Ivermectin is a drug very commonly used in veterinary medicine. This anti-parasitic drug is used in a number of species to control internal and external parasites. In dogs, it is incorporated in products to prevent heartworm disease as well as used off-label to treat both sarcoptic and demodectic mange. It is available in topical, injectable, and oral forms.
Like any drug, an overdose of ivermectin can cause serious side effects. Symptoms of overdose include lethargy and weakness, tremors, and dilated pupils. There is no specific antidote, so treatment of an affected dog involves intravenous fluids and managing symptoms until they recover.
Any dog can be affected by ivermectin toxicity if they’re given too much of the drug. However, some dogs have a genetic mutation that puts them at a greater risk. The gene is known as the multidrug resistant gene (MDR1) and is found in a number of breeds, including herding breeds such as the Australian Shepherd and Collies and the Long Haired Whippet. Mixed breed dogs can also be affected. According to the Washington State University College of Veterinary Medicine, 50% of Australian Shepherds carry a defective MDR1 gene.
This gene is involved in the production of a protein that plays an important role in controlling the metabolism of certain drugs. Dogs with the MDR1 mutation have reduced ability to control the drugs’ absorption and distribution around the body (particularly to the brain), and they’re also less able to excrete them. This makes them at risk of toxicity at much lower doses than normal dogs.
There is no risk associated with giving heartworm prevention products to a dog with the defective gene. All the products available in the U.S. have been tested and found to be safe. Problems occur if they are given ivermectin to treat mange. There are also a number of other medications that can affect these sensitive dogs, including pain relief medications, antibiotics, tranquilizers, and chemotherapy drugs.
Dog breeders are DNA testing their dogs to see if they carry the defective MDR1 gene before they are mated. It’s a fairly straightforward mode of inheritance. A dog receives one copy of the relevant gene from their sire, and another from their dam. If they have two normal genes, they’re not sensitive to ivermectin and the other drugs. However, if they have two defective genes, or even one normal and one defective gene, they’re at risk of toxicity. It makes sense to breed only from dogs that don’t carry the defective gene, but this raises its own issues. In some breeds, there are just not enough dogs with two normal genes from which to breed. Carrier dogs with one defective and one normal gene may need to be included in a breeding program just to prevent inbreeding. A responsible breeder will be aware of their dogs’ genetic status, and by carefully choosing parent dogs, they can reduce the frequency of the defective gene in future generations.
This DNA test is also available to pet parents of dogs. It’s simple and inexpensive. A blood sample or cheek swab of the dog is sent to Washington State University Veterinary Clinical Pathology Laboratory. The results are available within a week or two. If you share your life with a herding dog or another at-risk breed, give some thought to having them tested for the MDR1 gene. The results will help you to protect them from avoidable drug toxicities.