Veterinary Therapeutics • Vol. 5, No. 1, Spring 2004 34 that can cause encephalomyelitis in livestock and laboratory animals. Equine protozoal my- eloencephalitis (EPM) is predominantly caused by S. neurona infection of the central nervous system (CNS). 1–4 Approximately 50% of horses ■ INTRODUCTION Sarcocystis neurona, Neospora spp, and Toxo- plasma gondii are related protozoan parasites Evaluation of Immune Responses in Horses Immunized Using a Killed Sarcocystis neurona Vaccine* Antoinette E. Marsh, MS, PhD a Jeffrey Lakritz, DVM, PhD, DACVIM b Philip J. Johnson, BVSc(Hons), MS, MRCVS, DACVIM d Margaret A. Miller, DVM, PhD, DACVP c Yu-Wei Chiang, PhD e Hsien-Jue Chu, PhD e a Department of Veterinary Pathobiology College of Veterinary Medicine University of Missouri Columbia, MO 65211 b Department of Veterinary Clinical Sciences The Ohio State University Columbus, OH 54321 c Animal Disease Diagnostic Laboratory Purdue University 406 S. University Street West Lafayette, IN 47907 d Department of Veterinary Medicine and Surgery College of Veterinary Medicine University of Missouri Columbia, MO 65211 e Global Biological R&D Fort Dodge Animal Health Fort Dodge, IA 50501 CLINICAL RELEVANCE Clinically normal horses developed cellular immunity to Sarcocystis neurona following IM vaccination with a commercial killed S. neurona vaccine, as indi- cated by the development of measurable anti– S. neurona IgG antibodies and additional intradermal skin testing. Large-scale independent assessments of the vaccine’s performance and safety are in progress under field conditions. The next step in the evaluation of this vaccine would be to attempt experimental challenge after a reproducible reliable equine model of S. neurona encephalitis has been established that allows for reisolation of the pathogen after challenge. *Funding for this study was provided by Fort Dodge Animal Health, Fort Dodge, IA.