Accurate antemortem diagnosis of equine protozoal myeloencephalitis (EPM) based on detecting intrathecal antibodies against Sarcocystis neurona using the SnSAG2 and SnSAG4/3 ELISAs.
Authors: Reed S M, Howe D K, Morrow J K, Graves A, Yeargan M R, Johnson A L, MacKay R J, Furr M, Saville W J A, Williams N M
Journal: Journal of veterinary internal medicine
Summary
Equine protozoal myeloencephalitis (EPM) caused by *Sarcocystis neurona* remains diagnostically challenging in live horses, as clinical signs overlap considerably with other neurological conditions. This multicentre study evaluated whether a straightforward serum-to-cerebrospinal fluid (CSF) antibody titer ratio using SnSAG2 and SnSAG4/3 ELISAs could reliably diagnose EPM antemortem, building on earlier work showing the value of intrathecal antibody detection. Paired serum and CSF samples from 128 horses with postmortem diagnoses (44 EPM cases, 35 cervical vertebral malformation cases, 39 other neurological cases, and 10 non-neurological controls) were analysed, with serum and CSF titer ratios calculated and diagnostic accuracy assessed against confirmed diagnoses. A serum:CSF ratio threshold of ≤100 yielded 93.2% sensitivity and 81.1% specificity, whilst a more stringent cutoff of ≤50 achieved 86.4% sensitivity and 95.9% specificity; notably, serum titers alone performed poorly as diagnostic markers, whereas CSF antibody detection provided good accuracy. For practitioners, these findings validate the clinical utility of CSF analysis in suspected EPM cases and demonstrate that practitioners can employ a simple ratio calculation rather than more complex antibody indices, potentially improving diagnostic confidence and supporting treatment decisions in this difficult differential diagnosis.
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Practical Takeaways
- •Use paired serum and CSF samples with SnSAG2, 4/3 ELISA to diagnose EPM antemortem—a serum:CSF titer ratio ≤100 confirms EPM in 93% of cases, or ≤50 for higher specificity (96%)
- •Don't rely on serum antibody titers alone; intrathecal antibody production (detected via CSF) is the key diagnostic marker for active EPM
- •This simplified titer ratio method allows equine practitioners and diagnostic labs to accurately diagnose EPM without complex index calculations
Key Findings
- •SnSAG2, 4/3 serum:CSF titer ratio ≤100 achieved 93.2% sensitivity and 81.1% specificity for EPM diagnosis
- •More rigorous cutoff of ≤50 achieved 86.4% sensitivity and 95.9% specificity
- •Simple serum:CSF antibody titer ratio is as diagnostically accurate as complex antigen-specific antibody indices
- •CSF antibody titers alone provided good diagnostic accuracy, while serum titers alone were inadequate for EPM diagnosis