Q: I don’t give Alex, my Australian shepherd, heartworm medication, because I’ve heard it’s toxic to herding breeds. However, my veterinarian recommends that Alex take it every month throughout the year. What do you recommend?
A: You should feel comfortable giving Alex any heartworm medication approved by the Food and Drug Administration as safe and effective in dogs, including Heartgard Plus, Interceptor Plus, Sentinel and Trifexis.
Because these heartworm preventives also kill intestinal parasites, some of which cause disease in humans, most veterinarians recommend monthly treatment throughout the year to protect dogs and their people.
When you speak of drug toxicity in herding breeds, you probably are referring to dogs with mutations of the MDR1 (multi-drug resistance) gene. However, even in dogs with these mutations, monthly heartworm medications have proven safe. Drug side effects occur only at extremely high doses, such as those used to treat mange.
The MDR1 gene produces a protein called p-glycoprotein that carries certain drugs throughout the body. This protein promotes excretion of these drugs through the kidneys and liver, preventing their build-up in the body, and it helps prevent the drugs from crossing the blood-brain barrier, causing neurologic side effects.
Dogs with MDR1 mutations may experience side effects from high doses of some anti-parasitic medications as well as some antibiotics (doxycycline and erythromycin), pre-anesthetics (acepromazine and butorphanol), gastrointestinal medications (loperamide and ondansetron) and chemotherapy drugs.
Dogs carry two copies of the gene. If both copies are mutated, the dog may experience severe adverse reactions to these drugs when given in high doses. If only one of the two copies of the gene is mutated, the side effects will be less severe.
Breeds at increased risk include collies (70 percent affected), Australian and miniature Australian shepherds (50 percent), Shetland sheepdogs (15 percent), German shepherds (10 percent) and mixed-breed dogs (5 percent).
Consider genetic testing, which will tell you whether Alex carries one, two – or no – copies of the mutated MDR1 gene. This knowledge will help you and your veterinarian better manage his care in the future.
Lee Pickett, V.M.D. practices companion animal medicine in Pennsylvania. Contact her at firstname.lastname@example.org.
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