The impact of introduced toxic plant species, to which evolutionarily naïve native mammals have little tolerance, is poorly documented. Sudden blindness, icterus and subsequent mass mortalities of Eastern Grey Kangaroos (Macropus giganteus) occurred in the Wagga Wagga region of New South Wales in April 2014. Preliminary differential diagnoses of Wallal/Warrego virus, macropod herpesvirus or facial dermatitis were ruled out. The animals had various severities and phases of hepatopathy with lesions ranging from acute damage to chronic regeneration. Animals presented with marked corneal oedema with a ventral to dorsal progression in animals showing progressive stages of disease. This syndrome was suspected to be a plant toxicosis resulting in hepatogenous photosensitisation.
Clinical pathology investigation of the cases provides evidence for a difference in the sensitivity of enzymes used to assess cholestasis in macropods. Histopathology of the livers of the affected animals demonstrated intrabiliary acicular clefts, indicating possible saponin crystal formation, resulting in structural damage to the bile ducts and hepatocytes.
An analysis of pasture proportion and species indicated the areas where kangaroos were feeding in the evenings and early night contained a high proportion of the suspect toxic plant species Panicum gilvum when compared to a control location. P. gilvum likely contains steroidal saponins (a secondary plant metabolite) and has been associated with previous case reports of photosensitivity in lambs and other species. There are preliminary indications in metabolomics data that compounds with similarities to diosgenin are found in the leaves, stems and inflorescence of P. gilvum and trace metabolites of diosgenin were found in the livers of sampled kangaroos.
This disease has recurred in 2015 following climatic conditions favouring the growth of Panicum species in the Riverina. This study highlights the potential of introduced weeds to affect and damage vulnerable populations of native mammals.