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