1. A specific disease of cattle is described which occurs sporadically in the East Coast fever areas of eastern equatorial Africa. Clinically, acute cases are characterised by peculiar nervous symptoms, absence of fever and blindness. Death occurs in most cases within 2 to 21 days of the onset of illness. Chronic cases are encountered from time to time and may survive up to 6 months.
2. The disease is widespread in Uganda. It affects cattle of both sexes and all breeds, and is commonest in young stock between 6 months and 2 years of age. It has, however, been observed in older animals.
3. Pathologically, in acute cases petechial haemorrhages or extravasations are found in various parts of the brain and meninges. Generally the haemorrhages are small and do not seriously injure the adjacent brain tissue. In some cases the extravasations are responsible for a considerable amount of damage.
4. Microscopic examination shows that the brain lesions are due to extensive embolism of cerebral vessels. The lymphocytes which form these emboli contain in their cytoplasm protozoan bodies indistinguishable in structure from the schizonts of the genus Theileria. These bodies occur in great numbers in the brain, but are extremely rare or absent in the lymphocytes of other parts of the body.
5. In chronic cases the haemorrhages have been absorbed but the brain is permanently damaged by replacement changes, and by the appearance of small cysts or cavities which contain a citron-coloured fluid. These cavities when numerous seriously disorganise certain parts of the brain.
6. The disease has not yet been transmitted experimentally, and its connection with the theileriases is still uncertain. However, much evidence has been collected to show that cases of turning sickness have been infected with East Coast fever some time previously, generally on an average of 6–8 months.
7. The possible relationship between turning sickness and East Coast fever is discussed at some length during the course of the paper.
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