Using Ivermectin as a Heartworm Preventive in Felines: Ashley's Question Answered

Is Ivermectin Appropriate for Heartworm Prevention in Cats?

"Hello Dr. Mark. I recently read your guide on ivermectin as a heartworm preventative in dogs. My questions are as follows: Is this method safe for use in cats? And if so, since my cats are 9–10 lbs, would 0.1 ccs of the sheep drench be the correct amount to use, mixed with 0.3 ccs of propylene glycol? Thanks so much for the guide and advice." —Ashley

Safety of Ivermectin in Cats

Ivermectin can be used safely in both cats and dogs. The dose recommended on the label of commercial heartworm preventives for cats is 55 micrograms for cats weighing up to 5 pounds, and 165 micrograms for those between 5 and 15 pounds. That might seem like a large amount for a 5.5-pound cat, but according to Small Animal Toxicology (third edition), the dose for heartworm prevention in cats is 0.024 mg/kg, and the margin of safety is wide.

When using the sheep drench, that translates to roughly 0.13 ccs of the 0.08% ivermectin product for a 10-pound cat.

What Happens If a Cat Gets Too Much Ivermectin?

Measuring such a small dose for a cat can be tricky, but because ivermectin has a wide safety margin, giving slightly more is not a problem.

The same textbook reports that the toxic dose of ivermectin in cats ranges from 0.2 to 1.3 milligrams per kilogram. For a 12-pound cat, that would mean toxicity could occur only after receiving 1–8 ccs of the sheep drench. Obviously, you would not want to approach that amount, but if you accidentally give a bit extra—say, 0.2 cc—it is still a very low dose.

0.2 cc equals 160 micrograms, which is a safe amount and closely matches what is used in commercial heartworm preventives for cats. It is acceptable for your 10-pound cats.

There is no need to dilute the sheep drench before administering it to a cat.

Obtaining Sheep Drench

Because ivermectin has shown some antiviral properties, people began purchasing the veterinary formulation to treat COVID. It can now be bought from Amazon and is also stocked at most local feed stores.

Generic heartworm preventives are available without a prescription from certain pharmacies in Canada and Australia.

Heartworms Resistant to Ivermectin

Another factor to weigh is your geographic location. Cats are generally more resistant to heartworms than dogs, but in the Mississippi valley and parts of the southern United States, a strain of heartworms has emerged that is resistant to ivermectin preventives.

Research has been conducted to find alternatives for cats (1). If you live in a region where resistant heartworms are present, you should consult your local veterinarian about which medication they currently recommend for heartworm prevention in cats.

Since cats are typically resistant to heartworm infection, ivermectin remains a suitable option if you are not in such an area.

Reference

(1) Pullins A, McTier TL, Mahabir S, DeRose G, Hedges L. The efficacy of a topical formulation of selamectin plus sarolaner in preventing the development of a macrocyclic lactone-resistant strain of Dirofilaria immitis in cats. Vet Parasitol. 2020 Jun;282:109122. i.nlm.nih.gov/32460111/

This article is not intended to replace diagnosis, prognosis, treatment, prescription, or individualized guidance from your veterinarian. Animals showing signs of distress should be examined by a veterinarian without delay.