The Side Effects of Moxidectin in Herding Dogs

Seventy-five percent of collies test positive for the MDR1 gene mutation.
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Parasite prevention is the responsibility of every dog parent. Invasive internal and external organisms can wreak havoc on your dog's body, causing extreme discomfort, infection, disease and even death. Moxidectin is the active ingredient found in many parasite preventatives, and is generally safe for most dogs if used properly. For some herding breeds, however, moxidectin is an ingredient best avoided.

Moxidectin Basics

Moxidectin is an anthelmintic, or anti-parasitic, drug commonly found in heartworm preventatives and dewormers prescribed for dogs, cats, cows, sheep and horses. In dogs, it protects against heartworm disease, hookworms, roundworms, whipworms and mites. It's used in pills, liquids, injections and topical solutions. Moxidectin prevents infection by selectively binding with elements critical to the function of nerve and muscle cells in invertebrates, causing the disruption of neurotransmissions, paralysis and eventual death of the parasites.

The MDR-1 Mutation

Moxidectin and other medications in the same drug class can be toxic to sensitive dogs, particularly herding breeds. A study conducted by Washington State University researchers in the Veterinary Clinical Pharmacology Laboratory determined that many herding dogs are afflicted with a mutation to their MDR1 (multi-drug resistant) gene, making it impossible for them to properly metabolize these drugs. Affected breeds include collies, Old English sheepdogs, Shetland sheepdogs, Australian shepherds, English shepherds, McNabs, German shepherds, longhaired whippets, silken windhounds and many mixed dogs with herding breed heritage.

Moxidectin vs. MDR-1

The MDR1 gene encodes the protein P-glycoprotein, which acts to cleanse the brain of toxins and drugs like moxidectin. Dogs with the MDR1 defect cannot accomplish this task, resulting in toxic levels of moxidectin crossing the blood-brain barrier into the dogs' brains and central nervous systems. This can lead to neurological abnormalities, illness or death. Side effects of toxicity include lethargy, vomiting, respiratory distress, anorexia, extreme thirst, facial edema, pupil dilation, drooling, seizures, tremors, lack of coordination and coma.

How to Protect Your Dog

If you have a herding dog or a mixed dog with suspected herding breed heritage, there are steps you can take to protect him from moxidectin toxicity. Disregarding heartworm protection altogether is not an option, so speak with your veterinarian about the safest heartworm and parasite protection for your dog. Have your dog tested for the MDR1 gene mutation. Test results will specify whether your dog is positive for two, one or no copies of the MDR1 defect.

Additional Drug Dangers

Keep in mind that moxidectin is not the only drug that's dangerous to dogs afflicted with the MDR1 defect. Dangerous drugs include other anti-parasitic agents, certain chemotherapy agents, butorphanol, acepromazine, morphine, erythromycin and loperamide. Research is ongoing concerning additional drugs that may pose a hazard. Be sure to discuss moxidectin and all other possibly dangerous medications with your veterinarian.