Ivermectin has become one of the most widely used deworming medications worldwide since its introduction 25 years ago. It's usually considered to have a large margin of safety, but it can be highly toxic in rare circumstances, with one possibly being when a horse eats silver nightshade and is dewormed with ivermectin around the same time. Tamara Swor, DVM, Dipl. ACVS, clinical assistant professor of veterinary medicine at Texas A&M University, discussed three cases of ivermectin toxicity at the 2008 American Association of Equine Practitioners convention, held Dec. 6-10 in San Diego, Calif.

The three affected Quarter Horses were all dewormed with the recommended dosage of ivermectin (0.2 mg/kg) about 18 hours before clinical signs were noticed. Two other horses on the farm with identical management, except for not being dewormed, were unaffected (the owner had only three tubes left in the box of ivermectin). Dewormer from the same box had been used before on the same horses with no ill effects.

The affected horses ranged from 4 to 13 years old. All had essentially normal temperature, pulse, and respiration rates, but they also had progressive bilateral mydriasis (pupil dilation in both eyes), decreased pupillary light reflex, ataxia (incoordination) in all four limbs, and decreased response to menacing movement (they didn't "flinch" as readily). Two horses were depressed, mentally dull, and had flaccid lips, while one was hypersensitive and agitated; the latter was euthanized when her signs progressed to the point that she was dangerous to herself and her handlers. The remaining two horses made a full recovery (one within days, the other within a few months) with supportive fluid treatment and flunixin meglumine (Banamine).

Swor explained that ivermectin normally does not cross the blood-brain barrier and enter the brain, but if it does (as can happen in susceptible dog and cattle breeds), it can cause toxic signs such as those seen in these horses. The euthanized horse's brain had 131 parts per billion of ivermectin, which Swor noted was consistent with toxic levels in dogs. "No animal should have ivermectin in its brain," she commented.

The horses' hay and water, along with the ivermectin remaining in the used tubes, all tested clean of contaminants, and no toxic plants were found in the euthanized horse's stomach. But researchers still believe that nightshade ingestion might have been a factor, due to previous research--showing that nightshade impaired the blood-brain barrier of rabbits--that was published in a brief discussion in a toxicology textbook and a previous article in The Horse about ivermectin and silver nightshade (for more information visit #6882 at TheHorse.com).

"Ivermectin toxicosis is uncommon in mature horses, but it can occur even at the appropriate dosage and there may be similar implications for moxidectin and other related products as well," Swor concluded. "Further studies are needed to determine the role of toxic plants. Silver nightshade toxicity in these horses is unproven, but it could have been a factor. The magnitude of clinical signs in horses is variable; signs include those previously discussed. Consider ivermectin toxicosis in horses with acute neurologic impairment especially right after antiparasitic medications have been given. Long-term complications are unknown, but recovery is possible with supportive care."

Editor's note: Silver nightshade is primarily found in semiarid regions around the world. In the United States this includes the American Southwest and Pacific Northwest. It might also be found in hay produced in these regions.

About the Author

Christy M. West

Christy West has a BS in Equine Science from the University of Kentucky, and an MS in Agricultural Journalism from the University of Wisconsin-Madison.

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