Avermectin Safety In Dogs

Sammi Jo, RAHT*, writes from Central Alberta, Canada, “Hey Dr. Randolph! So I had a question today from a client at the clinic I work at, and it had to do with Revolution (selamectin) Topical Dewormer. A client who was told by her breeder (Australian shepherd) that Revolution or anything of the avermectin family was toxic to her dog. Now I have only ever heard the breeds to have this predisposition as collies and some shelties, but only some. I know Pfizer says it’s safe but would like to know if you have seen any cases of Australian shepherds having this breed predisposition? I must say curiosity definitely takes flight when I read the questions posted on here. Hope all is well!”

Good to hear from you again, Sammi Jo. With a double name like that you should be here in the South, not in the frozen north! Your questions are always thought-provoking.

Avermectin is the family of chemicals that encompasses Heartgard’s and Revolution heartworm preventive’s active ingredients. Ivermectin for Heartgard, selamectin for Revolution. It is known that any of the herding breeds can react to these ingredients, but the tendency, or degree of sensitivity is controlled by an individual pet’s genes, specifically a mutation of the MDR1 gene. Another way to define “degree of sensitivity” is to say that the likelihood of reaction is dose-dependent in each individual. The problem was first observed in collies, but researchers soon found that some collies in specific family lines reacted at “x” dose, while others required “2x” or higher to react. Testing of siblings, parents, grandparents, aunts and uncles showed that many relatives share the highest sensitivity gene mutation, while others may not, depending totally on whether the right gene mutation was inherited.

Some important points to recognize:

• Heartworm-preventive doses in formulations manufactured for dogs have a near-zero risk of causing reaction. The dose required for adverse reaction for even the most sensitive of collies is much higher than the correct-sized tablet of Heartgard for collies and Shetland Sheepdogs.
• Selamectin is a different molecule, albeit in the same avermectin family as ivermectin, and its likelihood of causing a reaction in a herding breed is much, much lower. Revolution’s product label states, “In a topical safety study conducted with avermectin-sensitive collies at 1, 3 and 5 times the recommended dose of Revolution, salivation was observed in all treatment groups, including the vehicle control. Revolution also was administered at 3 times the recommended dose to heartworm infected dogs, and no adverse effects were observed.”
• Selamectin (Revolution) provides flea control and flea prevention at safe dosage levels, in addition to excellent heartworm prevention.
• Revolution is not formulated as a tasty treat, therefore, the likelihood of a pet chewing open the package and consuming overdoses is minimal.

See you tomorrow, Dr. Randolph.

* (a Registered Animal Health Technician, meaning she went to school two or more years for specific training in assisting veterinarians in health care. Another term that means the same thing is RVT, Registered Veterinary Technician)