When the Cure Turns Into a Culprit: Mesalamine and the Eosinophilic Lung.

We report a case of a 74-year-old man with a one-year history of ulcerative colitis in remission who developed low-grade fever, dry cough, progressive dyspnea, loss of appetite, and an eight-kg weight loss over 12 weeks. Oral mesalamine (2.4 g/day) had been initiated 12 weeks before presentation, with respiratory symptoms developing approximately three weeks after initiation. Despite initial antibiotic therapy, his symptoms progressively worsened, and he developed hypoxemia requiring hospitalization. Investigations revealed leukocytosis with marked peripheral eosinophilia, elevated C-reactive protein (CRP) levels, and bilateral upper lobe- and peripheral-predominant pulmonary consolidations and ground-glass opacities. Infectious, parasitic, autoimmune, and malignant causes were investigated, and no alternative cause was identified. Transbronchial lung biopsy demonstrated eosinophil-rich subacute to chronic inflammation with organizing pneumonia-like changes, supporting eosinophilic pneumonia in the appropriate clinical context. The temporal relationship between mesalamine exposure and symptom onset, peripheral eosinophilia, compatible radiological and histopathological findings, and exclusion of alternative causes supported a diagnosis of mesalamine-associated eosinophilic pneumonia. Mesalamine was discontinued, and systemic corticosteroids were initiated concurrently, followed by rapid clinical improvement and subsequent complete radiological resolution. As both interventions were instituted simultaneously, the response could not be attributed independently to mesalamine withdrawal or corticosteroid therapy. This case highlights the importance of considering mesalamine-associated eosinophilic pneumonia in patients who develop respiratory symptoms and peripheral eosinophilia after mesalamine initiation. We also present a review of previously reported cases of 5-aminosalicylic acid (5-ASA)-associated eosinophilic respiratory disease.