This toxic reaction occurs especially in dogs that are genetically hypersensitive to ivermectin, an anti-parasite medication most commonly used for heartworm prevention, to treat ear mites and kill mange mites. Ivermectin prevents or kills parasites by causing neurological damage to the parasite, resulting in paralysis and death for the parasite. But dogs genetically sensitive to the medication have an anomaly that allows the ivermectin to pass the dog's blood-brain barrier and into its central nervous system, which can be lethal for the animal. The low dose used for commercial heartworm prevention is well below the toxic dose even for the most sensitive dogs.
While the sensitivity to this type of medication is not always guaranteed, the following breeds are most likely to be affected: Old English Sheepdog, English Sheepdog, Shetland Sheepdog (Sheltie), Australian Shepherd, German Shepherd, Long-haired Whippet, Silken Windhound, Skye Terrier and Collie and collie mixes
It is also seen in mixed-breed dogs, older dogs that have experienced a blow to the head, puppies, and dogs that have overdosed on similar types of drugs. Treating dogs that are susceptible to ivermectin toxicity with parasitic medication should be only be done under a veterinarian's supervision and with great caution.
Symptoms for the dog may be acute or mild. Acute signs will become apparent within 4 to 12 hours of the drug's administration Animals can absorb ivermectin through oral or topical exposure as well as injection. . In mild cases, symptoms will occur between 48 to 96 hours after your dog has been treated. Such symptoms include: Lethargy, depression, drooling, vomiting, dilation of the pupil, loss of appetite (anorexia), difficulty controlling voluntary movement, disorientation, tremors/seizures, inability to stand, blindness, slow heartbeat, respiratory distress and coma.
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 gen (MDR1 or ABCB1) that makes doses of ivermectin and other drugs dangerous to those dogs that carry the gene. A test is available to test your dog for these genes.
Treatment is not always successful. Your pet could require several weeks hospitalization. Please consult your veterinarian before giving any medications to your pet.
You may contact Dr. Norette L. Underwood of Trumann Animal Clinic and Best Friends Vet Mobile at email@example.com.