A Rare Case of Blastomycosis in a Psoriasis Patient Treated With Ustekinumab (Stelara)

ABSTRACT Psoriasis is a chronic inflammatory skin condition mediated by T‐helper 1 (Th1) and T‐helper 17 (Th17) cytokines, including interleukin (IL)‐12, IL‐23, IL‐17, and tumor necrosis factor alpha (TNF‐α). Ustekinumab, a human monoclonal antibody against the p40 subunit of IL‐12/23, has revolutionized treatment of moderate to severe psoriasis but carries a risk of opportunistic infections due to impaired cell‐mediated immunity. We describe a 54‐year‐old man from Kentucky with type 2 diabetes and chronic sinusitis, receiving ustekinumab for 1 year for plaque psoriasis. He presented with recurrent pneumonia unresponsive to multiple courses of amoxicillin‐clavulanate, azithromycin, and levofloxacin. Chest imaging consistently demonstrated left lower lobe airspace opacities, initially attributed to nonresolving bacterial pneumonia or possible malignancy. On bronchoscopy with bronchoalveolar lavage, scattered broad‐based budding yeast were visualized, and culture confirmed Blastomyces dermatitidis . The patient was initiated on oral itraconazole, resulting in symptomatic relief and gradual radiographic resolution over 6 months of follow‐up. Although pulmonary blastomycosis is rare (1–2 cases per 100,000 in endemic areas), clinicians should maintain a high index of suspicion for endemic mycoses in immunosuppressed patients on biologic therapies presenting with persistent pulmonary infiltrates. Early invasive diagnostics and targeted antifungal therapy are critical to prevent morbidity and improve outcomes.