Studies of polymorphonuclear leucocyte function and of gross and microscopic lesions in sheep experimentally infected with trypanosoma bruce] and.congolense
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Trypanosomiasis in animals is associated with immunosuppression that manifests clinically as increased susceptibility to opportunistic infections. Although immunosuppression has been shown to result from depression of antibody production and suppression of cell-mediated immunity, in some cases there is no obvious impairment of these functions. For effective immune response, all of the three effector systems; the mononuclear phagocytic system, the polymorphonuclear phagocytic system, and the complement cascade, must be intact. Polymorphonuclear leucocyte (PMN) function was assessed in trypanosome-infected sheep to establish its possible contribution to lowering of body defence. Trypanosoma congolense (Broden, 1904) and T brucei (Plimmer and Bradford, 1899) were obtained from American Type Culture Collection, Maryland USA, and amplified in rats. Fourteen castrated, eight-month old, male sheep were purchased locally. Initially, four Suffolk x Finnish sheep were inoculated with T congolense while two acted as controls. Later, six North Country Cheviot sheep were inoculated with T brucei while two acted as controls. Each infected sheep received eight million trypomastigotes intravenously. The sheep were monitored daily for clinical disease and tests of PMN function were assessed over two months. PMNs were isolated from ethylenediamine tetra-acetic acid anti-coagulated blood obtained from each sheep and superoxide anion production assessed by cytochrome C reduction, every second week in the first experiment and weekly in the second experiment. In addition, heparinized blood was obtained weekly and used to perform nylon wool adherence and phagocytosis of opsonized zymosan in the second experiment. Although sheep infected with T congolense developed parasitaemia, there were no clinical signs of trypanosomiasis, no gross or microscopic lesions and results of cytochrome C assay were not significantly different between infected and control sheep. Sheep infected with T brucei developed parasitaemia, clinical signs of trypanosomiasis and alteration in PMN function. PMNs obtained from infected sheep had significant decrease in superoxide anion production (p<0.01) and nylon wool adherence (p< 0.01), but enhanced phagocytosis of opsonized zymosan (p< 0.01). Clinical disease of T brucei infection in sheep was characterized by pyrexia, anorexia, weight loss, anaemia, and inflammatory edema of skin of the head, neck, brisket, distal limbs, and tail. At the end of the trial, the sheep were euthanised and examined for lesions. There was edema of the skin and enlargement and edema of superficial lymph nodes. Microscopic lesions were observed in the skin, superficial lymph nodes, spleen and kidneys. Skin lesions consisted of severe edema accompanied by diffuse perivascular and periadnexal dermatitis, perineuritis, lymphangitis, distension of lymphatic vessels with fluid, and lymphatic thrombosis in the dermis and subcutis. Inflammatory reaction extended deep into subcutis leading to severe panniculitis and myositis. Inflammatory cells were mainly macrophages, plasma cells, and lymphocytes although polymorphonuclear leucocytes were also present. Numerous trypanosomes were seen in skin lesions. Superficial lymph nodes had severe capsular lymphadenitis and lymphoid hyperplasia. The spleen had lymphoid hyperplasia. Lesions in the kidney consisted of mild to moderate glomerulonephritis and a few tubular casts. In summary, sheep infected with T congolense for two months were not clinically sick and had no alteration in PMN function. Sheep infected with T brucei for two months .were clinically sick and had significant alteration in PMN function. Such alteration may be responsible for the increased occurrence of opportunistic infections in trypanosomiasis. Microscopic appearance of skin lesions described in T brucei-infected sheep confirms that the inflammatory reaction is similar to that previously described in other organs.