Exercise 12 discussion

Clinical history and questions


H&E section (40x objective) of a transverse slice of white matter from a mid-cerebral gyrus of a mature standard-bred horse.

The animal had developed non-localising nervous signs while on pasture dominated by Echium plantagineum.

Describe the abnormalities, make a histopathological diagnosis, and comment on the pathogenesis of the changes.

An interpretation of Diagnostic Exercise No. 12 – Roger Kelly


Five or six astrocytes have markedly enlarged, pale-staining (“ballooned”) nuclei (compare with the more numerous oligodendrocyte nuclei), and either their cytoplasm is distended with non-staining fluid, or the nuclei are surrounded by an empty zone.

Histopathological diagnosis:

Degenerative cerebral leucoencephalopthy characterised by presence of Alzheimer type 2 astrocytes.


The history is suggestive of liver damage by the pyrrolizidine alkaloids in Echium plantagineum, which can result in hepatic encephalopathy. The hydropically swollen and reactive astrocytes are known as Alzheimer type 2 astrocytes and are suggestive of hepatic encephalopathy in humans and horses, but are not seen so well in our other species, in which HE is manifest more often by status spongiosis of central myelin, which seems not to occur in HE in horses (or humans).

Both myelinic vacuolation in other species and Alzheimer type 2 astrocyte formation in horses can occur in hyperammonaemia in the absence of hepatic encephalopathy.

Visit the Huxtable-Summers-deLahunta online Atlas of Veterinary Neuropathology at http://web.vet.cornell.edu/public/oed/neuropathology/index.asp to see other examples of Alzheimer type 2 astrocytes

The apparent expansion of the VR space is probably within the normal range, being an artefact of immersion fixation. Some of the apparent space around the astrocyte nuclei could have the same cause.