Enzyme-Linked Immunosorbent Assays (ELISAs) Based on Sarcocystis neurona Surface Antigens (SnSAGs)
Daniel K. Howe, Ph.D.
EPM is easy to suspect clinically, but difficult to diagnose accurately due to a variety of reasons. Horses suffering from EPM exhibit clinical signs that resemble other common neurological conditions, so ancillary tests that detect the presence of antibodies against S. neurona are routinely employed. However, asymptomatic S. neurona infection of horses is relatively common, with seroprevalence of 30-50% in areas where opossums occur. Therefore, simple detection of antibodies against the parasite in the blood of a horse is not very informative.
The S. neurona surface antigens (SnSAGs) are related, immunogenic proteins that elicit robust antibody responses in infected horses1. Recombinant forms of the SnSAGs have been incorporated into enzyme-linked immunosorbent assays (ELISAs), and extensive investigations have been conducted to assess and validate various permutations of the SnSAG ELISAs for their ability to accurately detect antibodies against S. neurona2. These studies suggested that optimal sensitivity and specificity could be achieved by running two independent ELISAs, one based on SnSAG2 and the second based on a chimeric fusion of SnSAG4 and SnSAG3 (designated SnSAG4/3)3. These assays can provide a quantitative assessment of the anti-S. neurona antibodies in both the blood (serum) and cerebrospinal fluid (CSF) of a horse.
Importantly, the accuracy of EPM serodiagnosis is greatly enhanced by using the SnSAG2 and SnSAG4/3 assays to compare the quantity of antigen-specific antibodies in the CSF relative to the quantity in the serum4. These measurements provide a reliable determination of whether antibodies present in the CSF are due to intrathecal antibody production, which is an indicator of active S. neurona infection in the central nervous system.
1. Howe, D.K., R. Gaji, M. Mroz-Barrett, M-J. Gubbels, B. Striepen, and S. Stamper. 2005. Sarcocystis neurona merozoites express a family of immunogenic surface antigens that are orthologues of the Toxoplasma gondii surface antigens (SAGs) and SAG-related sequences. Infection and Immunity 73(2):1023-1033. (pubmed15664946)
2. Hoane, J.S., J.K. Morrow, W.J. Saville, J.P. Dubey, D.E. Granstrom, and D.K. Howe. 2005. Enzyme-linked immunosorbent assays for the detection of equine antibodies specific to Sarcocystis neurona surface antigens. Clinical and Diagnostic Laboratory Immunology 12(9):1050-1056. (pubmed16148170)
3. Yeargan, M.R., and D.K. Howe. 2011. Improved detection of equine antibodies against Sarcocystis neurona using polyvalent ELISAs based on the parasite SnSAG surface antigens. Veterinary Parasitology 176:16-22. (pubmed21075532)
4. Furr, M., Howe, D., Reed, S., and Yeargan, M*. 2011. Antibody coefficients for the diagnosis of Equine Protozoal Myeloencephalitis. Journal of Veterinary Internal Medicine 25:138-142. (pubmed 21155894)
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