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Ivermectin Toxicity in Dogs

This toxic reaction occurs especially in dogs that are genetically hypersensitive to ivermectin, an anti-parasite medication most commonly used for heartworm prevention, or to treat ear and hair mites, which can lead to mange. 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.

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
• Collie

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

Symptoms for the dog may be acute or mild. Acute signs will become apparent within 4 to 12 hours of the drug's administration. 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
• Coma

Treatment

Unfortunately, ivermectin toxicity cannot be reversed. Therefore, it is best to make your pet comfortable and treat the symptoms to the best of your ability. If exposure has occurred within the past four to six hours, induce vomiting and/or administer activated charcoal for the purposes of minimizing absorption. Be on the lookout for signs of secondary complications.

Some or all of the following measures may also be recommended by your veterinarian: 

• Intravenous fluid therapy
• Keeping electrolytes in balance
• Intravenous nutritional support
• Turn the dog over frequently
• Appropriate bedding
• Physical therapy
• Ocular lubricants
• Ventilator in case of respiratory distress
• Heat support if body temperature is low
• Fans if body temperature is high
• If your dog can not stand up, urinary catheters may be needed
• Medication for seizures if appropriate

Much will depend on the severity of the dog's reaction, along with its initial overall health. It may take several weeks of dedicated care before the dog fully recovers.

Prevention 

There is a test available to check sensitivity to ivermectin. If your dog is one of the breeds that is prone to ivermectin toxicity, you might consider testing for it. If you decide not to have the testing done, be cautious about using ivermectin to prevent heartworm disease or for the treatment of mites.