United States Eventing Association

The U.S. Food and Drug Administration has released a statement that four horses have died after receiving a compounded drug containing pyrimethamine and toltrazuril, used to treat equine protozoal myeloencephalitis.

Reports of adverse events, including seizures, fever and death, involved two horses in Kentucky and eight horses in Florida. Four horses have died or been euthanized and six horses are recovering from an apparent reaction to the drug.

Two lots of the compounded medication, one in a paste form and one in an oral suspension, have been accounted for and are no longer in distribution.

"At this time, FDA testing indicates that one lot of product contained higher levels of pyrimethamine than the labeling indicates. Adverse events associated with high doses of pyrimethamine include seizures, fever and death.

Horse owners, caretakers and veterinarians should report to FDA any symptoms such as seizure, fever or collapse in horses who might have received high doses of pyrimethamine. During the course of this on-going investigation into the adverse reactions, it has come to the attention of FDA that elevated doses of pyrimethamine may have been used by some practitioners with negative results. The usual dose of pyrimethamine in horses is 1 mg/kg, which has been shown to be safe for the treatment of EPM in an FDA-approved combination product containing pyrimethamine and sulfadiazine.

These compounded combination products are not approved animal drugs. Additionally, toltrazuril is not approved for use in horses. In general, FDA has serious concerns about unapproved animal drugs, including certain compounded animal drugs. These drugs are not evaluated by FDA and may not meet FDA’s strict standards for safety and effectiveness. Unapproved animal drugs also may not be labeled or advertised appropriately. Horse owners or caretakers should consult with their veterinarians about other therapies.

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."