Ivermectin toxicity?

Dr. Marie replied:

I'm sorry to hear that Sunny is having these problems.

Years ago I did work with some clients who gave ivermectin instead of Heartgard as heartworm prevention. While it can be safe, my main concern is that it is very difficult to properly dose ivermectin. It comes in a bunch of different strengths and often it is very complicated to figure out how much to give. I often have to figure out dosing when I give ivermectin injections to small animals like guinea pigs or mice for parasitic issues and even though I have done the math several times I always have trouble with it! It would be quite easy to be giving too much by a factor of 10 or more.

On top of that, some dogs can be extremely sensitive to ivermectin if they have a defective MRD1 gene. This is much more common in herding breeds of dogs though.

Here are the symptoms of ivermectin toxicity:

• vomiting


• ataxia (wobbly walking)


• lethargy


• fast heart rate


• salivation


• mydriasis (large pupil size)


• seizures

In cats, blindness can be a side effect of receiving too much Ivermectin, but this is not common in dogs.

Sunny's case has me a little bit puzzled. The increase in his pupil size could be from an ivermectin overdose, but the increase in thirst and urination and actual blindness really should not.

Because I was puzzled by his symptoms I did some research for you and did find something that could cause blindness as well as an increase in thirst and urination. You may want to ask your vet about the possibility of a condition called Sudden Acquired Retinal Degeration (SARD). We don't know why dogs get this, but they can suddenly lose their ability to see and also, most dogs who have this will have an increase in thirst and urination. The symptoms are often similar to cushing's disease (hyperadrenocorticism).

There is no cure for SARD, but dogs can live quite happily with it.

There is also another rare condition called nephrotic syndrome that can cause these symptoms. Dogs with nephrotic syndrome usually have high levels of protein in the urine, so it is a little bit easier to diagnose. There can possibly be a treatment depending on what is causing the protein in the urine.

It sounds unlikely to me that Sunny's recent symptoms are caused by the Ivermectin. I would still stop the ivermectin for now. It's not a bad idea to mention to your vet how much you are giving so they can determine if it really was a toxic dose, just to be safe. Then your vet can advise you on when to start up on Heartgard.

It would be a great thing if your vet could refer you to a veterinary ophthalmologist. You will pay a little bit more for an appointment with this type of vet, but in the long run it will likely get you a diagnosis quicker. That way, if there is anything that can be done for Sunny's vision then the sooner treatment is started, the better!

I hope things improve soon!

Dr. Marie.



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Dr. Marie replied:

Hmmm....I don't think either of those issues would directly cause weight gain. That's got me puzzled as well.

I think a thyroid issue is certainly a possibility but can't really connect hypothyroidism to blindness.

If you do get some answers to what is going on with Sunny I would love to know, so feel free to post back again when you next talk to your vet.

Dr. Marie.