Toxicity to pets

A class of drugs commonly used in veterinary medicine called avermectins, includes the commonly well-known ivermectin drug.  These are anthelmintics (these drugs kill parasites) and are commonly used as an active ingredient for monthly heartworm preventatives in dogs and cats. Certain breeds of dogs (e.g., collies, Border collies, Australian shepherds, Old English sheepdogs, etc.) with a multidrug resistant (MDR) gene mutation, also known as ABCB1, are extremely sensitive to this class of drugs. Accidental poisoning can be seen in these sensitive breeds or when non-sensitive breeds get into a toxic amount. Toxic ingestions often occur when dogs accidentally ingest horse dewormer, which contains large amounts of ivermectin. Accidental poisoning has also been reported in dogs ingesting feces from horses that were recently dewormed. The amount of ivermectin in standard dog and cat heartworm prevention is not likely to cause  a concern in non-sensitive breeds if they were to ingest 1-2 tablets above their normal dosing.  However, ingesting a larger number of tablets or medication intended for a larger animal, may be a concern. Clinical signs of poisoning include dilated pupils, difficulty walking (ataxia), tremors, drooling, seizures, coma, inability to breath, and death. Without aggressive treatment, ivermectin poisoning can be deadly.