- When ivermectin, a macrocytic lactone, is present in sufficiently high concentrations to cross the blood–brain barrier, it can cause neurologic signs in dogs.
- Toxicity may occur with products administered orally, topically, or parenterally.
- The same dose will be absorbed faster when administered parenterally than when administered topically, but signs of toxicity can be seen in all routes with high enough doses.
- No gender or age predisposition.
- Very young animals may have an increased risk because of their immature blood–brain barriers.
- Dogs with a mutation in the multidrug resistance gene (ABCB1, formerly MDR1) are especially sensitive to ivermectin.1,2
- Dogs with an ABCB1 mutation are also predisposed to increased sensitivity to moxidectin, loperamide, milbemycin, and chemotherapeutic agents.
- Common breeds with this mutation include the border collie, Australian shepherd, long-haired whippet, silken windhound, rough- and smooth-coated collies, and associated mixed breeds.
- In sensitive breeds, ivermectin toxicosis can be seen in doses as low as 100 µg/kg, although doses of 6 µg/kg have been shown to be safe in sensitive breeds.
- In nonsensitive breeds, a dose of >2000 µg/kg is required to produce signs of toxicosis.3,4
- Most cases of ivermectin toxicosis result from administration of ivermectin-containing products.
- Dogs on farms or in rural settings are at greater risk for ivermectin toxicosis because they may be exposed to products formulated for large animals.
- Dogs can also be exposed to ivermectin through ingestion of feces from treated cows, horses, or pigs.