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Ivermectin Toxicity While Herding

An Austalian Shepherd is poisoned from eating the feces of a recently dewormed sheep.
By JoAnna Lou, October 2015, Updated June 2021
Coming from a family with Shetland Sheepdogs and Border Collies, I've always avoided heartworm medication with ivermectin, a drug that some herding breed dogs are sensitive to, due to a genetic mutation. There's a simple test your veterinarian can perform to know for sure, but most of us with herding breeds just use alternate medications.

So ivermectin toxicity wasn't even on Laura Liebenow's mind last month when she brought her Australian Shepherd, Bristol, to a farm to herd sheep. During the herding lesson, Bristol ingested sheep feces, something my dogs have done many times. It wouldn't have been a problem, except that these sheep had been recently dewormed with a product containing ivermectin.

Soon after, Bristol started seizing and eventually became unresponsive. The poor pup was in critical condition and at risk or developing neurological damage. Bristol was quickly transferred to the veterinary school at Tufts University and placed on a mechanical ventilator. It took 10 days for Bristol to begin to breathe on her own and three weeks for her to regain consciousness. Once awake, Bristol needed the assistance of a cart and leg splints to walk. Incredibly Bristol made a full recovery after 40 days in the hospital--no easy feat.

Tufts says that they only treat one or two ivermectin toxicity cases each year, and they're usually from dogs that are exposed to higher-dose ivermectin products intended for farm animals.

A simple cheek swab can be performed by veterinarians to find out if your dog has the genetic mutation behind ivermectin sensitivity, but Bristol's story highlights the need to be vigilant when your pets are exposed to horses or farm animals. Next time I bring my dogs herding, I'll definitely ask if any of the animals were treated with ivermectin recently.

JoAnna Lou is a New York City-based researcher, writer and agility enthusiast.