Photo: Noah D. Cohen, VMD, PPH, PhD, Dipl. ACVIM

Equine protozoal myeloencephalitis (EPM) is one of the most discussed diseases in the horse health industry. Many owners are familiar with the clinical signs horses display once affected, but how much do they know about Sarcocystis neurona, one of the disorder's causative agents?

During a presentation at the Advances in Equine Neurological Diseases Symposium, held Dec. 6 in Lexington, Ky., Daniel K. Howe, PhD, a molecular parasitologist at the University of Kentucky Gluck Equine Research Center, provided extensive insight into the parasite and its relationship to EPM.

Lifecycle and Infection

Howe first discussed S. neurona's two-host lifecycle--something many horse owners are familiar with. To complete its life cycle, this organism requires a definitive host (the opossum), which feeds on the muscles of a dead intermediate host (such as a raccoon, skunk, cat, or armadillo) containing S. neurona sarcocysts. Once ingested by the opossum, the parasites mature to their infective stage (sporocysts), which the opossum passes in its feces.

Horses, which are generally considered "dead-end hosts" (meaning they typically can't pass the protozoa on to other animals), contract the disease by ingesting infected matter, often grass or hay contaminated with opossum feces containing S. neurona sporocysts.

Once the horse ingests the sporocysts, the parasite essentially makes itself at home in the horse's body. At this point the horse becomes infected with (or exposed to) the organism. Studies suggest 30-50% of horses in the United States have been infected with S. neurona; fortunately, less than 1% of infected horses develop signs of disease.

What causes the variation between S. neurona infection and disease remains unclear, Howe noted. He said researchers have suggested that individual horses' immune competency when faced with the parasite and parasite strain virulence might be influential factors, but these remain untested. Regardless of the cause, the discrepancy creates challenges for researchers when developing effective diagnostic measures and when creating new measures for disease prevention, such as vaccinations.

Diagnostic Testing: Past and Present

Howe relayed that there are several diagnostic testing options available for EPM. Two common tests are the Western Blot and the immunoflourescence antibody test:

  • Western Blot--The first commercially-available EPM test, the Western blot detects S. neurona antibodies either in blood (serum) or in cerebrospinal fluid. A positive Western blot merely means the horse has been exposed to the parasite and has developed antibodies to S. neurona. A positive test does not necessarily mean the horse is actively infected. In contrast, a negative test suggests the horse is not infected with S. neurona. Howe relayed that drawbacks to this test include subjectivity (a person must decide whether the test is positive or negative as opposed to getting a 'yes' or 'no' answer) and nonquantitative results.
  • Immunoflourescence antibody test (IFAT)--This test measures antibody levels against S. neurona in horses' serum. Like the Western blot, a positive result simply indicates exposure to the parasite, but it does not necessarily mean the horse is suffering from disease caused by S. neurona. He added that the IFAT can also be subjective in its results.

Howe then described relatively new assays used for detecting S. neurona infection and disease. These enzyme-linked immunosorbent assays (ELISA) measure antibodies to the surface antigens (SAG) of S. neurona. (An antigen is a disease-causing substance that stimulates the immune system.)

Originally, the tests detected antibodies against the primary surface antigen (SAG1). Further developments, however, found that detection of antibodies against SAGs 2, 3, and 4 provide the most accurate indication of S. neurona exposure or disease. Howe explained that research has found strains of S. neurona had either SAG1 or SAG5, but all possessed SAGs 2, 3, and 4.

Another advantage to the SAG ELISA is that quantitative data is obtained by measuring optical density (which utilized color change in a solution to determine the amount of antibodies against the parasite in the infected horse). The ability to obtain quantitative data allows researchers to compare antibody levels in the blood versus the cerebrospinal fluid, which can indicate if infection is active in a horse's central nervous system (brain or spinal cord).

S. neurona Genome Project

Finally, Howe discussed a relatively new project nearing completion in his laboratory at the Gluck Center. To gain a better understanding of the EPM-causing parasite, Howe and colleagues are sequencing the S. neurona genome.

He currently estimates the parasite has about 125,000,000 DNA base pairs and between 9,000 and 10,000 genes.

A fully sequenced genome will provide "a resource for discover and characterization of S. neurona antigens and virulence factors" among other benefits, Howe explained.

Upon completion, the genome sequence will be available online.