Dog sickened by ivermectin through sheep feces

A dog with sensitivity to ivermectin became ill after eating feces from sheep treated with an ivermectin-based dewormer.

A veterinarian at Tufts University warned that the illness shows the need to test dogs for the associated genetic mutation.

Dr. Theresa E. O’Toole, a critical care specialist at the Tufts University School of Veterinary Medicine, said the dog, an Australian Shepherd named Bristol, likely ingested a large amount of ivermectin in the sheep’s feces during a herding lesson last September, but how much is unknown. Most dogs that she has seen become ill from ivermectin, other than those that had received ivermectin-based treatments, had eaten deworming medicine spit out by horses.

Dr. O’Toole thinks many veterinarians may not think to ask dog owners about possible indirect exposure to ivermectin when examining a dog with relevant signs. Without suggestions from owners or details about activities and environments,  she thinks dogs such as Bristol would be hospitalized with management for their seizures and given supportive care and neurologic analysis.

Ivermectin is used as a heartworm preventive in dogs but can be toxic to dogs of certain breeds, in particular, herding dogs with white feet.

Those breeds include Collies, Shetland Sheepdogs, Old English Sheepdogs, Australian Shepherds, Longhaired Whippets, and crossbreeds thereof, according to the 2011 edition of Plumb’s Veterinary Drug Handbook and the 2010 edition of the Merck Veterinary Manual.

Dr. O’Toole recommends testing all herding dogs for ivermectin sensitivity. Dogs with the genetic mutation are predisposed to toxicosis not only with ivermectin but also with a variety of other drugs, including some chemotherapy drugs, she said.

“Since dogs commonly develop cancers or have conditions for which common pain medication may be used, it’s best to know if the dog has sensitivity prior,” she said.