Potential role of multiple rectal biopsies in the diagnosis of equine grass sickness.

Summary

# Editorial Summary: Rectal Biopsy as a Diagnostic Tool for Equine Grass Sickness Wales and Whitwell investigated whether multiple rectal biopsies could reliably diagnose grass sickness in living horses by examining postmortem tissue from 14 affected and 10 control animals, taking two samples from each horse's rectum and preparing four histological sections per horse stained with haematoxylin and eosin. The presence of three or more chromatolytic neurons—indicating neuronal degeneration—identified 10 of the 14 grass sickness cases with perfect specificity (100%) but moderate sensitivity (71%), whilst control horses showed no positive findings. Although additional staining techniques using cresyl fast violet and methyl green-pyronin to highlight Nissl substance did not enhance diagnostic accuracy, the high specificity suggests rectal biopsy could be a valuable ancillary tool for confirming grass sickness in vivo, particularly when clinical suspicion is high. However, the 29% false-negative rate means a negative biopsy cannot exclude the disease, so this approach works best alongside clinical assessment rather than as a standalone diagnostic test. For practitioners considering antemortem rectal biopsy protocols, standardisation of sampling location and section preparation will be essential to translate these postmortem findings into reliable clinical practice.

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Key Findings

• •Rectal biopsy using chromatolytic neurons as diagnostic criterion achieved 71% sensitivity (10/14 cases positive) in grass sickness horses
• •Specificity was 100% with no positive results in 10 control horses
• •Cresyl fast violet and methyl green-pyronin staining for Nissl substance did not improve diagnostic sensitivity

Conditions Studied