I had an especially sad euthanasia appointment a while back. A litter of puppies had eaten feces from several horses that had been dewormed with ivermectin. Enough of the medication passed through the horse’s body and out in the manure that when the dogs ate the feces, they ingested toxic amounts of the drug.
The owners were unaware of what had happened until the dogs began to fall ill. Treatment had failed to save the puppy that developed symptoms first. Meanwhile two more had died, and the owners called me out to euthanize the sole remaining puppy that was in a coma.
My clients were obviously heartbroken and felt terrible that their puppies had died from a preventable poisoning. Let me take this opportunity to review some basic information about ivermectin.
Ivermectin is a member of the macrocytic lactone class of parasiticides. It is commonly used as a heartworm preventative in small animals and for the treatment of certain types of external (e.g., mites) and internal parasites in many different species. The difference between the safe use of ivermectin and poisoning is all about the dose and an animal’s inherent sensitivity to the drug. Some dogs carry a gene (MDR1 or ABCB1) that makes doses of ivermectin and other drugs that are safe for the general population dangerous to those individuals.
I’m going to focus on dogs from here on out since they were involved in 282 of the 318 potentially toxic ivermectin exposures reported to the ASPCA Animal Poison Control Center during 2008–2009. Typical doses for ivermectin in dogs are
6 ug/kg for heartworm prevention
300 ug/kg for treatment of sarcoptic mange
400-600 ug/kg for treatment of demodectic mange
Non-sensitive breeds generally need to be exposed to more than 2,000 ug/kg before significant symptoms develop, but the potentially toxic dose in MDR1 positive individuals can be as low as 100 ug/kg. Take note that the incredibly low dose used for heartworm prevention is well below the toxic dose even for even the most sensitive dogs. Before using higher doses of ivermectin, however, at risk dogs can be tested for the MDR1 gene mutation. This is especially important for breeds like Collies, Shetland Sheepdogs (Shelties), Australian Shepherds, Old English Sheepdogs, English Shepherds, German Shepherds, Long-haired Whippets, Silken Windhounds, and mutts that might be derived from these breeds.
Animals can absorb ivermectin through oral or topical exposures as well as via injection. Symptoms arise when the drug is present in the body at high enough concentrations that it crosses the blood-brain barrier and adversely affects neurologic function. Typical signs include
unsteadiness when walking
Treatment for an overdose of ivermectin is essentially symptomatic and supportive. If the poisoning is caught early enough, decontamination is helpful (e.g., washing pets after topical exposure or inducing vomiting and/or activated charcoal administration within a few hours of ingestion). Intravenous fluid therapy, endotracheal intubation, mechanical ventilation, extensive nursing care, seizure control, application of eye lubricants if the patient cannot blink, and nutritional support may all also be necessary. In some cases, intravenous lipid emulsion therapy, which is a new but promising option for certain types of poisoning, might be worth considering.
A pet’s prognosis can be quite good if aggressive treatment is initiated in a timely manner, but because severe cases of ivermectin overdose often require several weeks of therapy, the expense is often prohibitive … as was the unfortunate case with my clients who chose to euthanize the last puppy in what had been their much anticipated litter.
Dr. Jennifer Coates
Image: Ruby Baby in Grass by carterse / via Flickr