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Ivermectin is the active ingredient found in many popular (and effective) heartworm preventatives, such as Heartgard, Tri-Heart Plus, and Iverhart Plus. The compound lives up to its reputation as a “wonder drug.” Not only does it shield dogs from heartworm, but it also has emerging uses as a cancer and antiviral medication.
Key Points
As with any drug, there are potential risks, side effects, and dosage instructions dog owners should know. Most notably, some herding and mixed breeds carry a genetic mutation that makes them especially sensitive to the drug. If you ever have questions regarding heartworm prevention, ivermectin, or any medically related concern, consult a veterinarian.
What Is Ivermectin for Dogs?
Ivermectin was discovered in the late 1970s, harvested from a single organism (Streptomyces avermitilis) isolated at the Kitasato Institute in Tokyo. It’s available through a veterinary prescription, usually in chewable tablets given monthly.
“Ivermectin is used to treat and prevent certain types of parasitic infections,” explains Dr. Max Jones, DVM, a Partner at Bahama Road Veterinary Hospital in Bahama, North Carolina. “While not effective against all parasites, ivermectin and related drugs (macrocyclic lactones) are effective against a broad range of invertebrates, including many nematodes (roundworms), mites, and even ticks. Other common parasites, such as tapeworms, are not affected.”
The drug works by paralyzing the parasites’ body-wall and pharyngeal muscles, preventing them from maturing into adults and spreading to other bodily organs. It goes a long way in preventing heartworm disease and its subsequent complications — some of which can prove fatal. Using this drug is also often significantly less expensive than treating heartworm.
“Heartworm disease is the perfect example of ‘an ounce of prevention’ being worth ‘a pound of cure,'” says Dr. Jones. “Treatment for heartworm disease is involved, expensive, and carries the potential for serious complications. Prevention is safe, easy, and cheap.”
A Genetic Mutation Makes Some Dogs Sensitive to Ivermectin
Dr. Jones notes that ivermectin is generally safe for dogs, and even those with a current parasitic infestation can take the drug with some precautions. However, generally safe doesn’t mean 100% safe.
Dogs with a mutation of the multidrug resistance 1 (MDR1) gene can develop a reaction called “ivermectin toxicity,” a severe, life-threatening reaction. As the name suggests, this gene mutation makes affected dogs more sensitive to certain drugs, such as Imodium® and certain cancer treatments.
As noted, ivermectin works by paralyzing parasitic nematodes. It doesn’t affect mammals because the agent doesn’t cross the blood-brain barrier, a semi-permeable membrane that regulates what substances can pass from the bloodstream to the brain. The blood-brain barrier of dogs with the mutated MDR1 gene allows ivermectin to pass through, resulting in neurological distress.
“In the case of overdose, ivermectin toxicity generally causes neurologic signs, such as lethargy, disorientation, incoordination, blindness, and tremors. More severe cases can progress to seizures, coma, and death,” shares Dr. Jones. “There is no specific antidote, and treatment is largely supportive.”
The only way to diagnose a dog’s mutated MDR1 gene is through scientific testing. The good news is that it’s easy to get a sample. Owners can swab the inside of their dog’s cheek and send the sample away for testing. You can utilize an AKC DNA + Health Kit, or the Washington State University and UC Davis also offer testing services.
Which Dog Breeds Can Be Sensitive to Ivermectin?
Research has found that up to 75% of some herding breeds can have a mutated MDR1 gene. In the United States, three in four Collies have the mutation, with the same rate of occurrence in other countries.
Other breeds prone to the mutation include:
- Australian Shepherds
- Border Collies
- German Shepherds
- Miniature Australian Shepherds
- Old English Sheepdogs
- Shetland Sheepdogs
- Skye Terriers
Some All-American dogs with herding parentage may also be prone to the MDR1 mutation.
“Even with the MDR1 mutation, low doses of ivermectin, such as those used for monthly heartworm prevention, are generally well tolerated, but higher doses can cause toxicity issues,” Dr. Jones says. “Higher dosing for problems, like mange, should be monitored more closely, and alternative options should be considered for dogs likely to be carrying the MDR1 mutation or any dog that has shown sensitivity to ivermectin.”
What to Know About Ivermectin Toxicity
While ivermectin is extremely beneficial in preventing heartworm, it’s critical to ensure that dogs receive the appropriate dosage, even those without the MDR1 mutation.
“There is definitely such a thing as too much ivermectin, especially for dogs affected by the MDR1 mutation,” says Dr. Jones. “Accidental overdosage with extra-label use of large animal ivermectin is the most common cause of ivermectin toxicity in a dog. Dogs have also been treated for ivermectin toxicity after ingesting feces from large animals that had recently been dewormed.”
Monitoring your dog on walks and exercising caution in off-leash areas can go a long way in preventing ivermectin toxicity from ingesting contaminated feces from horses, cattle, and other farm animals prescribed ivermectin in large doses.
Symptoms of Ivermectin Toxicity
It’s critical to recognize the signs of ivermectin toxicity, so you can make informed medical decisions regarding your pet’s health. Symptoms of ivermectin toxicity can vary from acute to mild. Acute symptoms can manifest within 4 to 12 hours of ingesting ivermectin, while more serious symptoms may become apparent over two to three days.
Signs include:
- Lethargy or depression
- Disorientation
- Drooling
- Loss of appetite
- Slow heartbeat
- Dilation of pupils
- Trembling or seizures
- Inability to stand
- Difficulty breathing
- Sudden blindness
“Side effects are uncommon with appropriate dosing of ivermectin. Oral doses used for heartworm prevention are typically safe, even in a patient with the MDR1 mutation,” Dr. Jones notes.
Prevention Is Key When Combating Heartworm
There is no denying the effectiveness of ivermectin when preventing or combating parasitic infections in dogs. Your pet’s veterinarian is an amazing resource for everything regarding your dog’s health, whether that’s preventing heartworm or determining whether they have a genetic mutation and need other forms of prevention.
“Ivermectin has saved countless dog lives, mainly through its role in heartworm prevention,” Dr. Jones says. “Even Border Collie owners shouldn’t fear ivermectin-based heartworm prevention. Higher doses require more careful monitoring, especially in herding breeds and can often be avoided by utilizing other classes of medications.”
This article is intended solely as general guidance, and does not constitute health or other professional advice. Individual situations and applicable laws vary by jurisdiction, and you are encouraged to obtain appropriate advice from qualified professionals in the applicable jurisdictions. We make no representations or warranties concerning any course of action taken by any person following or otherwise using the information offered or provided in this article, including any such information associated with and provided in connection with third-party products, and we will not be liable for any direct, indirect, consequential, special, exemplary or other damages that may result, including but not limited to economic loss, injury, illness or death.