Just Ask the Expert: The best way to treat ivermectin toxicosis

Q. Is there an antidote for ivermectin toxicosis in dogs and cats? What is the recommended treatment protocol?

A. There is no specific antidote for ivermectin toxicosis, but we can take measures to minimize the clinical signs. The clinical signs are typically neurologic in nature (ataxia, lethargy, mydriasis, tremors, blindness, drooling, vomiting, disorientation) and are related to excessive gamma-aminobutyric acid (GABA) receptor activity in the central nervous system.

Jennifer L. Garcia, DVM, DACVIM

What is considered toxic?

The toxic dose of ivermectin in dogs and cats ranges between 0.1 and 2.5 mg/kg (100 and 2,500 μg/kg). Dog breeds with defective P-glycoprotein (a recessive mutation in the ABCB1 gene, previously called the MDR1 gene), can experience toxic effects at the lower end of the toxic dose range because of higher concentrations of certain drug being allowed to cross the blood-brain barrier. These breeds include collies, border collies, Australian shepherds, and Shetland sheepdogs.

Initial treatment

If the patient is alert enough, decontamination with activated charcoal is indicated and may need to be repeated every four to six hours as ivermectin undergoes enterohepatic circulation. There is anecdotal evidence that tremors may respond to benzodiazepines or methocarbamol.

Consider ILE

Recently, use of an intravenous lipid emulsion (ILE) has shown success in ameliorating clinical signs in patients with ivermectin toxicosis. The mechanism of action of lipid emulsion is not completely understood but is thought to provide a "lipid sink" that allows lipophilic agents to partition out of the plasma and accelerate elimination. The most commonly used ILE is a 20% emulsion with a tonicity of about 800 mOsm/L, which allows administration through a peripheral vein.

The suggested administration protocol is as follows:

• Slowly administer an initial bolus of 1.5 ml/kg of 20% lipid emulsion.

• Begin continuous-rate infusion at 0.25 ml/kg/min for 30 to 60 minutes. This may translate to a very high fluid rate, so pay attention to this calculation and consider administering over one to four hours to avoid fluid overload.

• Repeat this dose if necessary, but check the serum before administration. If the serum is lipemic, do not repeat as it is unlikely to provide any added benefit. Check the serum for lipemia again in two hours if another dose is still needed. Do not give more than two doses if no improvement in clinical signs is noted.

Outcome

Most patients with ivermectin toxicosis respond well to supportive care and return to normal within 48 to 72 hours. Resolution may be more rapid in patients treated with ILE therapy.

This expert answer was provided by Jennifer L. Garcia, DVM, DACVIM, Sugar Land Veterinary Specialists, Houston, Texas.

Suggested Reading

1. Merola V, Khan S, Gwaltney-Brant S. Ivermectin toxicosis in dogs: a retrospective study. J Am Anim Hosp Assoc 2009;45(3):106-111.

2. Pritchard J. Treating ivermectin toxicity in cats. Vet Rec 2010;166(24):766.

3. Epstein SE, Hollingsworth SR. Ivermectin-induced blindness treated with intravenous lipid therapy in a dog. J Vet Emerg Crit Care 2013;23(1):58-62.

4. Bates N, Chatterton J, Robbins C, et al. Lipid infusion in the management of poisoning: a report of 6 canine cases. Vet Rec 2013;172(13):339.