Pyrimethamine

Description

Pyrimethamine, along with a sulfa antibiotic, is used by veterinarians in the treatment of equine protozoal myeloencephalitis. The combination of drugs is effective in blocking the process of metabolism of the protozoa and a step in the protazoa's folic acid synthesis, creating a synergistic effect against Sarcocystis neurona. Although the combination of drugs does not kill the protozoa, it inhibits further growth or reproduction.

Usage

Pyrimethamine is used along with either sulfadiazine or sulfamethoxazole in the treatment of equine protozoal myeloencephalitis. An FDA-approved combination of sulfadiazine and pyrmethamine is now available.

Dosage and Administration

Side Effects

The combination of pyrimethamine and sulfa may cause anemia, decreased platelets, decreased white blood cell counts, and suppressed bone marrow. Loss of appetite, diarrhea, depression, or lethargy are other side effects. Temporary neurologic symptoms can occur in horses and is thought to be due to inflammation caused by dying parasites in the central nervous system.

Precautions

Pyrimethamine should be used with extreme caution in horses with blood or bone marrow problems. The diagnosis and treatment of equine protozoal myeloencephalitis is a controversial area, with newer drugs that work by killing the protazoa now available. In all cases, a veterinarian should determine what is best for a particular horse.

Pyrimethamine is not FDA-approved as a drug for horses, but it's use is commonly accepted practice. It is a prescription drug restricted to use by or on the lawfully written or oral order of a licensed veterinarian.

It is unlikely that these drugs would be used in a horse that is actively competing, but would be prohibited in any drug-free competition. It is important to check with the individual regulatory organization.

Interactions

No drug interactions are listed for horses.

Overdose

No information is available.