages thereby leading to an animal's ability to form a preference for food. Many available plants consumed by horses in natural habitats are known to contain secondary compounds referred to as toxins and all toxins are known to be drugs. Locoweed contains the toxin swainsonine and is known to cause the neurological condition described as locoism in large continuous doses. However, recent studies have shown that swainsonine has medicinal affects in humans and animals. The current study tested four chronically lame horses to examine their ability to form an association of a flavor, either carrots or apples, with a possible post-ingestive physiological consequence induced by a drug. There were two drugs utilized in this study; locoweed that contained swainsonine, and butorphanol tartrate a synthetic opiate analgesic (brand name Torbugesic). The horses were divided into two groups and each group was assigned a respective drug throughout the duration of two separate trials. The first trial associated a flavor with each group's respective drug treatment and the second trial involved the reversal of flavors while holding the drug treatments constant for each group. Each trial involved a conditioning period followed by test days when horses were challenged to make a decision between the treatment associated flavor or the non-treatment associated flavor. The horses were then challenged with the drugs returned to flavored feeds. The results suggest that horses do have the ability to associate a taste with a post-ingestive consequence induced by a drug. This suggestion gives insight into the horse's possible capability of self-medication.
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Subject
butorphanol tartrate
horses
locoweed
self-medication
swainsonine
psychobiology
behavioral psychology
physiology
range management