A horse charity has warned people to keep their pet dogs and cats away from ivermectin-based horse deworming products because of toxic effects such as blindness.

Ivermectin has risen to prominence during the COVID pandemic due to being controversially touted as a potential treatment for people infected with the virus.

The drug can come in various forms. Some formulations can be used to treat certain parasitic infections in humans, while others are used to get rid of worms in horses and other animals.

But the drug is only well tolerated when used in appropriate doses in approved situations, according to the U.S. Centers for Disease Control and Prevention (CDC).

This week, the British Horse Society said that the drug can cause sudden onset blindness and other symptoms in dogs even if they lick or eat a small amount of ivermectin-based horse deworming medicine.

In a Facebook post on Wednesday, the group wrote: "Be aware that some dogs belonging to certain breeds, for example Collies, can be more sensitive to ivermectin-based wormers and these dogs can show signs of poisoning even when a small amount is eaten.

"If you ever suspect your pet has eaten equine wormer or they seem unwell having been around your horse at the time of worming, contact your vet for advice."

The charity shared a photo of a dog from the Red Bluff Veterinary Clinic in California that had sudden blindness and dilated pupils after it ate a small amount of horse dewormer that a horse had spat out. The dog recovered.

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The charity advised horse owners to keep dogs and cats out of the area when deworming a horse, clean up any spillages, safely dispose of used de-worming syringes and safely store unused ones, and prevent access to stables afterwards until they have been cleaned.

The U.S. vet group VCA Hospitals notes that while some ivermectin formulations may be used as a heartworm medication in cats and dogs, it can have serious neurological side effects when doses are too high.

VCA Hospitals states ivermectin can cause a "shock-like reaction" in some dogs and advises owners to contact their vet if their animal experiences side effects like stomach upset, vomiting, dilated pupils, or a dazed demeanor.

Some dog breeds, like collies, are particularly sensitive to ivermectin and can experience side effects even at low doses due to a genetic mutation called MDR1.

The American College of Veterinary Pharmacists states that it is important to check the weight range and ensure pets are getting the correct dose of medication.

It adds that toxicity may occur even if pets are exposed to the feces of large animals that have recently been treated with ivermectin-based medications.

According to the CDC, a recent study that looked at trends in ivermectin dispensing from retail pharmacies in the U.S. showed that the number of weekly prescriptions had increased from an average of 3,600 before the pandemic to 88,000 prescriptions in the week ending August 13, 2021—a 24-fold increase.

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