Feline Histoplasmosis

Histoplasmosis, a systemic fungal infection caused by Histoplasma capsulatum, is the most common deep fungal disease of cats. In this retrospective study, findings of 22 feline histoplasmosis cases were reported. The disease was rare, comprising 0.2% of the hospital population. Median age at presentation was 9 years. The most common clinical signs were weakness (68%), weight loss (50%), anorexia (45%), and respiratory signs (45%). Definitive diagnosis was based on cytology or histopathology. The organism was found on cytologic examination, most commonly from lung aspirates. Fifteen of the cats were treated with sole or combination antifungal therapy (ketoconazole–itraconazole–fluconazole with amphotericin B). Median time of treatment was 5 months; 55% of cats survived.

CommentaryHistoplasmosis should remain a differential diagnosis for cats presenting with respiratory and/or GI signs in a region where Histoplasma is endemic. Cytology and histopathology are often considered standard for diagnosis; patients with cutaneous nodules or pulmonary and/or GI masses should undergo cytologic evaluation. A Histoplasma antigen test (miravistalabs.com) can be performed on serum or urine and may be a companion to cytology when attempting diagnosis. Because this test provides an antigen titer, positive results indicate active infection. It is likely that this titer could monitor therapeutic response and evaluate for relapse, but this has not been evaluated. Amphotericin B is a fungicidal drug that should be considered in life-threatening Histoplasma infections; it can be nephrotoxic, but this can be minimized with lipid complex formulations. Azole agents are fungistatic and are typically administered for several months to allow the infection to clear. Fluconazole can be equally effective as itraconazole in canine blastomycosis (although a slightly longer duration of therapy may be required) at a reduced cost1; it also has superior penetration into the prostate, eye, and brain. It may be that the same scenario is true for feline histoplasmosis, but this must be evaluated.—J.D. Foster, VMD

SourceFeline histoplasmosis: A retrospective study of 22 cases (1986-2009). Aulakh HK, Aulakh KS, Troy GC. JAAHA 48:182-187, 2012.

1. Retrospective comparison of the efficacy of fluconazole or itraconazole for the treatment of systemic blastomycosis in dogs. Mazepa AS, Trepanier LA, Foy DS. J Vet Intern Med 25:440–445, 2011.