Ivermectine intoxication in a dog

You have a very shaky dog, trembling all over, he seems hypersensitive and all these signs seem to have appeared quite suddent. Intoxication does not always present with vomiting but neurological signs are common in intoxications. This post will help you figuring out if your shaky and trembling dog could have an ivermectine intoxication.

What do we use Ivermectine for?

Ivermectin is widely used in veterinary medicine as a potent and board spectrum anthelminthic and it generally has a wide margin of safety. However, intoxication is possible when the animal is exposed to an overdose.

Ivermectin produces its anti -parasitic action by potentiating release and binding of gamma amino butyric acid (GABA) at certain nerve synapses. In nematodes and arthropod GABA mediated transmission occurs in peripheral synapses while in mammals, GABA mediated transmission occurs only in central nervous system where ivermectin normally could not enter due to its macro molecular size.

The therapeutic dose of ivermectine is 0.003 to 0.006 mg/kg once a month for heartworm prevention; 0.3 mg/kg once, then repeat in 14 days for skin parasites; and 0.2 mg/kg once for gastrointestinal parasites. For cats the dose is 0.024 mg/kg once monthly for heartworm prevention and it’s generally not indicated in animals below 3 months of age.

How will a dog get intoxicated with Ivermectine?

Toxicity can occur by ingestions of horse dewormer with a high concentration of the drug or by ingesting feces from horses that were recently dewormed. Increased permeability of blood brain barrier and even a specific protein receptor in the brain attracts the  ivermectin. Moreover, in ivermectin toxicity inflammation of central nervous system has been reported, so it could be possible that during inflammation of central nervous system the ivermectin or its metabolites entered into CNS and produced toxic effects.

Are certain dogs more sensitve to Ivermectine intoxication?

Certain breeds of dogs (e.g., collies, Border collies, Australian shepherds, Old English sheepdogs, etc.) with a multidrug resistant (MDR) gene mutation, also known as ABCB1, are extremely sensitive to this class of drugs: These dogs can show marked clinical signs on presentation with a much lower dose. In normal healthy dogs, we typically don’t see signs of ivermectin toxicosis until 2.5 mg/kg (where we can see mydriasis). At higher doses (e.g., 5 mg/kg), we can see signs of ataxia, tremoring and seizuring. Note that in normal healthy dogs, the LD50 is reported to be as high as 80 mg/kg (in healthy Beagles). In MDR-allele mutation dogs, the LD50 is reported to be as low as 0.12 mg/kg.

Clinical presentation for Ivermectine intoxication

Ivermectine intoxication can produce a variety of clinical signs which determined by the dog susceptibility and dose ingested. Clinical signs of poisoning include dilated pupils, ataxia, tremors, salivation, seizures, coma, respiratory difficulties and death. Without aggressive treatment, ivermectin poisoning can be deadly. The progression of these signs can vary from 1 to 6 days, when they tend to recover. Collies, that are more sensitive to this intoxication can take up to 3 weeks to completely recover.

On this video

You see this dog is ataxic on the 4  limbs: he’s losing the balance and he’s even a little bit hypermetric and dissymmetric. On the examination we could not observe proprioceptive deficits and the spinal reflexes were normal, but as you can see, when performing the cranial nerves examination  you  observe the non-responsive pupillary light reflexes: there is bilateral mydriasis. The dog also presented muscle tremors.

Recovery from Ivermectine can be as fast as on this video where the following day after hospitalization the dog was brighter, happier and the ataxia had almost completely disappeared.