EPM Diagnostics

The development of a semi-quantitative assay could help improve EPM diagnostics, researchers say.
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Antemortem (before death) diagnosis of equine protozoal myeloencephalitis (EPM) has been a long-standing source of frustration for equine veterinarians and horse owners. Typically, a diagnosis of EPM has been based on the presence of clinical signs consistent with neurologic disease along with a supportive serologic test demonstrating the presence of antibodies against the primary etiologic agent. Most often this is the protozoan parasite Sarcocystis neurona, although EPM is attributed infrequently to the related parasite Neospora hughesi.

Unfortunately, most any neurologic disease can cause clinical signs similar to those associated with EPM. Moreover, horses are frequently exposed to S. neurona, so simply the presence of antibodies in the blood has little diagnostic value. Given these challenges, it is little wonder that some veterinary practitioners have relied on “response to treatment” as a primary diagnostic criterion. This approach to EPM diagnosis is not only expensive; it can be misleading as well.

But it might now be possible to view EPM diagnosis with much greater confidence. The development of semi-quantitative assays that can measure antibodies against S. neurona allow for diagnostic methodology that identifies with high accuracy horses suffering from EPM. Specifically, the assays can be used to demonstrate S. neurona-specific intrathecal antibody production (i.e., antibodies produced in the central nervous system), which indicates that there is active infection. This approach has been used for decades in human medicine and is based on comparing the amount of antigen-specific antibody present in the cerebrospinal fluid (CSF) relative to the blood. Infection in the central nervous system is confirmed when the amount of antibody present in the CSF is greater than anticipated from normal passive transfer across the blood-brain barrier.

A recent multi-investigator collaboration examined 128 horses that were diagnosed by postmortem examination with either EPM or another neurologic disorder (e.g., cervical-vertebral malformation). Serum and cerebrospinal fluid (CSF) from each horse were tested with two enzyme-linked immunosorbent assays (ELISAs) that detect antibodies against the conserved S. neurona proteins SnSAG2, SnSAG3, and SnSAG4

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