This toxic reaction occurs especially in dogs that are genetically hypersensitive to ivermectin, an anti-parasite medication most commonly used for heartworm prevention, or to treat ear and hair mites, which can lead to mange. Ivermectin prevents or kills parasites by causing neurological damage to the parasite, resulting in paralysis and death for the parasite. But dogs genetically sensitive to the medication have an anomaly that allows the ivermectin to pass the dog’s blood-brain barrier and into its central nervous system, which can be lethal for the animal.
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While the sensitivity to this type of medication is not always guaranteed, the following breeds are most likely to be affected (although not every “sensitive” animal in the breed is affected):
Washington State University Veterinary School developed a DNA test for a mutant gene which produces adverse reaction to ivermectin and other medications in some dogs. They have discovered that 75% of Collies, 25% of Border Collies, 8% of Old English Sheep dogs, mixed dogs of these breeds, plus other breeds have this mutant gene.
It is also seen in mixed-breed dogs, older dogs that have experienced a blow to the head, puppies, dogs that have overdosed on similar types of drugs, or other central nervous system diseases that affects the blood-brain barrier. Treating dogs that are susceptible to ivermectin toxicity with parasitic medication should be only be done under a veterinarian’s supervision and with great caution.
For breeds predisposed to the disease, an autosomal recessive trait (MDR-1) gene has been identified that causes a defect in the p-glycoprotein multidrug transporter to the blood brain barrier. This allows the ivermectin to pass into the brain at low dosages thus causing toxicity. This trait may also cause toxicity from other related drugs and should also be avoided or used in lower doses in susceptible breeds.
Toxicity can occur in cats, although it is uncommon.
Symptoms for the dog may be acute or mild. Acute signs will become apparent within 4 to 12 hours of the drug’s administration. In mild cases, symptoms will occur between 48 to 96 hours after your dog has been treated. Such symptoms include:
Unfortunately, ivermectin toxicity cannot be reversed. Therefore, it is best to make your pet comfortable and treat the symptoms to the best of your ability. If exposure has occurred within the past four to six hours, induce vomiting and/or administer activated charcoal for the purposes of minimizing absorption. Be on the lookout for signs of secondary complications.
Some or all of the following measures may also be recommended by your veterinarian:
Much will depend on the severity of the dog’s reaction, along with its initial overall health. It may take several weeks of dedicated care before the dog fully recovers.
There is a test available to check sensitivity to ivermectin. If your dog is one of the breeds that is prone to ivermectin toxicity, you might consider testing for it. If you decide not to have the testing done, be cautious about using ivermectin to prevent heartworm disease or for the treatment of mites.
In dogs with MDR 1 gene mutation, the following drugs should be avoided or used with caution:
WHY TELL YOU ALL THIS?
I include all this scary stuff so you know what can go wrong when using ivermectin on your own to treat your pets. It’s the same ingredient used in the popular heartworm meds you can buy for cats and dogs, but I know someone whose dog died when she administered Zimecterin Gold, a horse wormer, to her dogs. I don’t know the particulars in her case — what breed her dog was, how much she dosed, etc. I offer this info so you know what can go wrong and are as informed as possible before going this route. I assume no responsibility. Zimecterin Gold contains ivermectin and praziquantel. The praziquantel kills tapeworms. I have used this product for several years on my cats and dogs with no ill effects (knock on wood). My dogs are mutts. But be aware of the hazards if you have any of the breeds mentioned above. Having several cats and dogs, this is a more affordable alternative for me. There are several generic versions of heartworm preventative on the market. I used to order them from PetShed out of Australia. That worked fine, too. However, I have a number of cats and dogs and even the generic version became quite expensive.
Animals at highest risk are very young dogs, multi-drug sensitive breeds, dogs that may be exposed to ivermectin-containing products for large animals and/or the feces of large animals (horses, cows, pigs) treated with ivermectin.
Zemecterin Gold comes in an oversized syringe with a dosage appropriate for horses. The plunger on the syringe is marked at 50 lb intervals. This is how I determine the appropriate dosage. One syringe treats approximately 1300 pounds and costs about $15.
You can also buy the cattle version of just plain 1% ivomec (without the praziquantel), but it requires a larger dosage ivermectin (1/10 of 1 cc per 10 pounds) and it tastes really nasty. So you need to mix it in something like orange juice or yogurt.
Note: Cats can experience ivermectin toxicity. A small percentage of cats (less than dogs) have a sensitivity to it and will react adversely to Ivermectin. It’s very easy to overdose them. Safe dewormers for cats are not that much more expensive than ivermectin. Praziquantel and pyrantel pamoate are 2 that can be used with less risk and the most common ones you’ll find sold for cats.
Ivermectin as a Heartworm Preventive Medication in Dogs
Ivermectin is approved by the United States Food and Drug Administration (FDA) to prevent heartworm infections in dogs. Products containing Ivermectin are normally administered monthly for heartworm prevention and include medications such as Heartgard®, Iverhart®, Tri-Heart® and many other generic ivermectin-based heartworm preventive medications.
When used as a heartworm preventive medicine, ivermectin is used at much lower dosages than when used for other purposes. When used at the lower heartworm prevention dosages, ivermectin has a much lower potential for side effects than when used at higher dosages and this is the reason that ivermectin is approved for use as a heartworm preventive medication but must be used off-label (in a fashion not approved by the FDA) for many other purposes.
Read more at Suite101: Ivermectin for Dogs: Usages, Safety and Side Effects of Ivermectin in Dogs
Brand Names and Other Names
Uses of Ivermectin
How Ivermectin Is Supplied