Ivermectin Toxicity With Appropriate Doses
A report in the September 1 edition (Swor et al, 2009) of the Journal of the American Veterinary Medical Association describes three horses with ivermectin toxicity. All three animals were adult Quarter horses from the same farm, and were among five horses that received a single tube of ivermectin paste (Bimectin) each. The tubes were the last in a box of tubes, the rest having been given to other horses on the farm with no problems.
Approximately 18 hours after all five horses were treated, three started to show signs of depression, drooling and ataxia (e.g. wobbly, unstable gait, poor balance). One horse was severely affected, and was sent to Texas A&M's equine hospital. The neurological abnormalities progressed in that horse, it became recumbent (i.e. unable to stand) and blind, and was euthanized. Nothing remarkable was detected on necropsy. Testing of the brain revealed an ivermectin concentration similar to the levels found in dogs with ivermectin toxicity. (Normally, ivermectin should not be detectable in the brain after deworming, because it should not be able to cross the blood-brain barrier.) The other two horses were less severity affected. They also ended up being hospitalized but recovered.
This is the first report of complications developing in adult horses treated with appropriate doses of ivermectin. Earlier reports of such complications have involved young horses receiving overdoses.
So what happened? A couple of possibilities were considered and dismissed:
Too much ivermectin in the product: Some residual paste was tested and it did not have excessive ivermectin levels. Also, other horses treated with tubes from the same box were fine. Therefore, it's very unlikely that too much ivermectin was given.
A genetic sensitivity to ivermectin: All of the affected horses were unrelated.
The most likely explanation is compromise of the blood-brain barrier. As mentioned above, the blood-brain barrier should keep ivermectin from reaching the brain, but if this important barrier was somehow damaged, then ivermectin could have reached the brain and caused the signs of toxicity. Although there was nothing to indicate that another condition was present that would affect the blood-brain barrier, this is still the most likely cause. The authors suspected that a toxin from a plant may have been responsible for weakening the barrier, such as toxicity from silverleaf nightshade. Some infectious diseases can also affect the blood-brain barrier, but there was no evidence of these. In the end, a definitive cause could not be identified.
This report shouldn't scare people away from using ivermectin or other dewormers. This is a rare complication from use of a very common drug. Rather, it should remind us that there are potential risks with virtually all drugs (including supplements), and that the cost-benefit of using them must always be considered. It's also another good reason to use dewormers prudently. Ivermectin toxicity should be considered in any horse that develops neurological abnormalities after treatment, and any potential adverse reactions should be reported so the extent of the problem can be determined.
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