Veterinary Medicine - May 2008 - (Page 254) Histoplasmosis include the tibia and tarsal, carpal, and metacarpal bones.26,27,56,57 Radiographic abnormalities include soft tissue swelling and joint effusion with osteolysis and proliferative sclerosis.26,56 Cats. In a review of 96 cases of feline histoplasmosis, all seven cats with lameness had changes consistent with osteomyelitis on long bone radiographs.1 Multiple long bones, as well as carpal and tarsal bones, may be affected.38,43,44,58 A predilection for sites distal to the elbow and sti e joints exists.43 Lesions are predominantly osteolytic, with periosteal and endosteal proliferation seen in some cases.43 Bone weakening can result in pathologic fractures that heal after treating the disease.43,44 Additional radiographic changes reported in one cat included soft tissue swelling and subchondral bone collapse.58 DEFINITIVE DIAGNOSIS Histoplasmosis is commonly diagnosed by identifying the H. capsulatum organism.7,11,51,59 Cytology and histology Histoplasma capsulatum organisms can be identi ed by using Wright’sGiemsa stain or a modi ed Wright’s stain.11,50,53 Histoplasma capsulatum are oval or round 2- to 4-µm-diameter organisms that are found within macrophages.10,11,35 The yeast cells have a basophilic center surrounded by a clear area caused by shrinkage of the cell away from the cell wall during xation (Figure 2).11,24,31,34 During the preparation process, H. capsulatum organisms may be released from cells and appear free on the slide.11,24 When a peripheral blood smear is examined, identifying H. capsulatum is enhanced by examining the buffy coat.7,10,18,38,53 Histoplasma capsulatum has been documented in the neutrophils (Figure 3), monocytes, and, rarely, eosinophils of affected dogs.10,30,35,46,49 A review of cats with histoplasmosis reported the organism within phago- 254 VETERINARY MEDICINE
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