Diagnosis: Acute osteomyelitis Osteomyelitis is an infectious inflammatory disease of bone and its marrow cavity. Animals with acute osteomyelitis usually exhibit localized inflammation and soft tissue swelling with an obvious lameness. Also, most bone infections in horses are of bacterial origin. Our patient exhibits all of these clinical signs. Acute bacterial osteomyelitis is seen most commonly in very young animals and is usually of hematogenous origin. In the neonate it is frequently secondary to other sites of infection such as gastrointestinal, respiratory, umbilical, or even in utero infection. Osteomyelitis in foals is typically associated with septic arthritis. Radiographic and clinical pathology evidence of osteomyelitis was observed and is discussed in the Specialty Exams section. Pathophysiology: It has been shown that the endothelial lining of the capillaries that pervade the primary spongiosa of neonates is incomplete which allows the extravasation of bacteria and erythrocytes. In the young animal, leukocytes are absent in this location so macrophages are the sole defense against bacteria being able to colonize in this location. However, the macrophages appear to be ineffective in eliminating the bacteria, which leads to the development of hematogenous osteomyelitis centering upon the growth plate. Once osteomyelitis has developed in the bone evidence of osteolysis will become evident. Destruction of cortical bone initially appears as focal radiolucent holes that subsequently enlarge and coalese to involve a region of bone. The destruction on of bone results in the widening of the joint space. The osteomyelitis eventually leads to ankylosis or breakdown of the bone, which results in a poor prognosis for the affected animal.