My horse was diagnosed with EPM last year. We tried Baycox for one month--no change. Since then, he has been on a daily dosage of sulfadiazine/pyrimethamine combination (SDZ/PYR) for approximately nine months. Would it be advisable to give him Marquis and SDZ/PYR together? One veterinarian says yes, another no. Also, isn't staying on the SDZ/PYR indefinitely compromising his gastrointestinal health? The horse has had two spinal taps and his numbers went from about 25 down to 10. Will his numbers ever reach "0?" He presently shows no symptoms. Sorry, I know I've asked quite a few questions here.
via e-mail

It's hard to make a one-size-fits-all comment on dose duration for SDZ/PYR, as some veterinarians base recommendations on duration and severity of disease as well as other treatment history and response.

It is safe to say that most veterinarians recommend a minimum of 60 to 90 days on SDZ/PYR, and many horses are treated for up to six months or longer (nine months in your case). It is recommended that the SDZ/PYR combination be given on an empty stomach (most veterinarians recommend giving no food for two hours before through two hours after treatment). The food interferes with the absorption and availability of the drugs.

Now, regarding your specific questions:

1. Can I use Marquis and SDZ/PYR concurrently?

Bayer does not have data on the safety or efficacy of Marquis when used in conjunction with other treatments. As such, we cannot support or encourage use of combination treatments.

2. One veterinarian says don't use them in combination and another says yes.

Certainly we respect opinions on both sides of this issue. As noted above, Bayer has no safety data on Marquis when used in combination with SDZ/PYR. The rationale behind this combination therapy is as follows: These two drugs--Marquis and SDZ/PYR--have completely different "modes of action" against the Sarcocystis neurona parasite. Based on this, some practitioners feel that you get a "one-two punch" effect on the parasite since you are "attacking" them in two different ways.

3. Isn't keeping a horse on the SDZ/ PYR medication combination indefinitely compromising his gastrointestinal health?

Many horses have been on this drug regimen for many, many months without any adverse side effects. Having said that, it is important to realize that there are some adverse side effects seen in a small percentage of horses receiving this medication. The most common is anemia, which is a drop in the number of the oxygen-carrying red blood cells in the horse's blood. When present, this is seen while the animal is on the drug--once the drug is removed, the animal's red cell count returns to normal.

The other side effects are rare, and include abortion in pregnant mares, birth defects, stallion dysfunction, and leukopenia (a drop in the animals' white blood cell count).

4. The "numbers" from two spinal taps went from 25 to 10; will his numbers ever reach "0?"

This is a good question and probably one that nobody is able to answer. In certain horses, the antibody levels within the spinal fluid might drop to zero following treatment. There are veterinarians who use this measurement as a criteria for when to discontinue drug therapy. In other words, some veterinarians recommend treating the animal with the SDZ/PYR combination until antibodies in the spinal fluid can no longer be detected. We know that antibodies to S. neurona in the spinal fluid can be present for many months and in some animals perhaps for years. The other issue to consider here is that if the horse continues to be exposed to this parasite, then the immune response (the production of antibodies) will continue.

5. He presently shows no symptoms.

This is perhaps the most important piece of information in the letter. If the horse is showing no symptoms, and has been treated with SDZ/PYR for nine months, then many veterinarians would ask why the horse is currently on medication and why we are considering adding more medication, i.e. Marquis. I would be inclined to take the horse off of the SDZ/PYR combination--especially if you have safety concerns about long-term therapy. If the clinical signs were to return, you could then start the horse back on the SDZ/PYR or Marquis.

About the Author

Kenton Morgan, DVM

Kenton Morgan, DVM, is the Senior Technical Services Veterinarian with Bayer Animal Health, which manufactures the equine protozoal myeloencephalitis (EPM) medication Marquis.

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