Published May, 2012

Dan Howe, PhD, a professor and molecular parasitologist at the University of Kentucky Gluck Equine Research Center, and colleagues are finishing up a three-yearSarcocystis neuronagenome project. The primary goal of the project, titled "Genome Sequence for the apicomplexanSarcocystis neurona," has been to sequence and assemble the genome of S. neurona, the protozoan (single-cell) parasite that causes protozoal myeloencephalitis (EPM) in horses.

Prior to the S. neurona genome project, Howe had been conducting preliminary sequencing studies in his lab funded by a gift from Thoroughbred breeders John and Jerry Amerman. In 2009 he received a $500,000 grant from the USDA-CSREES (U.S. Department of Agriculture-Cooperative State Research and Extension Service) competitive grants for his research.

S. neurona causes equine protozoal myeloencephalitis (EPM), which is one of the most important and commonly diagnosed neurologic diseases in the United States. EPM has a tremendous ongoing impact on the equine industry and equine health due to the considerable cost of diagnosis and care to fully recover an affected horse. Some horses might never recover entirely. Clinical signs vary from horse to horse but include loss of coordination, muscle atrophy, sore back, stumbling, locking of the stifle joint, and weakness.

The parasite's life cycle initially begins in the definitive host, which is the opossum that passes the S. neurona oocysts and sporocysts in its feces. To complete its life cycle, this parasite needs two hosts, one definitive (final) and one intermediate.

Horses serve as an accidental intermediate host of S. neurona. Horses represent a dead end for the parasite life cycle since they cannot pass the disease among themselves or contract the disease from another infected horse. When grazing on the pasture, ingesting feed or water contaminated with opossum feces, the infective stages of the organism are passed to the small intestine of the horse, where emergence from a cyst eventually takes place.

The infective stage of the organism undergoes several replicative cycles in the blood vessels. In some horses they migrate to the central nervous system where the parasites cause severe neurologic disease. This neurologic damage can cause various signs, all dependent on the area of the central nervous system parasitized.

This project's purpose has been to sequence S. neurona, to compare it to sequences from other related parasites, as well as to make the information available to the research community, Howe said.

According to Howe, the study has succeeded in providing new insight to the protozoan that might be useful for the closely related important human parasites such as Toxoplasma and Plasmodium (malaria) that are known to cause significant disease in both humans and other mammals.

The biology of S. neurona resembles other pathogenic apicomplexans and is fairly complex with multiple life cycle stages. Howe said the sequencing efforts have revealed molecular diversity among different S.neurona strains and otherSarcocystis species.

"We have been able to sequence one representative strain at this point, but we hope to obtain genome sequences from at least one, if not several, additional strains," Howe said. "The tremendous drop in the cost of sequencing now makes this feasible. But at this point, there is no evidence to suggest that any one strain type is more important (i.e., more pathogenic) in horses than others."

Howe said the genome sequence will contribute to an increased understanding of S. neurona and thereby enhances its value as a comparative model for other members of this important group of pathogens.

"A fully sequenced genome will also provide a resource for characterization of virulence factors and S. neurona antigens, which can lead to new protective immunizations or chemotherapeutics against this parasite," Howe said. "However, there is a tremendous amount of data to be interpreted before the initial lab work on potential vaccine candidates and chemotherapeutic agents can get started."

The information from the study will be posted to a public database available for research on EPM. Howe has been collaborating on the project with Chris Schardl, PhD, the Harry E. Wheeler Chair in Plant Mycology and director of the University of Kentucky Advanced Genetic Technologies Center (UK-AGTC), and Jessica Kissinger, PhD, of the University of Georgia.

Shaila Sigsgaard is a contributing writer for the Bluegrass Equine Digest.