Experimental studies of drug-induced impaction colic in the horse.

The research paper discusses the effects of introducing the drug amitraz to horses, and how it induces a condition called impaction colic, characterized by a stark cessation of intestinal sounds, stasis, and impaction in the large colon. Findings indicate that this condition could serve as a unique model to understand the pathophysiology of impaired motor activity in the equine large intestine.

Research Methods and Findings

• The researchers administered amitraz, a formamidine acaricide, either topically or intravenously to horses and ponies. The resulting condition was characterized by the rapid cessation of intestinal sounds, stasis, extensive impaction, and tympany throughout large colon.
• Upon necropsy of three animals, a hard lump of excrement known as a faecalith was found to be obstructing the proximal small colon. Notably, this disruption is located aboral, or downstream in the digestive tract, to the marked colonic impaction.
• The researchers were able to reproducibly induce a reversible case of impaction colic with the intravenous injection of 1 mg of amitraz per kg of body weight. Injection-induced symptoms included both central nervous system and intestinal signs.

Implications of the Study

• Large intestinal contents were observed to dry out rapidly, implying enhanced fluid absorption. The effects could be specifically attributed to amitraz rather than one of its known metabolites or to other formamidines.
• Despite the clear effects of the drug, its exact action in the horse remains unclear. The researchers posit that it could involve the mediation of neuromodulators from the intrinsic enteric nervous system. These neuromodulators could potentially affect the coordination of myoelectrical activity from putative pacemaker regions in the large intestine. They may also affect fluid and ion transport.
• Impaction colic that occurs naturally may have a similar pathogenesis, or manner of origin, as the drug-induced form of the condition observed in this study.
• The findings of this research suggest that drug-induced impaction colic provides a unique model for studying the pathophysiology — the study of how disease processes affect the body — of impaired motor activity and ion transport in the equine large intestine.