Isoxasoline Flea Treatment and Seizures

Recently published data (2020) suggests that adverse events (AE) - the new word for side effects - are higher than reported by the manufactures of isoxazoline flea treatments. Specifically, researchers looked at Bravecto (fluralane), Nexgard (afoxolaner) and Simparica (Sarolaner) in both the US and Europe.


What did they find?

This is a TidBit Tuesday-style abbreviated summary. For the full report see the link at the bottom. There is a lot of data in this report!

  • The survey (called Project Jake Survey) had 2751 respondents. It was compared to the data requested of the FDA and the EMA (European Medicines Agency).

  • Almost 69% gave Bravecto, 26% gave Nexgard and a small 5% gave Simparica.

  • Overall, 66.6% of dogs had a reaction to treatment

  • Of the 911 dogs getting Bravecto, 791 had an AE (86.8%)

  • Of the 342 dogs getting Nexgard, 235 had an AE (68.7%)

  • Of the 72 receiving Simparica, 44 had an AE (61.1%)

What about seizures??

Death and seizures were a focus for this report, however other side effects are certainly noted and were reported. I will focus on seizures.

FDA report: Overall, 5.34% of dogs experienced seizures

  • 20% of dogs receiving Simparica

  • 2.8% of dogs receiving Bravecto

  • 6.9% of dogs receiving Nexgard

EMA report: Over all 30.3% of dogs experienced seizures (!!)

  • 60.2% of dogs receiving Simparica

  • 18.7% of dogs receiving Bravecto

  • 46.69% of dogs receiving Nexgard

Project Jake Survey:

  • 4.5% of dogs receiving Simparica

  • 14.8% of dogs receiving Bravecto

  • 12% of dogs receiving Nexgard

Why are the numbers SO MUCH higher in the EMA?

Perhaps it is related to genetics in the dog population, better reporting habits, or reporting by vets vs owners (or vice versa). That question wasn't answered in the report. The take home message for me was that Simparica is blowing things out of the park with seizures but that the other two still have a substantial group of dogs with seizures as an AE.
Critically, this report did NOT state if the dogs had ever had seizures prior to administration of the drugs. They did state that at least some of the dogs had novel seizures after administration of one of these drugs...but alas we don't know the details on this.

When do the majority of Adverse Events occur?

Within the first 24 hours. These are supposed to be insect specific GABA mediated chloride channel blockers that are distributed around the body in the first 24 hours so it isn't surprising that this is the highest spike of possible toxicity to the dog. Also, AE were reported as a small spike between 8-14 days.

So, what is the take away?

  • Any of the drugs in this report (Bravecto, Simparica or Nexgard) can have seizures as a side effect. Are they a cause of seizures? I'd say probably yes, for at least some of the dogs. Perhaps they lowered the threshold in prone animals too?

  • It looks like Simparica may have a higher incidence of seizures

  • If a dog has a FIRST seizure (within 24 hours or at 8-14 days) after using an isoxazoline drug, discontinue use and try a different class or different drug in this class.

Link: https://onlinelibrary.wiley.com/doi/epdf/10.1002/vms3.285. Publication date 2020. This was a very interesting read and eye opening for me.


Keep those consults coming! The more we work together, the more we learn from each other.

This TidBit Tuesday grew from a recent spike in questions about flea/tick medications from some of YOU...so, thank you!

I look forward to working with you and your staff, soon!