Horse Side Vet Guide ®

Equine Health Resource

Anti-Protozoal Drugs



Anti-protozoal drugs are primarily used to treat Equine Protozoal Myeloencephalitis (EPM), a common neurologic disease usually affecting the spinal cord. There are several classes of these drugs.

Drugs that have been evaluated and approved by FDA for the treatment of EPM include ponazuril, diclazuril, and a combination drug of pyrimethamine and sulfadiazine. Other drugs are likely compounded. Generally, there is no reason for use of compounded medications, as the FDA approved drugs exist.

Anti-protozoal drugs kill or inhibit growth of protozoa in a variety of ways, depending on the specific drug.

Due to the difficulty in definitively diagnosing EPM, your vet may choose to use these drugs as a diagnostic tool. If your horse responds to the drug course, this may confirm the diagnosis of a protozoal infection and warrant longer term treatment. These medications are typically given daily for a month or more at a time.

Consider Potential Side Effects & Complications

Some of these drugs can cause anemia by interfering with the development of red blood cells. Some can cause diarrhea. Sometimes there is a worsening of signs of disease a few days into treatment. This may be caused by die off of parasites and associated inflammation.

In 2014, a number of horses died after being treated with a compounded anti-protozoal combination of toltrazuril and pyrimethamine.

Consider Reasons Not To Use This Treatment

These drugs should not be used without some diagnostic evidence that there is indeed a protozoal infection. When they are used diagnostically, it should be clear that they are being used this way.

Is It working? Timeframe for effect

In many cases of EPM, there will be noticeable signs of improvement within 1-2 weeks. In other cases, any change or improvement may be very vague, and hard to discern. In that case, the course of medication is usually completed.

Questions To Ask My Vet

  • Given my options, why have you chosen this particular drug?
  • How soon should we see the effect, if any?
  • How will we decide if the horse is improved or not, if the signs are so subtle to begin with?
  • What are the side effects and complications for the specific anti-protozoal drug you recommend?
PRN Pharmacal ReBalance® Antiprotozoal Oral Suspension
* Would you like to showcase your equine product or service in
Horse Side Vet Guide? Contact us to start the conversation.
Author: Doug Thal DVM Dipl. ABVP


We're not around right now. But you can send us an email and we'll get back to you, asap.