Veterinary Medicine - May 2008 - (Page 255) cytic 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 Histoplasma capsulatum organisms are also identi ed on cytologic evaluation of ne-needle aspirates or impression smears of tissue (Table 1; Figures 2, 45).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 identi ed on cytologic or histologic examination of rectal or intestinal mucosa in seven of 12 (58%) dogs with disseminated histoplasmosis.8 When indicated, cytologic analysis of pleural or peritoneal effusions, synovial or cerebrospinal uid, or tracheal or bronchoalveolar uid 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 in ammation 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 identi ed antemortem by histologic examination of several tissues (Table 1). incubation at room temperature can result in the growth of the mycelial phase within seven to 10 days, posing a risk for personnel.10 In addition, fungal cultures may be falsely negative,10,61-63 and growth of H. capsulatum may require up to four weeks. Polymerase chain reaction testing To date, polymerase chain reaction (PCR) testing has been used primarily in research settings to con rm a diagnosis of histoplasmosis. A nested PCR technique has been applied in situ to DNA extracted 15 µm 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). Fungal culture A de nitive diagnosis may be made through fungal culture of tissue, body uid, or ne-needle aspirates of affected organs.10,11 However, culturing H. capsulatum outside of professional diagnostic laboratories is not recommended, as 15 µm 3. A canine peripheral blood smear demonstrating two segmented neutrophils that contain several budding yeast (modified Wright’s stain, 100X). VETERINARY MEDICINE May 2008 255
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