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The drug combination of pyrimethamine and sulfadiazne is a commonly prescribed treatment for equine protozoal myeloencephalitis (EPM). Pyrimethamine is an anti-protozoal drug and sulfadiazine is a sulfa antibiotic. These drugs combined have a synergistic effect against Sarcocystis neurona, the protozoa that cause EPM. Each drug blocks a different step in the metabolism of the protozoa by inhibiting a different enzyme necessary for the synthesis of folic acid. This drug combination does not kill the protozoa; it only inhibits further growth or reproduction. Neither drug alone is considered effective for the treatment of EPM.
The diagnosis and treatment of EPM is a controversial and rapidly evolving field. A great deal of research effort is being focused on this disease, primarily because there is still controversy and disagreement surrounding what constitutes an accurate diagnosis. Treatment for EPM with pyrimethamine/sulfa has been shown to be effective but usually requires long-term medication (12 to 24 weeks). There are conflicting opinions among veterinarians regarding folic acid supplementation for horses on this drug combination. The concern is that long-term use of these drugs might inhibit folic acid metabolism in the horse.
The combination of pyrimethamine, and sulfa may cause anemia, decreased platelets, decreased white blood cell counts, and suppress bone marrow. Pyrimethamine should be used with extreme caution in animals with blood dyscrasias or bone marrow suppression.
• Because pyrimethamine and sulfadiazine combination have caused leukopenia, thrombocytopenia, and anemias, the regular monitoring of peripheral blood work is suggested.
• Pyrimethamine has been shown to cause birth defects in other species. Some veterinarians have reported cases in which they thought this EPM treatment caused abortions or reproductive losses in mares.
• Pyrimethamine is excreted in the milk of other species. This drug combination should only be used in pregnant or lactating mares when the benefits outweigh the potential risks.
• Limited research on pyrimethamine/sulfa combination showed no adverse effect on semen in stallions.
• Pyrimethamine should be safe to use in older horses with normal liver and kidney function. The package insert for pyrimethamine cautions against using this drug in humans with decreased kidney or liver function.
No drug interactions are listed for horses. Interestingly, the use of Sulfamethoxazole-Trimethoprim (SMZ-TMP) with pyrimethamine is not recommended in humans because of the risk of bone marrow suppression. This does not seem to be a common side effect in horses.
There was no information found on overdose of pyrimethamine in the horse. Acute overdose of pyrimethamine in humans caused GI distress, central nervous system stimulation including seizures and hematologic effects. No specific information is available on overdose of sulfa drugs in horses although GI signs including diarrhea are the most common adverse side effect.
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
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