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Ivermectin is a medication used to treat parasite infections in animals. It works by interfering with the nervous system of parasites to ultimately paralyze and kill them. Ivermectin is a dewormer used to treat many types of intestinal, lung, and external parasites. It can also be used in heartworm-prevention medications for dogs. Ivermectin comes in several formulations, including injectables, oral solutions, pastes, topicals, pour-ons, chewable tablets, pills, and more.
While ivermectin is prescribed for dogs, toxicity can occur when dogs ingest more ivermectin than they should. This can occur if a dog is administered a dosage that is intended for larger animals, such as horses, or if a dosage for a greater weight/size of dog is inadvertently administered to a smaller dog. In these cases, the excessive ivermectin from overdose begins to interfere with the dog’s own nervous system and can cause life-threatening symptoms, such as seizures and coma, and can even lead to death.
Ivermectin toxicity is considered a medical emergency. If you believe your dog may be experiencing ivermectin toxicity, contact your local animal hospital immediately.
In dogs, ivermetcin is typically prescribed as an ingredient in a commercially available heartworm prevention. While there are many popular brand names, Heartgard® is one of the most recognizable heartworm preventions on the market that utilizes ivermectin as a main active ingredient.
The therapeutic monthly dose for heartworm prevention that utilizes ivermectin is well below the established doses of ivermectin toxicity. Ivermectin is approved for use in dogs at oral doses up to 0.024 milligrams per kilogram, whereas most dogs can tolerate doses of up to 2.5 milligrams per kilogram without issue.
However, each animal’s reaction to a medication can vary and all medications have the possibility to have unexpected reactions or toxicities at doses far below anticipated in some pets. Always follow the specific instructions of all medications, including heartworm preventions, as indicated by your veterinarian. If you accidentally administer the wrong dose/weight range of medication or your pet ingests more than one monthly dose/pill, contact your veterinarian immediately.
Most dogs tolerate ivermectin well, but there is a subset of dogs with a genetic mutation that makes them much more sensitive to certain medications, including ivermectin. This mutation is called the multidrug resistant mutation, or MDR1 mutation. It is most commonly seen in herding breeds, such as Collies, Old English Sheepdogs, and Australian Shepherds, but can be found in any breed.
The normal MDR1 gene in dogs produces a protein (P-glycoprotein) that helps to excrete and eliminate drugs. In dogs with the MDR1 mutation, this process is not as effective so drugs may linger in the dog’s system, resulting in higher-than-normal levels, particularly in the brain, which can lead to neurologic effects. This means that dogs with the MDR1 mutation are more sensitive to some drugs compared to dogs with the normal MDR1 gene.
Genetic testing is readily available to screen for the MDR1 mutation. It is recommended to discuss the benefits of testing with your veterinarian, especially if your dog is a herding breed known to be more at-risk for this mutation.
Dogs with MDR1 mutation are significantly more sensitive to ivermectin and can be affected at doses as low as 0.1 milligrams per kilogram as compared to normal dogs at 2.5 milligrams per kilogram. However, all FDA-approved heartworm prevention products use doses lower than those known to affect dogs with the MDR1 mutation and have been tested for safety in dogs with the MDR1 mutation. Therefore, these products are considered safe for most dogs if used at the manufacturer’s recommended dose. This includes pregnant and breeding dogs; however, ivermectin is not recommended for use in puppies under 6 weeks old. Heartworm prevention should not be started before 6 weeks of age.
Clinical signs usually develop within a few hours but may be delayed for up to 24 hours. The main clinical signs associated with ivermectin toxicity in dogs are related to the drug’s effect on the central nervous system and are therefore neurologic. Common symptoms in dogs include:
Depression
Disorientation, including loss of balance or coordination
Excessive drooling or hypersalivation
Vomiting and/or diarrhea
Increased vocalization (whining, barking)
Dilated pupils
Weakness
Trouble breathing
Muscle twitching
Tremors or seizures
Coma
Ivermectin toxicity in dogs occurs primarily through accidental overdose. Sources of the overdose may include:
Ingestion of a livestock ivermectin product manufactured for large animals (i.e., higher dosage than recommended for dogs)
Overdose of a canine ivermectin product, either due to incorrect weight/size range or ingestion of multiple monthly doses at once
Ingestion of manure of livestock that were treated with ivermectin for parasite control. (The drug is excreted in feces and, given the much higher doses used in livestock compared to dogs, can still pose a risk of toxicity in this form.)
Your veterinarian will start with a thorough history of any toxins your pet may have been exposed to. It is always helpful to tell your vet if your pet was recently on a farm or could have been exposed to livestock or livestock medications, even if you did not specifically see them eat an ivermectin product. If you know your pet ingested an ivermectin medication or product, please bring the product and label with you to the vet.
Your veterinarian will then conduct a physical examination to assess your pet’s neurologic status. A complete blood count, serum blood chemistry, and urinalysis will all likely be recommended for a baseline evaluation. While there are laboratory confirmatory tests to screen for ivermectin concentrations in the blood, these tests are not readily available and cannot be completed quickly enough to be of clinical assistance. Therefore, a complete history of any possible medication or toxin ingestion is crucial.
Dogs should be treated as soon as possible for ivermectin toxicity. Early diagnosis and aggressive treatment are crucial for successful outcomes.
If your dog ingested ivermectin within the past two hours and is not showing clinical signs, your veterinarian will likely start by inducing vomiting at the hospital as a means of decontamination. They may also give active charcoal to decrease the absorption of any remaining drug in the digestive tract. However, if your dog has already developed any neurologic signs, such as disorientation, tremors, or seizures, induction of vomiting or administration of activated charcoal is no longer considered safe due to the risk of respiratory aspiration.
Unfortunately, there is no specific antidote for ivermectin toxicity. Most dogs with signs of ivermectin toxicity will need to be hospitalized and placed on IV fluids for hydration and supportive care. Medications will be given to treat the symptoms of vomiting and diarrhea, as well as for muscle twitching and seizures. Depending on the severity of their condition, your dog may need to be placed on a mechanical ventilator to help them breathe.
While in the hospital, the veterinary staff will monitor your dog’s mental status, temperature, blood pressure, heart rate, respiratory rate and effort, and ECG. Extensive nursing care will likely be needed, including feeding tubes and management of the recumbent or comatose patient.
A newer treatment called intralipid therapy may be recommended to help pull the ivermectin out of the tissues faster and shorten the symptoms.
Recovery from ivermectin toxicity can vary from days to weeks. Prognosis depends on the dose ingested, the time it takes to receive care, and if the patient had concurrent factors, such as other medications or MDR1 gene mutation. Patients requiring more intensive care, including mechanical ventilation and prolonged ICU stays, will have increased cost of care, which may affect the pet parent’s decision about treatment.
However, dogs that do recover typically have no long-term side effects and can go on to live normal, happy lives.
As with most toxicities, prevention is key! Never give any medication to your dog without the explicit recommendation by your veterinarian. And always store all medications in a safe and secure place that pets do not have access to.
It is very important to always give the appropriate size/weight range product per manufacturer’s instruction when treating your pet with any medication, especially heartworm prevention and dewormers. Never give a large-animal product (such as ivermectin for horses) to your dog. Always monitor your dog carefully when on a farm to ensure they do not gain access to livestock medications or ingest manure.
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