Nocardiosis

Nocardiosis is an opportunistic infection caused by Nocardia species, aerobic actinomycetes found in soil and decaying organic matter, that primarily affects individuals with immunocompromise.

Nocardia species are Gram-positive, partially acid-fast, lysozyme-resistant, catalase-positive aerobic actinomycetes transmitted via inhalation or direction inoculation. The bacteria can evade immune defenses due to their ability to survive within macrophages, leading to chronic infections.

Nocardiosis is more common in tropical and subtropical climates and may also occur more frequently in dry, windy regions, such as the southwestern US. The reported incidence of Nocardia infection among solid organ transplant recipients is 2.65%.

The most significant risk factor for nocardiosis is immunocompromise, particularly compromised cell-mediated immunity, as seen in HIV infection, solid organ transplantation, allogeneic HSCT, hematologic malignancies, prolonged corticosteroid use, and immunosuppressive therapy. Other risk factors include chronic pulmonary disease, alcohol use disorder, diabetes, and trauma for primary cutaneous infection.

Nocardiosis typically presents as a chronic infection, with symptoms varying based on the affected organs. The lungs are most commonly involved, but the infection can disseminate to other sites, including skin and soft tissues and the CNS. Pulmonary nocardiosis presents with nonspecific respiratory and constitutional symptoms. CNS involvement includes brain abscesses and meningitis, presenting with nonspecific generalized symptoms and focal neurological deficits. Secondary cutaneous infections may develop from lymphatic or hematogenous spread, presenting as painful erythematous subcutaneous nodules with ulcerated draining lesions (sporotrichoid nocardiosis) or larger painless masses (actinomycetoma). Primary cutaneous infection can also occur through direct inoculation.

In the US, the reported 90-day mortality rate is 17.5%.