When I got my first pony more than a dozen years ago, she came from a place that was far from a model of good horsekeeping. My friend who found her for me recommended that I immediately worm her with Ivermectin when I got her home. I dutifully did as directed, though when I read the label before dosing her, I realized it was far from a simple procedure. The label indicated that ivermectin is toxic to both canines and fish. My dog was my constant companion, and the pony paddock bordered a creek containing trout. I therefore was vigilant in picking up all my pony’s manure for a few days before my dog could get to it and taking it away from the creek and burying it so my dog wouldn’t ingest it and runoff from it wouldn’t get into the creek.
I follow the current recommended protocol for worming, which is to perform fecal tests and then deworm only if test results indicate it is necessary. About a year ago, after a positive fecal test, I asked my veterinarian if the wormer he was recommending had any side effects for dogs or fish. He looked at me like I was crazy but politely responded no. I then told him of my first use of a chemical wormer, but he said he’d never heard of canine or fish toxicity at all.
An article in the recent issue of Equus magazine helped me understand that my memory was accurate after all, despite the vet’s reaction. (1) The article’s author had received similar responses from veterinarians, so apparently canine ivermectin toxicosis isn’t a well-known problem.
It turns out that ivermectin is often used in canine worming medications as well as in the treatment of mange, so it’s understandable that vets wouldn’t consider horse dewormers based on ivermectin to be an issue for dogs unless they are consumed in very high doses. Unfortunately, there are certain breeds of dogs that carry a genetic mutation that makes them highly susceptible to ivermectin toxicosis. And many of us have these types of dogs – shepherds, collies, and other herding breeds – because they are such good companions in our equine lifestyle. The mutation is at its highest incidence in Collies at 70% and in Australian Shepherds at 50%. My loyal canine companion is an Aussie. In other breeds the incidence is much lower, from 5 to 30%.
The symptoms of canine ivermectin toxicity are alarming, including sudden blindness, aggression, and incoordination. If you use an ivermectin-based chemical wormer on your equine friends, and you count canines as companions, too, I highly recommend getting a copy of the article in Equus so you can take appropriate precautions for your situation.
1) Langmesser, Karenna. “A Danger to Dogs,” Equus, Issue 413, February 2012, p. 51.