Intoxication of cattle on kikuyu grass following army worm (Spodoptera exempta) invasion

Clinical features and pathological and mycological findings in a field outbreak of intoxication in dairy cattle grazing kikuyu grass are reported. The outbreak followed invasion of the grass by the army worm (Spodoptera exempta). Clinical signs included drooling of saliva, depression, apparent inco-ordination, sunken eyes, ruminal distension and atony, recumbency, moderate diarrhoea and "sham drinking". Seventy-seven cows (64%) were clinically affected over a period of 12 days. Of these, 37 died. Necropsies performed on 4 affected cattle revealed necrosis of the epithelium of the forestomach, which was consistently more severe in the omasum. Light microscopy showed extensive necrosis of the epithelium of the forestomach with associated fibrinopurulent inflammation. The stratum spinosum and s. granulosum were selectively involved, but the s. basale was generally preserved. Electron microscopical examination of ruminal and omasal epithelium from 2 of these cattle revealed cytopathological features in the s. spinosum and s. granulosum which were consistent with stages in an acute, anoxic type of injury. Mycological examination of the pastures revealed sparse growth of a mixed fungal population, which included Myrothecium verrucaria. There was no evidence of heavy fungal infestation. Previous evidence that M. verrucaria, or other fungi, may be involved in the aetiology of kikuyu grass poisoning of cattle in New Zealand is addressed. It appears improbable that any of the fungi isolated in this investigation could have played an important role in the aetiology of this outbreak.