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

Colic was induced in horses and ponies following topical or intravenous (iv) administration of amitraz, a formamidine acaricide. The condition was characterised by rapid cessation of intestinal sounds, stasis, extensive impaction and tympany throughout the large colon. Three animals that were necropsied had a faecalith obstructing the proximal small colon aboral to marked colonic impaction. A reproducible and reversible impaction colic syndrome could be induced by an iv injection of 1 mg amitraz/kg body weight in solvent. There were immediate central nervous system and intestinal signs. Large intestinal contents dried out rapidly, possibly indicating enhanced fluid absorption. The effects could be attributed to amitraz rather than to one of its known metabolites and was not shown by other formamidines. The action of amitraz in the horse is unclear. It could involve the mediation of neuromodulators (from the intrinsic enteric nervous system) which affect the coordination of myoelectrical activity from putative pacemaker regions in the large intestine and, possibly, fluid and ion transport. Naturally occurring impaction colic could have a similar pathogenesis. Drug-induced impaction colic might provide a unique model for studying the pathophysiology of impaired motor activity and ion transport in the equine large intestine.