Toxins: Problems of the Equine Gastrointestinal Tract

Poisonings occasionally happen in horses, usually because of inadvertent ingestion of toxic plants or chemicals. While poisons may harm multiple organ systems, one research team* recently confirmed that exposure to poisons can damage the gastrointestinal tract.

“Even though a toxin may ultimately result in nerve, kidney, or liver damage, the cells located within the intestinal tract may be wounded and must be considered during treatment in cases of toxicity,” explained Kathleen Crandell, Ph.D., a Kentucky Equine Research nutritionist.

Examples of poisonings provided by these researchers included:

Heavy metal toxicity. “Medicinal or dietary miscalculations and contaminated pastures or water are the most common ways excessive heavy metals, such as mercury, selenium, lead, and cadmium, find their way into a horse,” Crandell said.

As the most commonly encountered heavy metal contaminant, lead causes severe neurologic disease with pronounced laryngeal paralysis. In addition, excessive ingestion of lead and other heavy metals brings about severe colic and hemorrhagic gastroenteritis (bloody diarrhea).

Plants and forages causing physical damage. Various grasses contain spiny awns. When ingested, awns can damage the soft tissues in the oral cavity and esophagus. While grazing, the needlelike awns can puncture the skin on the horse’s face and around the eyes. If they penetrate the skin and snap off from the plant, the awns can embed and then migrate to distant locations, causing ulcerations and abscesses.

“These horses can have difficulty eating, salivate excessively, and lose condition” Crandell said. “Because of the potential to migrate, the awns are sometimes difficult to find and surgically remove.”

Plants and forages causing obstruction. Some plants with indigestible fibers accumulate in the gastrointestinal tract, binding with other plant materials, hair, debris, and magnesium ammonium phosphate to create concretions, or solid masses, that can obstruct the gastrointestinal tract and lead to colic. Some of those concretions, also referred to as phytobezoars, can weigh up to 22 lb (10 kg).

Plants containing toxins. Many grasses and weeds contain chemicals that are poisonous to horses. Examples include certain nightshades, pokeweed, castor bean, morning glory, foxglove, and a myriad of others. While the specific toxins in these plants differ, many induce inflammation and cause sores on the lips and in the mouth, colic, and diarrhea. Those toxins can cause other severe health problems.

Easy ways to minimize exposure of these diet-related toxins and irritants include pasture rotation to minimize overgrazing and analyzing baled hay for contaminants such as grasses with awns, large seeds, and burrs.

“When pastures are overgrazed, weeds can overtake the area, choking out grasses and legumes. Horses preferentially consume nutritious forage, but when none is available, they will graze plants that aren’t typically suitable for horses,” noted Crandell.

For information on toxic plants in your area, contact your local extension office. In times of hay shortages, contact a Kentucky Equine Research nutrition advisor for help choosing hay alternatives.

*Stegelmeier, B.L., and T.Z. Davis. Toxic causes of intestinal disease in horses. Veterinary Clinics of North America: Equine Practice. In press.