DEFINITIVE DIAGNOSIS
Histoplasmosis is commonly diagnosed by identifying the H. capsulatum organism.7,11,51,59
Cytology and histology
2. A lymph node aspirate from a cat. A single large macrophage is densely packed with yeast (arrow), and numerous yeast organisms
are free in the background. Note the clear area surrounding the yeast, caused by shrinkage that occurs during fixation. Numerous
small lymphocytes and a few bare nuclei are also present (Diff-Quik, 100X).
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Histoplasma capsulatum organisms can be identified by using Wright's-Giemsa stain or a modified 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 fixation (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 phagocytic cells in peripheral blood smears in 11 of 56
(19.6%) cases.1 In a separate study, examination of peripheral blood smears revealed the organism in four of 12 (33%) feline cases.15
3. A canine peripheral blood smear demonstrating two segmented neutrophils that contain several budding yeast (modified Wright's
stain, 100X).
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