Malignant catarrhal fever is briefly reviewed and recent findings are described. Initially the disease was observed as a disease of cattle in Europe where, although no cause could be identified, circumstantial evidence implicated sheep as a source of infection and it was thus designated 'sheep-associated' malignant catarrhal fever. Subsequently the disease was observed in Africa where it became evident that a herpesvirus which normally infects wildebeest was the cause. It is now apparent that deer are highly susceptible to both forms of the disease, the sheep-associated form being a serious problem in farmed deer. The wide spectrum of clinical and pathological changes that occur in affected deer are described. A major constraint to studies of sheep-associated malignant catarrhal fever has been the absence of an experimental laboratory system. However, from affected deer it has been possible to transmit the disease to rabbits and thus has allowed detailed pathogenesis studies to be made which are summarised in this paper. It is suggested that the agent of sheep-associated malignant catarrhal fever is a virus and that when a particular subpopulation of T-lymphocytes is infected a profound immunological perturbation results; the lesions of malignant catarrhal fever being explained by a benign T-lymphocyte hyperplasia accompanied by a deregulation of cytotoxic natural killer lymphocytes that gives rise to tissue necrosis.

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A farmed red deer in contact with a flock of lambing ewes developed malignant catarrhal fever (MCF). Tissues from this deer were homogenised and inoculated into two rabbits one of which developed a febrile response on the 11th day. This rabbit was killed on the following day after developing conjunctivitis and hyperaemia of the nostrils. The condition was transmitted from this rabbit through a further three rabbit passes. Of 21 rabbits inoculated 15 reacted after three to 29 days. The clinical and pathological disease that developed was indistinguishable from the response of rabbits to infection with the virus of MCF of wildebeest origin. Infectivity was retained in homogenised tissues stored at -80 degrees C in 10% dimethyl sulphoxide but could not be detected in supernatant fluid of a lightly centrifuged tissue homogenate, which suggests that the agent is cell associated.

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