Equine Protozoal Myelitis (EPM) has recently become recognized as a frequent cause of neurologic disease in horses. With the discovery of the organism that causes EPM, Sarcocystis neurona (SN) (or perhaps Sarcocystis falcatula), and subsequent development of the immunoblot test for SN antibodies (IgG class) in the CSF, it is now possible to diagnosis EPM in horses with neurologic disease. A positive test in the serum means that specific antibodies are present and the horses has been exposed to SN,whereas a positive test in CSF means that specific antibodies to SN are present in the central nervous system. Unfortunately, antibodies in the CSF may be present due to intrathecal production (local central nervous system production) or from leakage ofantibodies from the peripheral blood into the CSF. The immunoblot test cannot differentiate antibodies that are locally produced from antibodies that have leaked from the blood into the CSF. Thus, the CSF indices can be used to differentiate antibodies locally produced in the central nervous system (denoting infection of SN) from antibodies leaking from the blood. The CSF indices, Albumin Quotient and IgG index, are tests that can help in determining damage to the blood-brain barrier and intrathecal (locally produced) antibody production, respectively. These indices can be calculated from measurements made on consequent serum and CSF samples to confirm if the immunoblot test is a true positive or a false positive. Using these indices along with the Western blot test for SN antibodies is helpful in differentiating horses the have EPM from horses that have damage to the blood brain barrier or blood contaminated CSF and do not have EPM. Since pharmacologic agents used to treat EPM are expensive and potential adverse side effects (anemia, depression of WBC, abortion, and colitis), establishing a diagnosis prior to continued treatment is important.
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