Canine and feline histoplasmosis: A review of a widespread fungus - Veterinary Medicine
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Canine and feline histoplasmosis: A review of a widespread fungus
Infection with this pathogenic fungus most commonly results from inhaling spores from contaminated soil. The infection should be treated promptly to avoid dissemination, which carries a poorer prognosis.


VETERINARY MEDICINE



4. A liver aspirate from a cat. Three dark-purple hepatocytes are seen among multiple macrophages, some of which contain yeast (arrows). Several yeast are noted free in the background (arrowheads) of erythrocytes and a small amount of finely granular protein (Diff-Quik, 60X).
Histoplasma capsulatum organisms are also identified on cytologic evaluation of fine-needle aspirates or impression smears of tissue (Table 1; Figures 2, 4-5).8,15,38,58 Bone marrow aspirates can be diagnostic in feline disseminated histoplasmosis. In retrospective studies, diagnoses were obtained through bone marrow aspirates in 14 of 16 (87.5%) and 11 of 12 (91.7%) cats.1,15 Rectal scrapings (Figure 6) may be diagnostic in dogs with disseminated histoplasmosis; a diagnosis was made in seven of 24 (29%) dogs based on cytologic evaluation of rectal mucosa.8,9 Similarly, H. capsulatum was identified on cytologic or histologic examination of rectal or intestinal mucosa in seven of 12 (58%) dogs with disseminated histoplasmosis.8


5. A lung aspirate from a cat. Multiple foamy activated macrophages are noted, including a single giant cell on the far left. One macrophage contains several yeast (arrow). Several yeast are noted in the background (arrowheads) of erythrocytes and a small amount of finely granular protein. Also note the morphologically normal respiratory epithelial cells with their tufts of apical cilia to the right of the micrometer (Diff-Quik, 60X).
When indicated, cytologic analysis of pleural or peritoneal effusions, synovial or cerebrospinal fluid, or tracheal or bronchoalveolar fluid can be helpful.11,23,25,33,38,41,58,60 During bronchoscopy of affected dogs, mild to severe compression of the mainstem bronchus may be seen secondary to hilar lymphadenopathy, affecting both the left and right principal bronchi.17

Histologic evaluation of affected tissues often shows granulomatous or pyogranulomatous inflammation with intracellular organisms. Histoplasma species organisms are not well-visualized with routine hematoxylin-eosin stain. Periodic acid-Schiff, Gridley's fungal, or Gomori methenamine silver stains should be used to enhance detection.10,11 Histoplasma capsulatum has been identified antemortem by histologic examination of several tissues (Table 1).


6. A rectal scraping from a dog. A mixture of individualized epithelial cells and macrophages are seen in a moderately heavy background of mixed bacteria. Yeast are seen within macrophages (arrow) and are free in the background (arrowhead) (Diff-Quik, 100X).


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