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  Discussion on Case Study: Ivermectin toxicosis in three adult horses
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DrO

Posted on Friday, Nov 6, 2009 - 10:47 am:

Here we go again. A second reported cluster of ivermectin poisonings in horses over the entire history of ivermectin use in horses. I think it is important to put this in perspective of toxicological studies in horses: Symptoms of toxicosis did not occur until the dosage rate reached 10X the labeled dose and those were generally mild and the horses recovered. I don't think water is that safe.

So what is happening here that 3 horses on one farm become ill? The product was checked for concentration and found to be OK and the brain of the horse that died was checked and yes higher than expected concentrations of ivermectin were there.

For those who remember the last case (1999?) where 8 of 14 horses became ill there was an association with exposure to silverleaf nightshade (Solanum elaeagnifolium) plant. A plant known to cause neurological signs also. In the article on this case report it was noted that no exposure to silver nightshade was found. However it was noted that the hay was of poor quality. Is it possible that there was just one area of the hay had silver nightshade that was all consumed? I hope someone plans to visit the hay field next summer to see if any is growing there.

The take home message here is that this is a very safe product that makes a remarkable improvement in your horses health. However in very rare (certainly less than one time in 10 million considering how many horses receive ivermectin everyday) instances of toxicosis outbreak have occurred. The common factors seem to be exposure to weeds or overall poor nutrition. see ivermectin article for more
DrO

J Am Vet Med Assoc. 2009 Sep 1;235(5):558-62.
Ivermectin toxicosis in three adult horses.

Sword TM, Whittenburg JL, Chaffin MK.
Department of Large Animal Clinical Sciences, College of Veterinary Medicine and Biomedical Sciences, Texas A&M University, College Station, TX 77843.

Case Description-3 adult Quarter Horses were evaluated for acute, progressive neurologic signs 18 hours after oral administration of 1 dose of 1.87% ivermectin paste. Clinical Findings-Clinical signs included depression, forelimb and hind limb ataxia, DrOoping of the superior and inferior lips, and muscle fasciculations. Bilateral mydriasis, decreased pupillary light reflexes, and absent menace reflexes were evident. Clinical signs progressed in severity for 36 hours after administration of the ivermectin. Treatment and Outcome-All horses were treated supportively with IV administration of fluids and anti-inflammatory medications. Two horses survived with no apparent long-term sequelae. One horse was euthanized, and a high concentration of ivermectin was detected in its brain tissue at postmortem examination. Analysis of the ivermectin concentration in the paste product revealed that the concentration was approximately that indicated on the packaging. Clinical Relevance-Ivermectin toxicosis is an uncommonly reported condition in equids that should be considered when acute neurologic impairment develops after ivermectin administration. Recovery is possible with supportive care and time.
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