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Annual incidence in the United States is estimated at around 1/250,000 inhabitants, but may be underestimated. The exact incidence in Europe is unknown. Men are more frequently affected than women (sex ratio: 3:1).

Nocardiosis may occur in any age group, but it is more common in middle-aged adults. Clinical manifestations depend on the site of infection. Most patients present with pulmonary disease. Frequent signs are fatigue, fever, chills, coughing (similar to the cough in pneumonia or tuberculosis), dyspnea, pleural chest pain, and weight loss. Primary cutaneous nocardiosis may present as cutaneous, lymphocutaneous or subcutaneous infection. Cutaneous infection manifests as cellulitis, pustules, pyoderma, paronychia, ulcerations or localized abscesses. Similar lesions are present with lymphocutaneous infection but are associated with ascending regional lymphadenopathy. Subcutaneous infection manifests as the apparition of mycetoma (see this term), predominantly affecting the extremities. Disseminated nocardiosis typically affects immunocompromised hosts. Lesions in the brain or meninges are frequent.

The disease is caused by Nocardia infections. Nocardia is an aerobic, Gram-positive branching filamentous bacteria found in soil, decaying vegetable matter, and aquatic environments. Infection may occur through direct inhalation or inoculation. At least 13 species of Nocardia are reported to cause human disease, with the most frequently identified causes being N. asteroids complex and N. brasiliensis. Nocardiosis is often associated with a patient history of trauma (puncture wounds or cat scratches), immunodeficiency, resistance to previous antibiotic therapy, or fever.

The diagnosis is based on analysis of cultures of the causative organism from the infection site and on identification of Nocardia throughmolecular techniques (RFLP or PCR assays). Chest radiography may also be required.

The differential diagnosis should include tuberculosis, aspergillosis, histoplasmosis, Kaposi sarcoma, sporotrichosis, non-Hodgkin lymphoma (see these terms), lung abscess and pneumonia.

Management includes antibiotic therapy, in particular with trimethoprim-sulfamethoxazole (TMP-SMX). The duration of therapy (1 to 12 months) depends on the type of Nocardia infection. Surgical resection may be feasible for localized abscesses.

Prognosis is generally favorable, except in cases of disseminated nocardiosis in immunocompromised patients.