Pulmonary Nocardia paucivorans Infection Mimicking Lung Cancer in an Immunocompetent Patient: A Case Report

Pulmonary nocardiosis is uncommon and usually affects immunocompromised hosts, but it can occur in immunocompetent patients and mimic primary lung malignancy clinically and radiographically.

A 52-year-old immunocompetent woman presented with one month of cough, hemoptysis, and mild shortness of breath after repeated hiking exposures in Mexico and Central California. Chest CT showed bilateral apical ground-glass opacities and a spiculated, pleural-based mass involving the right upper and lower lobes, raising concern for primary lung cancer. HIV testing was negative; HbA1c was 5.1%; and initial sputum, blood, and acid-fast bacilli studies were negative. CT-guided biopsy showed acute and chronic inflammation, necrosis, and stromal fibrosis without well-formed granulomas. Gram stain identified Gram-positive rods. Tissue culture grew Nocardia paucivorans, identified by MALDI-TOF MS and confirmed by DNA sequencing. Susceptibility testing showed TMP-SMX susceptibility. She received prolonged oral TMP-SMX from November 2022 through December 2024, with resolution of cough and hemoptysis. Serum creatinine rose mildly during therapy but normalized without interruption. Follow-up CT showed improvement in the dominant mass, with residual bilateral upper-lobe abnormalities favored to represent postinflammatory fibrosis.

Pulmonary Nocardia paucivorans infection can closely mimic lung cancer, even in immunocompetent hosts. When noninvasive testing is unrevealing, tissue biopsy may be necessary for diagnosis. Radiological follow-up after treatment is warranted to exclude occult malignancy masked by infection.