Myocarditis associated with Theileria spp. in calves.

Two 3-week-old Limousin x Retinta calves from extensive farms in the south of Spain died suddenly. Microscopic examination of the spleen and lymph nodes revealed an intense lymphoid proliferation; the germinal centres were prominent and were surrounded by areas of haemorrhage. Parasitic forms, similar to Koch's bodies, were observed in only a small number of lymphoid cells and macrophages. In the myocardium a severe myocarditis was found. Inflammatory cells were in close contact with damaged myocytes, sometimes surrounding fragments of cells. In some areas the vessels had intraluminal deposits of fibrin. The endothelial damage and thrombosis may have contributed to the damage observed in the myocytes, together with the cytotoxic mechanism associated with the presence of an intense inflammatory infiltrate. The active multiplication of parasites may be due to the age of the animals and their breed, since they are not an autochthonous breed and present low resistance to infection.