Equine & Livestock

Fall/Winter 2022 Equine & Livestock Solutions

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10 Covetrus | ph: 855.724.3461 | fx: 888.329.3861 | northamerica.covetrus.com A fork in the road: Antibody testing The question I often get asked, "What is the right biological sample to use? Blood or cerebrospinal fluid (CSF)?" Evidence of intrathecal antibody production is the most accurate way to support an EPM diagnosis. However, blood is a good screening tool. If serum comes back negative, likely there is no infection or recent infection. EPM can generally be ruled out and you can proceed down your list of differentials. (See Figure 1.) A rare exception would be a very acute onset prior to antibody production, in which case a retest in 10 to 14 days is warranted. A positive serum test result, however, presents a "gray zone" because it doesn't necessarily equate to an EPM diagnosis. Knowing there is a high seroprevalence to the causative organisms of EPM in healthy U.S. horses, now what? SERUM POSITIVE? PROCEED WITH CAUTION: 78% of healthy U.S. horses are seropositive to Sarcocystis neurona and 34% to Neospora hughesi. 1 So, while you can find a seropositive horse in almost every pasture, a positive result doesn't mean that horse has EPM. To more definitively rule out (or in) EPM, a CSF tap is recommended. If the CSF sample is negative, EPM is ruled out and the practitioner should proceed to the next differential diagnosis. If the CSF sample is positive, consider it a pretty strong case for an EPM diagnosis. Be aware, a positive CSF result can happen for reasons other than antibody production within the CNS. For one, blood contamination. If there is a high blood titer to S. neurona, for example, this could give a positive result from blood-derived antibody and not production within the CNS. Therefore, current best practice consensus is to collect spinal fluid and a blood sample and compare the antibody titers in each to determine if there is evidence of a CNS infection. This is done by evaluating the ratio of antibody titer in serum divided by antibody titer in CSF. The only way to definitively diagnose EPM is on necropsy. Authors of the Updated ACVIM Consensus Statement on EPM (2016) defined the 'gold standard' for diagnosing EPM in a living horse: 2 • Observable neurologic signs consistent with EPM • A serum:CSF ratio that is within the range of established serological tests (e.g., SAG 2,4/3 < 100, IFAT ≤ 64), which indicates intrathecal (within the central nervous system) antibody production against S. neurona or N. hughesi • Ruling out diseases with similar neurologic signs Treating the horse with an antiprotozoal drug for two weeks and then reassessing may be a practical approach if you feel strongly this is an EPM case and CSF sampling is not accepted. However, this is ONLY recommended if you can physically reassess the horse in two weeks to evaluate progress. At that time, critically evaluate the improvement by repeating the same thorough physical and neurologic workup. A significant improvement must be seen to continue with unfinished treatment (I like to see at least a 25% improvement). If the horse responds to treatment, you have further evidence to support an EPM diagnosis. If the horse does not respond, it's back to the drawing board. This is a critical communication point with the owner to help them understand next steps and avoid potential frustration of treating for EPM with no improvement. (See sidebar on client communication tips.) Tricky disease requires careful diagnosis EPM got you scratching your head? Take heart. Each clinical presentation is different, and the best way to outwit this disease mimicker is with a solid dose of due diligence. There are no shortcuts when it comes to doing a proper EPM diagnostic workup. Look for hallmarks of clinical disease in your physical exam and don't shy away from the need for immunodiagnostics to get to the root cause. THE GOOD NEWS: If EPM is diagnosed, there are safe and effective EPM treatment options.

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