Aetiology
Nocardia species are ubiquitous soil organisms that often infect immunocompromised patients.[15] The aerosol route is the main portal of entry, and the lungs are the most frequently affected organs. As Nocardia species are not part of the normal human flora, any isolate from tissue or a normally sterile site must be carefully evaluated.[7]
Nocardiosis is due to microorganisms of the genus Nocardia, which includes gram-positive, acid-fast bacilli, with >90 species identified, 54 of which have been reported as clinically relevant.[16][17] Pulmonary nocardiosis and disseminated nocardiosis can be considered opportunistic diseases because both occur mainly in patients with deficient cell-mediated immunity, such as solid organ transplant and haematopoietic stem cell transplant recipients, patients living with HIV, and patients taking corticosteroids or those with malignancies.[18] Patients with structural lung diseases, such as cystic fibrosis or bronchiectasis, are also more susceptible to Nocardia infection, especially if receiving corticosteroids.[6]
Primary cutaneous nocardiosis may occur in immunocompetent patients with direct inoculation of the microorganism. Most of these patients have a particular exposure to risk factors, such as working in rural areas or being involved in agricultural activities.[8][11][12][13]
Pathophysiology
Nocardia species are easily inhaled with dust, especially in dry areas. After inhalation, the bacteria may colonise lungs or produce an acute, sub-acute, or chronic respiratory disease, depending on the immune status of the patient. The majority of disseminated cases are produced by haematogenous spread from the lungs. The most frequently affected organs are the central nervous system (CNS) and the skin, but almost every organ can be involved. Rarely, dissemination can occur by continuity from one body part to an adjacent body part.[8] Both immunocompetent and immunocompromised patients can present with CNS nocardiosis, even if the primary involvement is not evident.[19]
In patients with primary cutaneous nocardiosis, infection occurs after direct inoculation of the skin. There the infection may evolve as a superficial infection or progress in depth and affect the subcutaneous tissues. Lymphatic involvement of both nodes and lymphatic vessels can develop, as well as a nodular-lymphangitic form. Infection can spread into even deeper skin tissues and evolve over months or years as a mycetoma (actinomycetoma).[8][11][12][13] There is no definitive evidence of person-to-person transmission of Nocardia infection.[20]
Classification
Clinical categories
No internationally accepted disease classification is available. Commonly accepted clinical categories are as follows:
Pulmonary nocardiosis
Caused by numerous species of Nocardia. Acute, sub-acute, or chronic pneumonia with different radiographic patterns (alveolar filling process, cavities, empyema, and/or abscesses). May occur in patients with structural pulmonary disease.
Cutaneous nocardiosis
Caused mostly by N brasiliensis. Immunocompetent patients with traumatic inoculation can develop superficial cutaneous disease (primary cutaneous nocardiosis), lymphocutaneous disease (sporotrichoid nocardiosis), or mycetomas (actinomycetomas).
Disseminated nocardiosis
Caused by numerous species of Nocardia. In most cases, dissemination is from the lungs and more frequently involves the central nervous system, skin, and soft tissues. This is typically seen in immunocompromised patients (e.g., organ transplant recipients and patients with AIDS).
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