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