Indirect fluorescent antibody test and surface antigen ELISAs for antemortem diagnosis of equine protozoal myeloencephalitis.
Authors: Johnson A L, Morrow J K, Sweeney R W
Journal: Journal of veterinary internal medicine
Summary
# Editorial Summary: Antemortem Diagnosis of Equine Protozoal Myeloencephalitis Sarcocystis neurona-induced EPM remains diagnostically challenging in live horses, prompting evaluation of two commercially available serological tests—the indirect fluorescent antibody test (IFAT) and surface antigen SAG2, 4/3 ELISA—against paired serum and cerebrospinal fluid samples from horses with confirmed EPM (n=9), confirmed negative cases (n=28), suspected positive cases (n=6), and suspected negative cases (n=14). The SAG2, 4/3 ELISA serum : CSF titer ratio demonstrated superior diagnostic accuracy (0.97; 95% CI 0.88–0.99) with excellent specificity (1.0) and good sensitivity (0.88), whilst IFAT performed less reliably with lower sensitivity (0.65) despite comparable overall accuracy when CSF samples alone or titer ratios were analysed. Critically, both tests showed substantially reduced diagnostic utility when serum alone was assessed, underscoring the importance of paired fluid analysis. Equine practitioners should prioritise the SAG2, 4/3 ELISA utilising serum : CSF titer ratios as the most reliable antemortem diagnostic approach, as serum-only testing risks both false negatives and false positives that could delay appropriate treatment or lead to unnecessary therapy.
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Practical Takeaways
- •Request paired serum and CSF samples with SAG2, 4/3 ELISA titer ratio analysis for most accurate antemortem EPM diagnosis—serum alone is unreliable
- •SAG2, 4/3 ELISA titer ratio is preferred over IFAT for diagnosing EPM due to higher sensitivity and specificity
- •Horses with neurologic deficits should have paired samples tested; a negative titer ratio has high confidence for ruling out EPM
Key Findings
- •SAG2, 4/3 ELISA serum : CSF titer ratio achieved highest accuracy of 0.97 (95% CI 0.88-0.99) with sensitivity of 0.88 and specificity of 1.0
- •IFAT CSF and titer ratio showed good accuracy of 0.88 (95% CI 0.77-0.94) but lower sensitivity of 0.65 (95% CI 0.41-0.83)
- •Serum results alone were least accurate for both test types
- •Paired serum : CSF titer ratios provide superior diagnostic accuracy compared to single-sample testing for antemortem EPM diagnosis