Central neuropathology of equine grass sickness

Equine grass sickness (EGS) is an acquired disease of unknown aetiology aff ecting horses kept at grass. The disease is characterised by postganglionic sympathetic and parasympathetic neuronal pathology and is categorised as a dysautonomia. This study undertook a systematic examination of brain stem cranial nerve nuclei in 59 cases of EGS. Pathology consisting of neuronal c hromatolysis was most consistently noted in the lower motor neurons of the general visceral efferent nucleus of CN III and X and the general somatic e fferent nuclei of CN III, V, VII and XII. The prevalence of chromatolysis d iffered significantly between the diagnostic categories and was inversely r elated to the age of the animal. Duration of disease had a small but signif icant negative effect on the proportion of nuclei with chromatolytic neuron s in chronic cases. The distribution of chromatolytic neurons is unlike tha t reported in any other equine or human disease; however, it appears to be equivalent to that in other animals with primary dysautonomias. EGS should be classified as a multisystem disease.