Nocardiosis: clinical and pathologic examination of 140 cases representing 18 species in 6 complexes

Before molecular taxonomy, Nocardia species infections were predominantly attributed to Nocardia asteroides; now, more than 50 species are known to infect humans. To better study the range of associated clinicopathologic features, Nocardia species can be grouped into complexes based on gene sequencing, antibiotic susceptibilities, and matrix-assisted laser desorption/ionization with time-of-flight mass spectrometry profiling.

A 20-year retrospective study of laboratory-detected Nocardia species cases from 2 US tertiary-care academic medical centers was performed by review of anatomic pathology slides and electronic health records.

From 2004 to 2024, 140 patients with 151 Nocardia species detections were grouped into N abscessus (n = 15), N brevicatena/paucivorans (n = 3), N cyriacigeorgica (n = 13), N farcinica (n = 21), N nova (n = 60), N transvalensis (n = 6) complexes, N asteroides (n = 28) complex, and Nocardia species not further classified (n = 5). The majority of patients were immunocompromised, most commonly due to steroids. Only 16% of cases were clinically suspected, and 15% of the patients died with disease. Nocardiosis predominantly affected lungs, followed by brain and skin/soft tissue. Ninety anatomic pathology specimens from 61 patients were reviewed, identifying abscess as the most common inflammatory pattern across all complexes and methenamine silver stains as the most useful ancillary test for visualizing bacilli.

Although Nocardia species cases are rare and clinically diverse and can be difficult to diagnose, this study highlights associations between specific Nocardia complexes and patient risk factors, sites of involvement, and antibiotic susceptibility, providing new insights to aid in the diagnosis and treatment of patients with nocardiosis.