Antigen Concentrations as an Indicator of Clinical Remission and Disease Relapse in Cats with Histoplasmosis

BACKGROUND: Treatment monitoring is subjective and disease relapse is common in cats with histoplasmosis. The Histoplasma antigen enzyme immunoassay (EIA) is a noninvasive test used for determining disease remission and detecting disease relapse in humans with histoplasmosis. The utility of the antigen EIA for these purposes in cats remains unknown. HYPOTHESIS/OBJECTIVES: Those Histoplasma antigen concentrations in urine and serum would decline with antifungal treatment and that antigen elimination would be an indicator of clinical remission in cats with histoplasmosis treated with antifungal treatment. ANIMALS: Fifteen client‐owned cats with histoplasmosis. METHODS: Masked observational study. Cats were monitored monthly during antifungal treatment. Time of clinical remission and serum and urine antigen elimination were determined for each cat. RESULTS: Twelve of 15 cats achieved clinical remission. At the time of diagnosis, antigen was detectable in urine in 14/15 (93%) cats and in serum in 11/15 (73%) cats. Both serum (P < .0005) and urine (P < .0001) antigen concentrations significantly decreased over time with effective treatment. Antigen elimination was sensitive [urine, 90.0% (95% CI 72.3–97.4%); serum, 90.4% (68.2–98.3%)] but less specific [urine, 64.6% (51.7–75.8%); serum, 52.1% (37.4–66.5%)] for disease remission. Urine antigen was positive in both cats and serum antigen was positive in 1 cat at the time of disease relapse. CONCLUSIONS AND CLINICAL IMPORTANCE: Measurement of Histoplasma antigen in urine and serum might be useful tests for determining disease remission and relapse in cats with histoplasmosis. Further research is needed to investigate the importance of low‐level antigenemia and antigenuria.