Antigenemia without antigenuria in a cat with histoplasmosis

Based on demonstration of the yeast phase of Histoplasma capsulatum on fine-needle aspirate cytology of the kidney, a 5-year-old cat was diagnosed with histoplasmosis. Urine and serum were tested for antigen via a Histoplasma antigen enzyme immunoassay. At the time of diagnosis, and on multiple occasions during antifungal treatment, antigenemia was detected without antigenuria. The cat was treated with standard therapy and achieved clinical remission.

Diagnosis is most commonly made by finding the yeast phase of H capsulatum via cytology of fluid samples or cytology or histopathology of infected tissues. In certain cases this may require invasive tests. Recently, a non-invasive test, a Histoplasma antigen enzyme immunoassay, has been shown to be a sensitive test for supporting the diagnosis of histoplasmosis in cats. Urine has been considered the biologic specimen of choice for antigen testing and there is a paucity of information concerning the use of other specimens such as serum. The case herein reports a cat with antigenemia without antigenuria. These findings suggest that further research is necessary to better understand the ideal biologic sample or combination of samples as it pertains to antigen testing in cats. It also suggests that to maximize sensitivity both urine and serum may need to be tested in cats with suspected histoplasmosis.