Moxidectin Poisoning in Dogs

Moxidectin is a drug that is used to treat parasites in dogs, cats, and large farm animals. It works similarly to other anthelmintic drugs like ivermectin and milbemycin, by interfering with important neurotransmissions in parasites so that they become paralyzed and die. Moxidectin comes in oral, topical and injectable forms which may be used to treat a number of different parasites. It is frequently found in heartworm medication for dogs and other animals. Moxidectin is safe for dogs in the correct dosage, but it can have significant side-effects. High doses that are able to cross the blood-brain barrier have a neurotoxic effect and untreated moxidectin poisoning can result in severe symptoms or death. Exposure can take place through ingestion or topical exposure. 

Some breeds of herding dogs have avermectin sensitivity which makes them particularly susceptible to moxidectin poisoning. These dogs carry a mutation that allows drugs to move more easily from the blood to the brain, so even a small dose can have a toxic effect. Most cases of toxicity in breeds that are not sensitive are a result of exposure to large doses of parasite medication for horses and other farm animals. Dogs that ingest horse feces directly after the horses were dewormed have also been known to develop symptoms. Signs of moxidectin poisoning include vomiting, excessive salivation, and generalized tremors or seizures. Most dogs respond well to treatment.

Moxidectin is used to kill parasites in dogs and other animals. It is safe in the recommended doses, but overdose can cause moxidectin poisoning with life-threatening neurological symptoms.

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These are signs you will see in a dog with moxidectin poisoning. Get immediate veterinary treatment if you notice any of these symptoms.

• Vomiting
• Drooling
• Shortness of breath
• Dilated pupils
• Generalized tremors
• Lack of muscle control (ataxia)
• Dilated pupils (mydriasis)
• Blindness
• Seizures
• Coma

Types

These are some products that contain moxidectin. Many over the counter moxidectin products are not for sale in the United States.

• Advantage Multi for dogs – topical solution
• Advantage multi for cats – topical solution
• CYDECTIN for cattle and sheep
• Quest Gel for horses
• Advocate for Dogs  - not for sale in the US
• Advocate for cats – not for sale in the US
• ProHeart6 – sold in Canada and Japan
• ProHeart10 – sold in Australia

These are some of the risk factors for moxidectin poisoning.

• Some breeds are more sensitive (Collies, Border Collies, English Sheepdogs, Australian Shepherds)
• Dogs that live on a farm
• Ingestion of worm medication intended for horses, cows, pigs or another large animals
• Ingestion of horse feces after deworming
• Overdose of a medication intended for dogs

A history of moxidectin exposure is the easiest way to diagnose the condition. The veterinarian will examine your dog physically and take blood and urine samples. Moxidectin poisoning can be confirmed by a serum blood test, but this result may need to be processed by a lab so it may not be immediately available. Other diagnoses will be symptomatic and based on your dog’s response to stimuli. If your dog belongs to a breed that is typically sensitive, this will make moxidectin poisoning more likely. Any possible exposure to moxidectin or other similar medications, either from the dog’s own medications for other large farm animals is relevant.

If you are concerned that your dog may be avermectin sensitive, testing is available at some specialist veterinary institution including Washington State University in the United States. Breeds that are known to be sensitive should avoid moxidectin medications if possible.

If poisoning took place recently (within 1-4 hours), vomiting will be induced to reduce absorption. Activated charcoal may also be given if the incident took place in the last 24-36 hours ago. This medication binds to toxic drugs in the gastrointestinal tract and can further reduce absorption. If the exposure was primarily topical, initial treatment will involve washing the dog with warm water and detergent.

The veterinarian will monitor electrolyte levels and give IV fluids as needed. External heat may be necessary if hypothermia is present. Medication will be prescribed as needed to treat seizures, either diazepam or an anesthetic agent such propofol depending on the response of the dog (some studies have found that dogs with moxidectin poisoning don’t respond well to diazepam). Some more novel treatments like lipid injection have also been successful at reducing the severity of neurological symptoms. Symptoms can last for up to a week in some cases. Depending on the severity, the veterinarian may want to keep your dog in a veterinary hospital during that time, or he may be discharged with careful monitoring instructions.

Dogs typically respond well to treatment and there is a good chance of recovery from moxidectin poisoning. Prompt treatment is necessary however, otherwise symptoms often become fatal. To avoid exposure, keep all your dog’s medications out of reach and only give the doses prescribed by a veterinarian or suggested on the packaging. Call your veterinarian immediately if you notice side-effects. 

Take precautions when giving medications to large farm animals as these doses will be toxic for your dog. Keep your dog away from feces, especially after worm medication has just been given. If your dog belongs to a breed that is known to be sensitive, special care will need to be taken with any medication containing moxidectin. Discuss this condition with your veterinarian to determine which medications are safe for your dog.