American College of Veterinary Pharmacists

Ivermectin, a macrocyclic lactone derivative, is used in veterinary practice as a heartworm preventative medication and to treat pets infected by parasites. This class of medications also includes milbemycin and moxidectin.

Intoxication occurs in pets when small animals ingest medication intended for larger animals. It is very important to check the weight range and ensure your pet is receiving the correct dosage of medication. Dogs on farms or in rural environments are at a greater risk of toxicosis due to exposure of medication formulated for animals of greater weight/size. Toxicity may occur if your pet is exposed to the feces of large animals recently treated with these medications.

There are particular breeds of dogs that are sensitive to this drug class due to a genetic mutation (referred to as MDR1). Toxicosis may occur in these dogs even when therapeutic amounts are ingested. Dogs that are classified with this sensitivity include Australian Shepherds, Collies, English Shepherds, German Shepherds, Longhaired Whippets, McNabs, Old English Sheepdogs, Shetland Sheepdogs, and Silken Windhounds.

Signs and symptoms of toxicity: Clinical signs of toxicity include fatigue, difficulty walking, increased saliva production, dilated pupils, blindness, and seizures.

Toxic consumption:
It is important to note that each animals’ reaction to medication will vary and thus the dosages at which animals have shown clinical signs of toxicity will also differ.

Ivermectin:
– For most dogs signs of toxicosis may be seen at doses greater than 2 mg/kg (0.9 mg/lb). Dogs affected by genetic sensitivity can have toxicosis with as little as 0.1 mg/kg (0.04 mg/lb).
– Clinical signs of toxicosis in cats have been reported at doses of 0.3-0.4 mg/kg (0.1 mg/lb).

Moxidectin:
– The minimum toxicity threshold for most dogs is unknown. All incidences of exposure should be reported immediately. For dogs with genetic sensitivity toxicity has been reported after ingestion of 0.09 mg/kg (0.04 mg/lb).

Milbemycin:
– The minimum toxicity threshold for most dogs is unknown. All incidences of exposure should be reported immediately. For dogs with genetic sensitivity toxicity has been reported after ingestion of 5 mg/kg (2.2 mg/lb).

Dogs: Ivermectin Toxic Consumption X-Small
Yorkie, Chihuahua Small
Pug, Boston Terrier, Poodle Medium
Beagle, Scottish Terrier Large
Boxer, Cocker Spaniel X-Large
Retriever, German Shepherd XX-Large
Great Dane, St. Bernard 1 – 10 lbs.
(0.45 – 4.6 kg) 11 – 25 lbs.
(5 – 11.4 kg) 26 – 40 lbs.
(11.8 – 18.2 kg) 41 – 70 lbs.
(18.6 – 31.8 kg) 71 – 90 lbs.
(32.3 – 40.9 kg) 91 – 110 lbs.
(41.4 – 50 kg) > 0.8 mg > 9.9 mg > 23 mg > 37 mg > 64 mg > 82 mg Dogs with MDR1 mutation: Ivermectin Toxic Consumption 1 – 10 lbs.
(0.45 – 4.6 kg) 11 – 25 lbs.
(5 – 11.4 kg) 26 – 40 lbs.
(11.8 – 18.2 kg) 41 – 70 lbs.
(18.6 – 31.8 kg) 71 – 90 lbs.
(32.3 – 40.9 kg) 91 – 110 lbs.
(41.4 – 50 kg) > 0.04 mg > 0.5 mg > 1.1 mg > 1.8 mg > 3.2 mg > 4.1 mg Cats: Ivermectin Toxic Consumption Most Cats Large Cats 1 – 10 lbs.
(0.45 – 4.6 kg) 11 – 25 lbs.
(5 – 11.4 kg) > 0.1 mg > 1.4 mg

References:
– Al-Azzam SI, Fleckstein L, Cheng K, et al. Comparison of the pharmacokinetics of moxidectin and ivermectin after oral administration to beagle dogs. Biopharm Drug Disposition 2007;28:431-438.
– Mealey KL. Ivermectin: macrolide antiparasitic agents. In: Peterson ME, Talcott PA, ed. Small Animal Toxicology, 2nd ed. St. Louis: Elsevier, 2006.
– Osweiler, G, et al. (2011). Blackwell’s five-minute veterinary consult clinical companion. Small Animal Toxicology. [Kindle version]. Retrieved from Amazon.com