A case of Nocardia asiatica pneumonia: Relapse caused by trimethoprim-sulfamethoxazole treatment nonadherence.

Nonadherence to trimethoprim-sulfamethoxazole (TMP-SMX) is a known risk factor for nocardiosis treatment failure, often due to adverse effects. However, reports linking relapse to treatment interruption caused by poor adherence are scarce. A 70-year-old male with chronic bronchitis presented with persistent fever, cough, and sputum. Nocardia asiatica was identified. Initial improvement followed a 28-day TMP-SMX course, but the patient discontinued therapy. Multiple readmissions over three years resulted from relapses due to poor adherence. Whole-genome sequencing of isolates from the first and third episodes revealed 99.9% genetic similarity, confirming relapse from the original strain. Only after completing a full six-month TMP-SMX course did the patient achieve sustained remission with no recurrence for five years. This case provides molecular evidence supporting that premature TMP-SMX discontinuation contributed to relapse, emphasizing that adherence is as critical as accurate diagnosis for cure.