Do Popular Dog Tick Treatments Cause Seizures?

Updated November 28, 2020

There’s a new health scare in dogs and cats. Four common flea and tick treatments are being linked with nervous system disorders like muscle tremors, twitching, unsteadiness and even seizures.

Most of the discussion so far has been from the USA, but Australians have a unique perspective on this. The drugs we’re talking about are the best chance we’ve ever had to end the greatest fear of many pet owners: tick paralysis.

Tick paralysis? If like me, you live outside a tick area this might surprise you.

What Is Tick Paralysis?

Ticks are tiny bloodsucking parasites known to spread many blood-borne infections. Tick paralysis is caused by a toxin from the saliva of only certain species of Australian ticks. The map shows where they are found.

Adapted from Roberts FHS (1970) Australian Ticks. Yeerongpilly QLD by TAGS Inc, Bill Conroy & Norbert Fischer [Attribution], via Wikimedia Commons

Click here to learn how to recognise the signs of tick paralysis in dogs & cats.

Tick prevention options are found at our review of flea and tick treatments for dogs. However, it’s not as simple as it looks. The products that rely on being put on the outside of the animal (Frontline®, Advantix® and Seresto®) aren’t thought by most vets to be reliable enough, and are easily disturbed by bathing.

It’s the newer tablet-based Isoxazoline products that are the real game changers. Currently, these are Nexgard®, Bravecto® and Simparica®, and Credelio®. The regular use of these products is thought to give nearly 100% protection against ticks, although checking is still recommended.

But what, I hear you ask, about the side effects?

Simparica, Bravecto, NexGard & Seizures

Seizures in dogs are common, but it does appear that the risk increases when using Nexgard, Bravecto and Simparica. The only question is by how much. This is where life gets complicated.

The US Food and Drug Administration has issued the following public notice:

  • Isoxazoline products have been associated with neurologic adverse reactions, including muscle tremors, ataxia, and seizures in some dogs and cats
  • Although most dogs and cats haven’t had neurologic adverse reactions, seizures may occur in animals without a prior history
  • The FDA considers products in the isoxazoline class to be safe and effective for dogs and cats but is providing this information so that pet owners and veterinarians can take it into consideration when choosing flea and tick products for their pets

Personally, I have seen one dog develop intermittent twitches for two weeks after the second dose, and a puppy have a seizure after a first dose. Both dogs had other significant health issues, but I suspect the isoxazoline was at least partly responsible.

So when do we use these drugs? There are two groups of dogs to consider:

Dogs In Paralysis Tick Areas

For dogs with no history of seizures: use. The risk and consequences of tick paralysis are much higher than the risk of side effects. Other alternative drugs are likely to offer poorer protection.

For dogs with a history of seizures: consider carefully. Simparica, NexGard, Bravecto & Credelio may not increase the number of seizures, but the risk is real. Whether you use it will depend on how likely it is that your dog will get a tick and your confidence in other methods of prevention.

Dogs Not In Paralysis Tick Areas

For dogs with no history of seizures: consider carefully. Here you are using isoxazolines for the prevention of fleas and mites only. Personally, I consider it to be so far ahead of the rest that the low risk of seizures is easily justified by better results. I use it on both my dogs but you are welcome to do otherwise.

For dogs with a history of seizures: don’t use. Other products exist which can give nearly the same results. An exception might be for dogs with demodex mites, for which there is no good alternative to isoxazolines.

If a dog does have a seizure while on these drugs, it does not appear to be a disaster. These dogs are likely to be the ones who were always prone to seizures, and should go back to having none or very few once the medication is stopped.

The Cause vs Correlation Issue

We will never know how many of the dogs reported to have twitching or seizures would have started having them regardless. This is hard to say to an owner placed in such a situation, and of course I can never be certain. For a view of a similar debate with a different issue, visit our page called Does Bravecto Kill Dogs?

But the reality is that all drugs do have side effects. Vets are often guilty of not talking about them just because it’s such a can of worms. Without wanting to sound complacent, the reality is that everything has a potential downside. It’s up to vets and manufacturers to supply the information so you can make the best informed decision for your dog.

If you live in a tick area, or travel to one, please use these products if you can. Used correctly, they save countless lives. Vets in tick paralysis areas are seeing a big reduction in the number of cases being treated now that the oral treatments have arrived. That’s fewer prolonged stays in vet hospitals and fewer deaths.

Related: Treatment Of Seizures In Dogs | Causes Of Tremors & Twitching

Valuable advice was provided by Dr Jakki Yeomans, a vet working in a paralysis tick area.

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Have something to add? Comments are welcome below and will appear within 24 hours.
By Andrew Spanner BVSc(Hons) MVetStud, a vet in Adelaide, Australia. These articles are from a series regularly posted on email and Twitter. Subscribe via email here to never miss a story!


17 Replies to “Do Popular Dog Tick Treatments Cause Seizures?”

  1. Hi Andrew
    I just wanted to say thank you for your unbiased and thorough information on flea, tick and worm treatments.

    I am concerned in moving from a topical treatment (which alienates my dog from me monthly when she sees me coming with it!) to an oral tablet. She was on Nexguard Spectra as recommended by the vet for her 3rd and 4th month, and then I switched her off it given she was vomiting after each treatment.

