A six-year-old domestic short hair (DSH) cat was examined by a veterinarian for ocular and nasal discharge, anorexia, and weight loss that had persisted for two months. Bilateral enlarged popliteal lymph nodes were found on the initial examination. Fine needle aspiration of one of the enlarged lymph nodes was performed and a digital image of the aspirated material was captured. The image was submitted to the Texas A&M Veterinary Medical Diagnostic Laboratory (TVMDL) Digital Cytology service for evaluation. The image was cellular with a heterogeneous population of lymphocytes and mildly increased numbers of macrophages and non-degenerate neutrophils. The macrophages were actively phagocytizing numerous fungal yeast. The yeast were small (approximately 3 microns in diameter) with a thin clear staining capsule and a round purple staining center. The features of the yeast were consistent with Histoplasma spp.
Histoplasmosis is common in many areas of the United States with temperate climates, including Texas. This fungus prefers moist, humid conditions and tends to flourish in soil containing nitrogen-rich organic matter, such as bird and bat excrement. Infection often occurs via inhalation of microconidia so the lungs are often involved. Systemic involvement may occur by hematogenous or lymphatic dissemination. Diagnosis of the disease can be made by finding the yeast on cytology or in biopsy specimens or by antigen or antibody testing. A culture of the fungus is also possible, but generally less likely to be used as the other tests are quicker. Both animals and humans are susceptible to the fungus under the right conditions. Animal to animal and animal to human transmission has not been documented.
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