Evaluation of synaptophysin as an immunohistochemical marker for equine grass sickness

It has been proposed that synaptophysin, an abundant integral membrane protein of synaptic vesicles, is an immunohistochemical marker for degenerating neurons in equine grass sickness (GS). In the present study, a statistically generated decision tree based on assessment of synaptophysin-immunolabelled ileal sections facilitated correct differentiation of all 20 cases of GS and 24 cases of non-GS disease (comprising eight horses with colic, six with neuroparalytic botulism and 10 controls). This technique also facilitated correct diagnosis of GS in all three cases that had been erroneously classified as having non-GS disease based on conventional interpretation of haematoxylin and eosin-stained cryostat sections of ileal surgical biopsies. Further prospective studies involving larger numbers of horses are required to fully validate this decision tree. In contrast to GS, botulism did not alter ileal neuron density or synaptophysin labelling, indicating that different mechanisms cause neuronal damage and/or dysfunction in GS and botulism.