EPM Vaccine? Fact or Fiction
By Dr. Daniel K Howe
Gluck Equine Research Center

During the past decade, equine protozoal myeloencephalitis (EPM) caused by the protozoan parasite Sarcocystis neurona, has been one of the most visible and controversial diseases in the area of equine health. While disease awareness is always viewed as advantageous, it unfortunately has been somewhat counter-beneficial in the case of EPM since misdiagnoses with this disease have become common in horses that exhibit signs of neurological deficit.

In addition to the diagnostic challenges, chemotherapeutics for EPM are expensive, somewhat unreliable, and may not provide a satisfactory outcome since treatment is often initiated after irreversible damage to the CNS has occurred. Consequently, EPM is too often a death sentence for a beloved and or valuable animal.

These problems illustrate the significant need for an effective vaccine that can protect against S.neurona-induced CNS pathology, thereby reducing the concerns associated with diagnosis and treatment of this disease.

Vaccination is a prophylactic tool that is designed to stimulate an immune response to a particular infectious agent such that a subsequent exposure to the pathogen results in minimal infection and  little or no disease. In simple terms, development of a vaccine involves identifying pathogen antigens that will elicit protective immunity and determining how best to present these antigens to the animal in order to induce the proper response. 

However, the mammalian immune system and the molecular composition of most pathogens (including s.neurona) are far from simple; as a consequence, the interplay between the immune system and the parasite is exceedingly complex. Therefore, considerable aid in designing an effective vaccine is gained by obtaining a basic knowledge of the infectious agent and the pathogenesis associated with the disease. 

Unfortunately, there remains a distinct lack of information about s.neurona, the events that occur in the horse during exposure to the parasite, and the factors that influence the progression from simple infection to full blown neurological disease. While a comprehensive understanding of EPM is not an absolute necessity for vaccine development, the current deficiency in data about this disease represents a considerable barrier to the ration design of an effective vaccine.

Consequently, efforts toward identifying immunization methods that protect against EPM will be significantly benefited by basic research on s.neurona and the equine immune system. 

So what are the prospects for the development of an effective EPM vaccine?  Although the problem can be viewed with some optimism, it is revealing that no commercial vaccine against an apicomplexan parasite (the phylogenetic group that contains s.neurona) is currently available

It is possible that protection against EPM will be provided by crude vaccines that have been developed with a minimum of effort and little additional knowledge about the disease, and there is just reason to pursue these simple approaches. 

Past experiences with vaccine development for related parasite, however, suggest that these attempts will have limited success because of incomplete protection and/or undesirable side-effects. More likely, the development of a safe and effective EPM vaccine will come from a substantial foundation of basic research that provides a better understanding of the disease, some trial and error in the laboratory and in the field, and perhaps a small measure of luck.

Dr. Daniel K. Howe
859-257-2437