Flea, Tick and Heartworm medications and Seizures

Introduction to Flea, Tick And Heartworm medications and Seizures

Our neurology team frequently gets questions about flea, tick and heartworm medications and seizures. There is a lot information on the internet that flea medication can cause seizures, and that is partially true. A small amount of pets can have seizures due to flea medications, however it is not anywhere as common as some websites would have you believe. Reports are that this happens in 0.01% of dogs, which makes it very rare.

Why Do Some Dogs have seizures and some don’t?

We don’t fully understand why this toxicity happens, however it’s possible that it’s due to a problem with a system in the body called the blood brain barrier. The blood brain barrier keeps your brain protected from all sorts of things, including some medications. Flea, tick and heartworm medication should not be able to cross a dog’s blood-brain barrier, and therefore it should not get in to the brain to cause seizures.

The vast majority of dog’s have a normal blood brain barrier. When the blood brain barrier is not normal, either genetically or due to disease, these medications may get in to the brain and cause seizures. There is a test for one of these abnormalities, MDR-1, and depending on your dogs breed this test maybe recommended.

One class of medications, called isoxazolines, has been associated with seizures and other neurologic symptoms in some cats and dogs. Products that include isoxazolines include Bravecto, Nexgard, and Simparica and Credelio. In one report, Simparica had the highest incidence of reports of seizures. Opinions are mixed as to whether pets who have been on these medications long term can continue them should they develop a seizure disorder.

How can I tell if my dogs seizures are due to this medication

If your pet is sensitive to these medications, they will likely have a seizure or other neurologic signs within a day of the very first application or dose of these medications. There are rare reports of delayed reactions. If you change flea medication in the last three months, and consequently your dog has developed a seizure condition, this is a good indication that they should not take that medication. Good news though! As long as they do not take this medication, they should not have further seizures.

If your pet has seizures at times other than around medication adminstration, it’s unlikely that the medications are related to the seizures. Along those same lines, if your pet has been on this medication for years without problems in the past, it’s likely that this medication is still safe to use.

Think of this sort of like an allergy. If you are allergic to peanuts, your body will react poorly to them. However, if you aren’t allergic to peanuts, then you can eat them without any problems.

What if my dog has epilepsy or another disease causing seizures?

A dog with epilepsy is no more likely than a dog without epilepsy to have this adverse drug reaction as far as we know. So if your pet has been on a medication safely for years, and later develops a seizure condition, it’s unlikely that these drugs will have an effect. The majority of our patients have not had a change in their seizure frequency when starting or stopping these medications. However, if you are nervous, it’s always reasonable to try a medication that does not contain isoxazolines.