Tips for Diagnosing EPM in Horses 

Here’s how to use serology and CSF tests to diagnose EPM.
Serology test on horse as a diagnostic tool for EPM
The best use of serology when diagnosing EPM in horses is its negative predictive value. | Getty Images

Diagnosing equine protozoal myeloencephalitis (EPM) is complicated by the high prevalence of exposure and low prevalence of disease. Serology is often the first-line test, but the best use of serology is its negative predictive value.   

“If the test is negative, then EPM is not likely the problem … unless it’s an acute onset and the horse has not seroconverted yet, which is uncommon,” said Emily Berryhill, DVM, Dipl. ACVIM, assistant clinical professor of equine internal medicine at the University of California, Davis, during a Burst Session at the 2023 AAEP Convention in San Diego.  

If the original screening test for EPM is positive, however, the next step is differentiating between exposure and disease. To do this, it is important to think about the recommended interpretations of the following two validated tests: 

  • IFAT. A titer above 80 increases the chances of EPM being likely if you’ve already ruled out other causes of disease. “Consistent clinical signs should also be present, such as progressive asymmetric muscle atrophy,” said Berryhill. “If the clinical signs are more nebulous or there is concern for concurrent diseases, then perform a CSF tap.”  
  • SAG-2, 4/3 ELISA. Positive serum results for this test have poor correlation with disease, so always recommend a CSF titer.  

If the serum/CSF ratio is ≤ 100 (i.e., antibody levels are higher in the CSF than the serum), then it supports disease not just exposure.  

Unfortunately, it’s common to have values around 100, which is a gray zone.  

“There could be blood contamination, for example (giving a false positive if there really isn’t any antibody being produced in the CSF),” Berryhill explained. “Also, interpreting the CSF tap will be difficult if treatment was already initiated. If the CSF is negative for antibodies, then one could feel comfortable stopping treatment. If it’s positive, however, it won’t be possible to know how high the titer originally was, if the horse actually had the disease, or if it’s just a ‘background exposure’ titer.”  

As a sidenote, Berryhill advised against using the serum titer as a monitoring tool for treatment success.  

“Antibodies persist in the bloodstream for a long time. It is best to use clinical signs to monitor treatment success,” she stated.  

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