    Whilst I know there are no guarantees, your information has helped me feel a little better about making the switch and to watch out for symptoms after her first couple of treatments with Simparica Trio.

  2. Hi Andrew
    Can Simparica cause bloody diarrhoea and bloody vomit? My dog has had both for 24 hours.
    I swapped from Nexgard to Simparica for the first time yesterday but my dog was also on the anti inflammatory Previcox for his CCL he ruptured 2 days ago. We have stopped the Previcox in case it is causing the upset but now I am wondering about the Simparica.
    My dog is a gorgeous 3 year old Australian Terrier.
    Many thanks

    1. Hi Anna. Definitely, if it’s drug-related, the Previcox is the most likely culprit. NSAIDs are known for this and you are right to stop it.

  3. Hi Andrew, thank you your website is excellent and a great support for pet owners.
    Our Greyhound developed head tremors three days after a dose of Bravecto which he had been on for 3 years. It lasted 10 days and I haven’t seen it since (3 months). I am aware that Nexgard and Bravecto both contain isoxazolines but it’s been suggested that Nexgard may still be safe for him as he gets a smaller amount in each dose. Do you think a regular monthly dose might lessen his risk of reaction? Are there any internal medications that don’t contain isoxazolines? I can’t work that out from the table. I don’t want to use an external treatment that will rub off on humans.

    It’s been three weeks without flea protection now and I am concerned about the risk of fleas/tapeworm for my baby and children!

    Thank you

    1. Hi Rebecca. NexGard, Credelio and Simparica are all monthly isoxazolines. You are probably correct that these will have a reduced rate of adverse effects but of course, there’s no evidence for this. Comfortis is a nearly equally effective oral alternative (as long as you don’t need tick or mite control too). Its major adverse effect is vomiting, but only in a percentage of dogs.

  4. Hi Andrew can you please suggest what would be the most appropriate course of action since I want to avoid products containing isoxazolines as my dog has epilepsy and takes phenobarbital. Would Adavantix be appropriate and which wormer would pair well? Thank you for your time.
    Kind regards

    1. Hi Susana. It all depends on what you’re trying to prevent. For ticks, there’s clearly no equivalently effective product, so you need a balanced mix of preventative strategies and products like Advantix, Frontline or Seresto. For fleas and mites on the other hand you have plenty of options listed here.

  5. Hi Andrew,
    I have a four year old border collie, he has recently had two seizures within the last 6 months, both happening a day or two after giving him nexguard.
    He once happily took it but around a year or so ago he stopped wilfully taking it and we had to hide it in treats. The first was a horrible site to see we took him to the vet and he said to monitor it thinking another would occur within a day or two. He didn’t have another for 7 weeks, two days after his next dose, since then it’s been around 9 weeks, we haven’t given him another dose and he’s been seizure free.
    We live on the far south coast of nsw in heavy tick areas, I want him to be protected but am wary at doing that at the cost of more seizures. What are your recommendations?
    Thank you.

  6. Hi Andrew I bought some heart guard by Mistake. Can mix Heart guard with simparica for the monthly treatment of ticks and heart worms. Cheers Nino

  7. Thank you for these Andrew. I live in North Qld and visit regularly the Atherton Tablelands where paralysis ticks are extremely common. I give brevecto to my dog and have for 4 years without issues. So it is nice to read that he is unlikely to suddenly develop issues.

    The breeder I am looking at getting a pup from is heavily against the use of any tablet type tick prevention. But they are in a lower risk area and with the seresco collar have had a paralysis tick and almost lost their dog. You mentioned maybe aiming for something with a monthly dose instead of the 3 monthly. My vet favours brevecto, but likely due to the commonality of people forgetting to give monthlies. What products are monthly and would be most recommended to balance the breeders reluctance with my concern re: shaggy dog and ticks!

    1. Hi Lulu. The two monthly options are Simparica and Nexgard. Neither alas is likely to be favoured by your breeder, but then I’m not much good at breeding dogs either!

  8. Hi Andrew, thank you for your very informative article. I have a 3 & 1/2 year old Westie who had heart surgery at 6 months old and still has a serious heart murmur and is on 1/3 of 50mg tablet twice a day of Atenolol. We live in SA but travel at least twice a year to Qld and NSW with our dogs and so far have not used any prevention but we are vigilant with where they go. Each time we travel I worry, what is your opinion on using one of these 3 products on my boy to keep him safe? Thank you and regards, Sandy

    1. Hi Sandy. My opinion is that they are as safe as any other product, and much safer when in a tick area due to their increased efficacy. In the rare event of a seizure, I have never seen one cause problems in a dog with heart disease.

  9. Hi Andrew, just wanted to say thank you for your article which I found thorough and balanced. I see a FB article regarding Simparica and was concerned for my small one year old dog who uses the brand. We live on the back of the bush in a tick prone area so treatment is VIP. Quick question, in your opinion or experience…. if they have been using this and havent presented any adverse reactions is it likely that it wont develop in the future?? Ps. I had a lab with horrible seizures, just dont want to go back there…it was so awful for the poor dog. Thanks for your time

    1. Hi Angela. Our experience is that dogs that develop side-effects get them with the first or second dose. This is no doubt not universal but seems true in the majority of cases. My dogs are both on Simparica and I have very little concern as they have been taking it for some time.

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