COLIC AND DIARRHEA INDUCING PLANTS
Poisonous plants are but one of many causes of colic and diarrhea in horses. The action of various plant toxins may either have a direct irritant effect on the intestine, causing increased motility, colic, and diarrhea, or they cause the same effects by acting on the nervous system. Yet other plants may cause severe colic through obstruction or impaction of the small or large intestine, respectively. Fruits of plants such as Cockspur hawthorn (Crataegus crusgalli), mesquite (Prosopis glandulosa), and persimmon (Diospyros virginiana) when eaten by horses can cause obstruction of the small or large intestine. Plants such as halogeton (Halogeton glomeratus), greasewood (Sarcobatus vermiculatus), and shamrock, soursob, or sorrel (Oxalis spp.) contain high amounts of oxalate, which have the potential to cause stomach and intestinal inflammation and diarrhea. Prolonged consumption of low amounts of oxalate from these plants, or other plants containing low amounts of oxalate, may cause a calcium-deficiency.
Horses eating the fruit, seeds, or leaves of avocado (Persea americana) trees usually die within a few days or less, depending on the amount consumed, as described later in this chapter in the section on ''Sudden Death-Inducing Plants.'' Prior to death, however, colic, diarrhea, and signs of acute congestive heart failure occur. A variety of other common plants also may be incriminated as a cause of colic and diarrhea if horses are deprived of normal forages. Invasive pasture plants such as leafy spurge (Euphorbia esula), wild iris (Iris missouriensis), horsetail or scouring rush (Equisetum arvense), bitter weeds (Helenium spp.), and a variety of mustard plants (Brassica spp.) should be considered as a cause of colic and diarrhea when other causes are not found.
Determining that colic or diarrhea are due to a plant is difficult because generally there are no specific lesions of plant poisoning detectable in the gastrointestinal tract at postmortem examination, and it is difficult to identify plants in the gastrointestinal tract once they have been chewed and acted upon by digestive enzymes. When plants are suspected of causing colic and diarrhea, a careful history and a thorough examination of the horse's pasture and food should be made in an attempt to identify plants given in Table 18-2.
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