The U.S. Department of Agriculture announced on Dec. 18 that it had issued a conditional license to Fort Dodge Laboratories, Inc., of Fort Dodge, Iowa, a division of American Home Products, Inc., for a vaccine intended to aid in the prevention of equine protozoal myeloencephalitis (EPM). The Animal and Plant Health Inspection Service (APHIS) issues conditional product licenses under its regulatory authority to meet an emergency condition, limited market, local situation, or other special circumstances.

“Under these regulations, a product that is shown to be pure and safe and demonstrates a reasonable expectation of efficacy may be licensed while data to establish efficacy and potency are obtained,” said Alfonso Torres, deputy administrator for APHIS’ veterinary services program. The center for veterinary biologics, licensing, and policy development staff of veterinary services issued the license.

EPM is a debilitating and sometimes fatal neurologic disease of horses linked to infection by a protozoal parasite most often identified as Sarcosystis neurona. Opossums and birds are the normal hosts for S. neurona; horses can become infected by ingesting the organism in food or water contaminated by opossum fecal material. In some cases, infection by the parasite might result in damage to the central nervous system, particularly the spinal cord.

Diagnostic procedures currently available for use on living animals are difficult and often unreliable. Treatment regimens for EPM are costly, often of limited effectiveness, and in a small number of treated animals, symptoms recur when therapy is discontinued.

Nationwide, exposure of horses to S. neurona has been estimated to be as high as 50% based on the presence of antibodies to the organism in tested horses, with higher rates reported in certain areas. Evidence of exposure and subsequent development of disease, however, are not well correlated and most estimates of