Macrocyclic Lactone Poisoning in Dogs

What are macrocyclic lactones?

Macrocyclic lactones are a class of deworming medication. They are frequently used as large animal dewormers and are found in many heartworm preventatives for dogs and cats, in small doses. Macrocyclic lactones include ivermectin, milbemycin, and moxidectin.

What is macrocyclic lactone poisoning?

Macrocyclic lactone poisoning occurs when a dog receives a medication dose that is too high. This can happen if the dog ingests dewormer not meant for their size or species, such as horse dewormer, or even if they eat manure of horses that were recently dewormed.

Certain dog breeds are at a higher risk of having a genetic mutation (MDR1 mutation) that causes increased sensitivity to this class of drugs. The increased sensitivity results in serious side effects at doses that are safe for most dogs.  Heartworm preventives contain very low doses of macrocyclic lactones, and they are generally considered safe for most dogs with this mutation when used appropriately.

How does poisoning occur?

If a dog receives a toxic dose of macrocyclic lactone, the drug can enter the brain, interfering with normal function.

What are the clinical signs of macrocyclic lactone poisoning?

Clinical signs of poisoning may occur within hours of exposure and can include:

• Dilated pupils
• Blindness
• Difficulty walking (ataxia)
• Tremors
• Drooling
• Vomiting
• Seizures
• Coma
• Difficulty breathing
• Death

If you suspect your pet has ingested an inappropriate amount of macrocyclic lactone, it is important to immediately contact your veterinarian or Pet Poison Helpline, a 24/7 pet poison center, at 1-800-213-6680 to help determine the risk of poisoning to your pet.

How is macrocyclic lactone poisoning diagnosed?

Diagnosis is usually based on a history of macrocyclic lactone exposure, development of expected clinical signs, and physical examination findings. Your veterinarian may recommend bloodwork depending on the amount your dog was exposed to and clinical signs.

How is macrocyclic lactone poisoning treated?

As with all poisoning cases, early intervention is key to successful treatment. Treatment may include decontamination and supportive care to address any clinical signs that have developed.

What is the prognosis for macrocyclic lactone poisoning?

A dog’s ability to recover from macrocyclic lactone poisoning depends on how much they ingested relative to their size and whether they have the MDR1 mutation. Most dogs recover, but they may require weeks of intensive supportive care.

How can I prevent macrocyclic lactone poisoning?

You can help reduce the likelihood of accidental poisoning in a few ways:

• Never administer deworming medication unless prescribed by your veterinarian.
• Follow label dosing instructions exactly.
• Store any deworming medications where dogs cannot access them.
• Restrict your dog’s access to animals that have recently received deworming medications.
• Test your dog for the MDR1 mutation.

Pet Poison Helpline, an animal poison center based out of Minneapolis, MN, is available 24/7 for pet owners and veterinary professionals that require assistance treating a potentially poisoned pet. The staff provides treatment advice for poisoning cases of all species, including dogs, cats, birds, small mammals, large animals, and exotic species. As the most cost-effective option for animal poison control care, Pet Poison Helpline’s per-incident fee includes follow-up consultations for the duration of the poison case. Pet Poison Helpline is available in North America by calling 800-213-6680. Additional information can be found online at www.petpoisonhelpline.com.