Influence of collection site on cerebrospinal fluid test results in horses with equine protozoal myeloencephalitis.

Influence of collection site on cerebrospinal fluid test results in horses with equine protozoal myeloencephalitis

Overview of the Study

• This study investigated how the location from which cerebrospinal fluid (CSF) is collected in horses affects test results used to diagnose equine protozoal myeloencephalitis (EPM).
• The research compared results from two common CSF collection sites—the lumbosacral (LS) area and cranial sites (atlantooccipital or C1-2)—in both EPM-affected and non-EPM affected horses.

Background and Importance

• Equine protozoal myeloencephalitis (EPM) is a serious neurological disease in horses caused primarily by the parasite Sarcocystis neurona.
• Diagnosis involves testing the cerebrospinal fluid (CSF) to detect antibodies against the parasite, which helps confirm infection in the central nervous system.
• CSF can be collected from different anatomical sites, but the impact of collection location on the accuracy of diagnostic test results was unclear prior to this study.

Study Design and Methods

• The study was a prospective observational design involving 27 horses: 20 control horses without EPM and 7 horses affected by EPM.
• CSF samples were collected from two sites on each horse:
  • Lumbosacral (LS) site – lower back area near spine and pelvis.
  • Cranial sites – either atlantooccipital (between skull and first cervical vertebra) or C1-C2 (between first and second cervical vertebrae).
• Key tests performed on CSF included:
  • Anti-Sarcocystis neurona antibody titers, which measure the concentration of antibodies specifically targeting the parasite in the CSF.
  • Serum/CSF titer ratios, which compare antibody levels in blood serum versus CSF to help identify intrathecal (within the spinal canal) antibody production.
  • Antibody index (AI), a calculated value that corrects for total IgG and albumin concentrations and helps determine if antibody production is localized in the central nervous system.
• Data summarization and statistical comparisons were made using:
  • Wilcoxon signed rank test for paired comparisons between CSF collection sites within each group (EPM-affected and non-EPM horses).
  • Wald test statistic to assess differences in medians.

Key Findings

• In non-EPM horses:
  • Test results for CSF anti-S. neurona antibody titers, serum/CSF titer ratios, and antibody index did not significantly differ between the LS and cranial collection sites.
  • This indicates that collection site has minimal impact on test results when the horse is not affected by EPM.
• In EPM-affected horses:
  • CSF antibody titers were significantly higher in samples collected from the LS site compared to the cranial sites. For example, median titers were 160 (with an interquartile range of 1240) from LS samples versus 80 (IQR 600) from cranial sites (P=0.03).
  • The serum/CSF titer ratio was lower in LS samples (median 25) versus cranial samples (median 50), indicating higher relative antibody concentration in LS-collected CSF (P=0.03).
  • Despite differences in raw titers and ratios, the antibody index (AI) values did not significantly differ based on collection site.

Interpretation and Conclusions

• The site of CSF collection can influence some EPM-specific test results, especially in horses that are confirmed to have EPM.
• Higher antibody titers and lower serum/CSF ratios from the lumbosacral site indicate that antibody concentrations may be elevated in CSF sampled lower in the spinal canal compared to cranial samples.
• Diagnostic tests relying solely on antibody titers or serum/CSF titer ratios may be affected by collection site, potentially leading to diagnostic errors or misinterpretation.
• The antibody index (AI), which corrects for total IgG and albumin, appears to be a more stable and reliable measure across different collection sites.
• Clinicians should consider the CSF collection site when interpreting EPM test results, and preferentially use the antibody index to minimize variability and reduce risk of misdiagnosis.

Implications for Veterinary Practice

• Selection of CSF collection site is an important consideration in the diagnosis of EPM.
• Using the antibody index may provide more consistent diagnostic results regardless of collection site.
• Understanding the variability induced by collection site can help veterinarians improve test interpretation and provide more accurate diagnoses.
• Future diagnostic protocols might standardize collection sites or emphasize AI measurements for consistency.