The malignant ovine theileriosis due to Theileria lestoquardi is highly pathogenic in sheep. During an experimental study, clinical and pathological findings were investigated in sheep infected with T. lestoquardi. The common clinical signs were high fever, emaciation, enlarged lymph nodes, anemia, icterus, nasal discharge, cough, severe pneumonia, circling, opisthotonus, recumbency, and death. Schizonts of Theileria were detected in impression smears of the lymph nodes, spleen, and liver. Gross lesions include pale mucous membranes, enlarged lymph nodes, splenomegaly, and hepatomegaly with gall bladder distention. Petechial and ecchymotic hemorrhages were observed on most of the surface of the serous membranes of the small and large intestines. In some cases, small ulcers were also present in the mucosal membrane of the abomasum. The lungs were congested and edematous, and petechial and ecchymotic hemorrhages were presented. The main microscopic lesions were characterized by lymphocytic infiltration with edema, congestion, and hemorrhage in many tissue sections. It was concluded that T. lestoquardi is highly pathogenic in sheep and the clinical signs and pathological lesions could be associated with a marked lymphoproliferative reaction in the different organs

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