Histopathologic and electron microscopic studies of cutaneous lesions in calves with experimentally induced East Coast fever (theileriosis).

Skin lesions of bovine East Coast fever were examined by light and electron microscopy at 120 hours after attachment of Rhipicephalus appendiculatus ticks infected with Theileria parva. Lesions included epidermal ulcer, hemorrhage, edema, necrosis, and inflammatory cell infiltration by mostly polymorphonuclear and mononuclear (lymphoid) cells. The trophozoite state of T parva was observed in a parasitophorous vacuole and feeding on neutrophil granules. Schizonts, some in budding process, and merozoites were extracellular and among many mononuclear phagocytes (lymphoid cells). Some merozoites were in mononuclear phagocytes and a granulocyte. Some of the cells had already transformed into lymphoblasts with pseudopodia.