Severe lameness caused by metastatic renal adenocarcinoma of the third phalanx in a Warmblood mare

Malignant osseous neoplasia has been extremely rarely reported in the horse and, therefore, it is not commonly considered as a differential diagnosis for acute lameness. In the present report, a 19‐year‐old Warmblood mare in poor body condition and with a history of chronic haematuria, presumably attributable to renal neoplasia, was referred with acute, progressive, unilateral right forelimb lameness with marked unilateral digital pulsation and metacarpal oedema. No hoof abscess was found. Radiographic examination revealed a well defined osteolytic area in the dorsomedial half of the third phalanx. Dermal laminae at the dorsomedial aspect of the hoof showed a dark red appearance with poor vascularisation. Thoracic radiographs revealed pulmonary nodules. As osseous metastasis of the third phalanx was suspected, the mare was subjected to euthanasia. Post mortem histopathological examination confirmed the diagnosis of primary renal adenocarcinoma with metastasis to the lungs, myocardium, along the perirenal lymphatic tract and to the third phalanx, with extensive osteolysis in the latter location. Post mortem computed tomography of the hoof provided excellent visualisation of the osseous metastasis. The rare possibility of osseous metastatic neoplasia should be included in the differential diagnosis of severe lameness, especially in aged horses with a history of chronic renal disease.