Studies on the lesions and pathogenesis of East Coast fever (Theileria parva infection) in cattle, with special reference to the lymphoid tissue

Abstract

1. Regressive changes in the lymphoid tissue appeared to be the striking pathological feature in cattle that died of East Coast fever. 2. The earliest stages of Theileria parva seemed to occur mainly in the immature lymphocytic cells. The developing stages of the schizonts brought about a disintegration of most of the lymphocytic cells and the nodules lost their identity. The meshes of the reticulum, and the dilated subcapsular and medullary sinuses showed the presence of infected and disintegrated lymphocytic cells and dislodged schizonts, as well as a certain amount of fibrin, oedema, debris of cells, and infrequently, macrophages. 3. During the last stages of the disease, the impression smears of the lymphoid tissue showed practically all the lymphocytic cells infected with schizonts, chiefly gamonts. Due to the overlapping of the agamogenous and gamogenous cycles, it was not possible to say how long each cycle lasted, or how frequently it was repeated. 4. The damaged lymphocytic cells were apparently eliminated by a process of dissolution, whereas phagocytosis did not seem to play a significant part. 5. Schizonts, carried away in the circulation, probably infected lymphoid tissue, or newly-formed aggregates of lymphocytic cells in other parts of the body. The merozoites, during the last stages of the disease, were frequently present in large numbers in the impression smears of the spleen. It was, therefore, possible that many erythrocytes might have become infected in the ted pulp of the spleen. 6. Regular and often extensive aggregates of lymphocytic cells appeared as a secondary involvement in the kidneys, liver, adrenals, etc., and less frequently and less extensively in such organs as the lungs, myocardium, skeletal muscle, pancreas, hypophysis and thyroid. The central nervous system was not involved in these cases. 7. These secondary aggregates of lymphocytic cells were found associated with the walls of the blood vessels, of the bile-ducts or of the bronchi, or bronchioles. They were lodged in a reticulum, and mitosis was not infrequent. These newly formed lymphocytic cells became infected with schizonts and disintegrated, in the same way as described in the lymph nodes. 8. It seemed doubtful whether these infected and damaged lymphocytic cells present in the circulation, were able to emigrate and proliferate to form aggregates in the so-called secondary localities. 9. There appeared to be a "potential stem cell" widely distributed throughout the body, which under stimulation could proliferate to produce cells found on the lymphoid tissue. It would, therefore, seem to be more likely that "stem cells" in various parts of the body were stimulated to form aggregates of lymphocytic cells in situ. 10. The elimination of the lymphocytic cells, in the primary lymphoid depots, probably led to a compensatory formation of these cells in other parts of the body. 11. From the brief discussion of a number of papers dealing with the function of the lymphoid tissue, it was possible that in the extensive dissolution of lymphocytic cells in East Coast fever, a vital function in the animal body was interfered with. This apparently was an important contributory factor to the rapid wasting, the emaciation, and the subsequent death of the animal. 12. At present it is not understood how the many infected erythrocytes at the peak of infection were eliminated in the body.