Ivermectin dewormer is considered safe for horses, even at up to 10 times the recommended dosage. But results of a recent case series documented by researchers at Texas A&M University (TAMU) revealed that horses consuming plants from the toxic Solanum (nightshade) family could be in danger of ivermectin poisoning, even when the anthelmintic is dosed appropriately.
The case study focused on six horses residing at one farm, all of which rapidly developed neurologic signs less than 24 hours after ivermectin paste administration. When the horses were presented to Texas A&M's Large Animal Clinic, the staff investigated further to determine the exact cause of the signs.
"With the history of recent deworming and a herd outbreak, we strongly suspected ivermectin toxicosis," explained Tracy Norman, VMD, Dipl. ACVIM, a clinical assistant professor in the TAMU Department of Large Animal Clinical Sciences and lead author in the case study. "We also wanted to look at the hay ... since the toxicology folks (that work at the Texas Veterinary Medical Diagnostic Laboratory) ... have long suspected an interaction with the toxic weeds and ivermectin."
Norman relayed that the six horses displayed clinical signs including:
- Lack of muscle coordination;
- Facial nerve dysfunction, including drooping lips, dilated pupils, and weak tongue tone;
- Muscle twitching and head tremors;
- Mild fever;
- Fecal and/or urinary retention; and
- Head/facial edema.
All six horses regained normal neurologic function within four days of hospitalization.
Analysis results revealed traces of the administered anthelmintic to contain 1.2% ivermectin, which is far below toxic limits. Serum ivermectin concentrations in the six horses ranged from 39-110 ng/mL, which was higher than expected, but still not considered toxic.
Analysis of the round bale available to all affected horses revealed large amounts of both silverleaf nightshade (Solanum eleagnifolium) and Western horse nettle (Solanum dimidiatum).
"Solanum toxins are known gastrointestinal irritants and might have ... enhanced absorption of ivermectin," the research team reported, noting they suspect that toxins from the plants might have led to "accumulation of ivermectin in the brain."
To reduce the risk of ivermectin/Solanum species poisoning, Norman recommends that horse owners "maintain their pastures by mowing and judicious weed control. Buying hay from a reputable dealer and checking hay for weeds would helpful as well."
The study, "Concurrent Ivermectin and Solanum Species Toxicosis in a Herd of Horses," appeared in the Journal of Veterinary Internal Medicine in November 2012. The abstract is available online.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.