Disseminated peritoneal coccidioidomycosis mimicking malignancy in an immunosuppressed patient: a case report

Disseminated peritoneal coccidioidomycosis is rare and can mimic peritoneal carcinomatosis. Ustekinumab, an IL-12/23 inhibitor, impairs Th1/Th17 immunity which is critical for fungal control. No prior cases link ustekinumab use to disseminated fungal disease. We report a 45-year-old woman from an endemic region with prior pulmonary coccidioidomycosis and psoriatic arthritis on ustekinumab who presented with malignant-appearing ascites and omental caking. Fluid studies were non-diagnostic, but laparoscopic biopsies confirmed disseminated Coccidioides infection. She improved with liposomal amphotericin B followed by oral fluconazole suppression. This case highlights the need to consider peritoneal coccidioidomycosis in immunosuppressed patients, particularly on IL-12/23 blockade, and underscores the role of tissue diagnosis when fluid studies are negative.