Characterisation of localised pigment accumulation in brains of eastern grey kangaroos (<i>Macropus giganteus</i>) after clinical disease due to chronic Phalaris species toxicosis

A progressive tremorgenic syndrome characterised by ataxia and head nodding is well documented in sheep and cattle affected by chronic Phalaris toxicosis (CPT), and is increasingly documented in the eastern grey kangaroo {(EGK), Macropus giganteus} in southeastern Australia. It is characterised on gross necropsy by areas of acquired localised pigment deposits within the brain. This pigment was previously considered a storage disease, but more recently has been determined to be a metabolic breakdown product of tryptamine alkaloids within Phalaris species (spp) of introduced grasses. The study included 61 EGKs that were euthanased after a diagnosis of clinically advanced CPT, histopathological studies were performed on all cases and transmission electron microscopic studies on six brains. Histological examination of the brains from EGKs revealed brown pigmentation of neurons, particularly of large motor neurons, with accumulations of discrete granules in the cytoplasm that stained positive with stains used to identify melanin. This feature and the characteristic ultrastructural appearance of the pigment granules leaves little doubt that the pigment is primarily melanin in nature. Specifically, ultrastructural detail of the granules was consistent with neuromelanin present in the brains of higher order primates and humans and has been associated with susceptibility to neurodegenerative diseases in man including Parkinson's disease. Given greater urbanisation and reduced access to native pasture a greater understanding of pathogenesis of CPT is of major importance not only for kangaroo welfare but potentially as a model for neurodegenerative diseases in humans.