Grass sickness – postmortem ganglion sampling

While a postmortem diagnosis of equine grass sickness (EGS) can often be made from samples of the ileum, in some cases examination of the major autonomic ganglia remains the ‘gold standard’.  In cases with mild clinical signs or chronic EGS cases, or if there is marked postmortem autolysis, then examination of the ganglia is often required.

The cranial cervical and coeliacomesenteric sympathetic ganglia are the most commonly sampled, and the following guide describes a method to locate the coeliacomesenteric ganglion:

Protocol:

1. The gastrointestinal tract should be removed carefully, sectioning the root of the cranial mesenteric artery as far distally as possible and making sure that the kidneys, adrenal glands, associated aorta and the root of the mesenteric artery are left in situ (Fig 1).
2. The kidneys should be removed carefully from the carcass making sure that the adrenal glands and the adjacent aorta remain in situ and their relationships are not disturbed (Fig 2 ) .
3. Two deep cuts are made through the tissues cranially and caudally to the cranial and caudal poles of the adrenal glands (Fig 2).
4. With a finger inside the aorta applying traction (Fig 3), a block of tissue is removed from the carcass including the aorta from the area of the root of the cranial mesenteric artery, the vena cava, the two adrenal glands and the surrounding back musculature (Fig 4).
5. The musculature is dissected away, leaving the aorta, vena cava and adrenal glands undisturbed.
6. This block of tissue is placed in an ample volume of 10% neutral buffered formalin for fixation prior to further dissection.

The tissue block can be wrapped in formalin soaked tissue/cotton wool, packaged appropriately and submitted to the laboratory at this stage for further dissection.

Alternatively, the left adrenal gland is dissected carefully, elevating it from the aorta and revealing the coeliacomesenteric sympathetic nerve ganglia underneath. The ganglion can be hard to differentiate from surrounding tissues in its fresh state but after 24h of fixation it can be identified more easily, both visually and palpably. It varies in size in adult horses but is usually approximately 40 mm long by 15 mm wide with a flattened cut surface, with a characteristic cream-brown neural appearance. Thin transverse and longitudinal sections of the ganglion are then placed in fresh 10% formalin to complete fixation prior to processing and can then be submitted to the laboratory in a smaller container and packaged appropriately.