Selamectin Toxicity in Dogs

Selamectin is a medication that is both an insecticide and dewormer that is used for both dogs and cats. Although this drug is usually administered topically, it can also be administered by injection or by oral tablet, and it works by paralyzing the nervous system of the parasite. When the medication invades the system in large enough quantities, it can have an adverse effect on the central nervous system. Dogs in the herding breeds, like Collies, are more susceptible to selamectin toxicity than other breeds.

Selamectin, the active ingredient in the insecticide and dewormer Revolution, can have a toxic effect on the central nervous system if introduced in large enough quantities.

The symptoms of selamectin toxicity or overdose are primarily related to its effects on the central nervous system.

• Blindness
• Coma
• Depression
• Diarrhea
• Disorientation
• Exaggerated movements
• Excessive drooling
• Hair loss at application site
• Inability to rise
• Lethargy
• Loss of coordination 
• Muscle tremors
• Pupil dilation 
• Rapid breathing
• Sensitivity to touch 
• Sudden death
• Vomiting 

Types

• Imidacloprid - The primary active ingredient in medications such as Advantage Multi, this dewormer paralyzes the parasites, eventually killing them; herding breeds may have a higher susceptibility to toxicity due to this drug
• Ivermectin- Used in the commonly used heartworm medication Heartgard, Ivermectin also uses paralyzation to kill the parasites infecting your dog; this drug can be more dangerous for herding breeds like collies than other dogs
• Lufenuron- This ingredient is used in conjunction with the other dewormers and prevents the parasite eggs from hatching
• Milbemycin oxime- Found in both Interceptor and Sentinel, this dewormer targets the nerves of the parasites to kill them

The medication Selamectin targets the GABA receptors in nerve cells, which blocks the signals of the neurons in the parasites, paralyzing and then killing them. The only GABA receptors in mammals are in the central nervous system, which is somewhat protected from the effects of the medication by the brain-blood barrier, but excessive concentrations of selamectin can cause toxicity. This can happen either because the dog receives substantial overdoses from the oral or injectable version of the medication, or due to the animal licking off the topical solutions.

If your pet is showing indications that they have ingested any of the selamectin, which is usually administered topically, your veterinarian will likely order a biochemistry profile, complete blood count, and urinalysis as well performing as a full physical examination. If the ingestion of the medication was not witnessed, your veterinarian would take particular note of any opportunistic eating that was suspected or observed as there are other medications and plants that can cause similar symptoms.

In addition to environmental dangers, data regarding concurrent supplements or prescriptions that are being administered to your dog will be needed in order to reveal other toxins or drug interactions that may be at the root of the symptoms. Evaluation of the stomach contents and feces will often reveal residue from the medication, as will close examination of samples from the animal’s fat or liver. Supportive treatment will often start before a definitive diagnosis is made due to the severity of the symptoms.

Preliminary treatments will be supportive, including IV fluids to prevent dehydration and combinations of electrolytes and sugars to adjust for any imbalances. Further therapies

will depend upon the original method of administration, the amount of the medication that made it into the dog’s system and if any symptoms have already become apparent. If the Selamectin was ingested and was consumed recently, then vomiting will be induced as soon as possible to prevent the absorption of the toxins into the bloodstream.

If the exposure was more than an hour or two before treatment, the attending veterinarian might choose to perform a gastric irrigation under general anesthetic to remove the neurotoxin from the patient’s digestive system. Activated charcoal will then be dispensed to the patient in an attempt to soak up as much of the toxin as possible. This step may be helpful not just when the dewormer is ingested, but also in cases of overdose by injection or topical exposure. Supportive measures will be continued until the symptoms are eliminated. Antiseizure remedies may also be utilized if tremors become uncontrollable.

Plenty of fresh water should be made available for patients recovering from poisoning, and extra bathroom breaks will be needed as both the toxins and the medications make their way through the patient’s digestive system. Dogs that are recovering from anesthesia, as would be required  for gastric irrigation, may have coordination difficulties and be confused and disoriented until the anesthesia has cleared their system. Isolation from other pets and from children is generally advised until the medication has a chance to fully clear from your companion’s system. In cases of poisoning, your veterinarian may recommend more frequent monitoring of your pet’s blood chemistry levels in the future, particularly in relation to kidney and liver functionality or impairment.