Pyrimethamine/Sulfadiazine Combination for Horses
Contents
General Drug Information and Indications
How to Give this Medication
Side Effects
Precautions
Drug Interactions
Overdose
Storage
Search for Available Dosage Forms
Pyrimethamine/sulfadiazine combination is used to treat equine protozoal myeloencephalitis (EPM) in horses. These drugs combined have a synergistic effect because each one blocks a different step of the protozoa’s metabolism. Treatment of EPM with pyrimethamine/sulfadiazine has been shown to be effective although it usually requires long term medication (3-6 months of treatment). This combination is FDA approved for the treatment of EPM in horses. When the appropriate form or dose of this drug is not available through a veterinary pharmaceutical manufacturer, it may be compounded by a specialty pharmacy.
Give this medication to your horse exactly as your veterinarian prescribes. If you miss giving your horse a dose of pyrimethamine/sulfadiazine combination, give the next dose as soon as you remember or, if it is close to the next scheduled dose, return to the regular schedule. Do not double dose to catch up.
Wash your hands after giving your horse this medication.
Be sure to discuss any side effects with your veterinarian immediately.
Some horses may show a worsening of clinical signs at the beginning of treatment. This is thought to be caused by inflammation in the central nervous system due to the dying off of the protozoal parasites. This “crisis” may go on for as long as 5 weeks.
Pyrimethamine/sulfadiazine combination may cause changes in your horse’s blood counts or bone-marrow suppression. Your veterinarian may choose to follow your horse’s blood counts on a regular basis while it is on treatment. Some veterinarians recommend folic acid supplements for horses that are being treated with pyrimethamine/sulfadiazine combination.
Other side effects are related to the digestive tract. They include loss of appetite, and diarrhea. Occasionally a horse will develop hives.
Keep this and all drugs out of reach of children. Pyrimethamine/sulfadiazine is a prescription drug and should be used according to your veterinarian’s directions. It should only be given to the animal for which it was prescribed. Do not give this medication to a person.
Pyrimethamine has been shown to cause birth defects in other species. This drug combination should not be used in pregnant or nursing mares unless the benefits outweigh the potential risks.
Limited research in stallions on pyrimethamine/sulfadiazine combination showed no adverse effect on semen.
Be sure to review with your veterinarian any medications or supplements your horse may be receiving.
The following drugs may have interactions with pyrimethamine/sulfadiazine combination: methotrexate, phenylbutazone, thiazide diuretics, salicylates, probenicid, warfarin, PABA, trimethoprim, and antacids.
If you suspect your horse or another animal was overdosed accidentally or has eaten this medication inadvertently, contact your veterinarian or the A.S.P.C.A.’s Animal Poison Control Center at 888.426.4435. Always bring the prescription container with you when you take your pet for treatment.
If you or someone else has accidentally ingested this medication call the National Capital Poison Center at 800.222.1222.
Different strengths or dosage forms of pyrimethamine/sulfadiazine combination may have different storage requirements. Read the labeling or ask your pharmacist for the storage requirements of the prescription you receive.
About the Author
Dr. Barbara Forney is a veterinary practitioner in Chester County, Pennsylvania. She has a master's degree in animal science from the University of Delaware and graduated from the University of Pennsylvania School of Veterinary Medicine in 1982.
She began to develop her interest in client education and medical writing 1997. Recent publications include portions of The Pill Book Guide to Medication for Your Dog and Cat, and most recently Understanding Equine Medications published by the Bloodhorse.
Dr. Forney is an FEI veterinarian and an active member of the AAEP, AVMA, and AMWA.
You can purchase books by Dr. Forney at www.exclusivelyequine.com