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Histoplasmosis in Cats

By: Dr. Arnold Plotnick

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  • Complete blood count. There may be several findings on a complete blood count that support a diagnosis of histoplasmosis. Anemia is a common finding in dogs with histoplasmosis. The anemia is mostly due to suppression of red blood cell production by the infected bone marrow. In dogs, blood loss in the stool, due to severe Histoplasma infiltration of the intestinal tract, contributes to the anemia. Thrombocytopenia (a low platelet count) is commonly seen. There is no consistent pattern to the white blood cell count in affected dogs; the white count may be high, low, or normal.

  • Chemistry panel and urinalysis. Serum biochemical profiles often reveal a low albumin in dogs with the disease. Much of the low albumin is attributed to loss of albumin in the stool. Urinalysis shows no specific abnormalities.

  • Radiographs. X-rays of the chest often reveal a nodular pattern that is characteristic for fungal infections. Pulmonary lymph nodes are often visibly enlarged. Abdominal films may show an enlarged liver and spleen. X-rays of the bones in animals presenting with lameness may show various degrees of bone destruction and new bone formation.

  • Complete ophthalmic exam. An examination of the eye, especially the retina, may show abnormalities characteristic of fungal disease, lending further support to a diagnosis of histoplasmosis.

  • Rectal scrapings. A rectal scraping, obtained using a small metal spatula and stained and examined under a microscope often reveals Histoplasma organisms inside many of the cells. This allows a definitive diagnosis of the disease.

  • Bone marrow examination. Examination of stained bone marrow specimens often reveals cells containing the Histoplasma organism, allowing for a definitive diagnosis.

  • Cytology of fine-needle aspirates. Obtaining a tissue sample by aspirating it through a fine needle and spreading it onto a slide, staining, and examining under a microscope may reveal fungal organism and allow a definitive diagnosis. The liver, lung, spleen, and lymph nodes are good tissues to sample, and may be better than rectal scrapings in dogs that do not have signs of gastrointestinal involvement.

  • Biopsy. If examination of aspirated tissue cells is not diagnostic, examination of a biopsy specimen may be required. A specific type of inflammation is often seen in fungal infections and may be highly suggestive of the disorder, but finding actual fungal organisms may be difficult. Special fungal stains are often used to increase the chances of identifying the organism.

  • Serologic tests. Blood tests that look for antibodies against the organism may be performed. A positive test means that the animal was exposed to the organism. It does not necessarily mean that the animal is infected. Results of serologic tests are inconsistent and are not very useful in making a diagnosis.

  • Culture. Because of the large numbers of organism present in affected tissues, the Histoplasma organism can often be cultured from fine needle aspirates; however, the organism grows slowly and it may take 7 to 10 days before results are available. The organism, when grown in culture, also poses a significant hazard to human health, and laboratories involved need to be notified before submitting samples.


    Treatment of disseminated histoplasmosis is difficult. It requires the use of antifungal agents and supportive therapy like adequate nutrition, hydration, and control of secondary bacterial infections.

  • Antifungal drugs. A number of antifungal drugs such as ketaconazole, itraconazole, amphotericin B have shown some efficacy against Histoplasma infection, either alone or in combination. These drugs have to be given for prolonged periods of time, and are often fairly expensive.

  • Supportive care. Intravenous fluids, good nutrition, and possible antibiotic therapy to control or prevent secondary bacterial infections may be necessary as part of the overall therapy for histoplasmosis.


    Optimal treatment for your pet requires a combination of home and professional veterinary care. Follow-up can be critical, especially if your pet does not rapidly improve.

    Administer all prescribed medications as directed. Alert your veterinarian if you are experiencing problems treating your pet

    The prognosis for cats with disseminated histoplasmosis is guarded. Treatment is frequently unrewarding because the patients are often quite debilitated at the time of diagnosis, and the disease may already be fairly widespread. Newer antifungal drugs have improved the success rate of treatment.

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