The Harry M. Zweig Memorial Fund for Equine Research

Equine Protozoal Myeloencephalitis: Isolation of Potential Agents from Wildlife

Dr. Dwight D. Bowman

Several years ago it was believed that the source of the agent causing equine protozoal myeloencephalitis had been discovered. That is, it was believed that the cycle into which the horse was being unfortunately introduced was the life cycle of a Sarcocystis species, Sarcocystisfalcatula, wherein the opossum served as the source of environmental contamination and the grackle served as the reservoir through which opossums became infected. Unfortunately, this does not appear to be the case. Although, there was one initial report from Kentucky of disease being caused in horses fed sporocysts from opossum feces, this has not been replicated in two trials in Florida, nor in a trial here. Thus, it would appear that the initial jubilation over having discovered the source of equine infection was misplaced, and we now find ourselves almost exactly where we were when the organism was first described as Sarcocystis neurons in 1991.

Advances have been made, however, relative to the development of tools to look for the infection in the wild. Our lab, and several others, now have expertise in maintaining Sarcocystis species in culture. We have avian models, the parakeet, which allows the growth of pathogenic tissue-culture infective stages that can be used for parasite isolation. We now have the technology that allows the molecular separation of species. Also, we are now developing the tools that can be used to differentiate the organisms both morphologically and antigenetically. Thus, although we still do not know what the source of infection is for the horse, we now have in place many tools that did not exist for this purpose in 1991.

Thus, the goal of this research will be to examine a series of North American wildlife to identify the host that serves to produce the sporocysts that are infecting horses. The work will simply consist of reproducing what has been done with the opossum-grackle model system with other carnivore(or omnivore)-prey systems. The work is being carried forward in conjunction with similar studies being performed by the USDA in Beltsville under the direction of Dr. Dubey. We believe that application of the tools which have been developed should allow us to rapidly identify several other potential species that are likely to be sources of infection in the horse.