Nocardiosis

Updated: Feb 16, 2016
  • Author: George Kurdgelashvili, MD; Chief Editor: Michael Stuart Bronze, MD  more...
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Overview

Background

Nocardiosis is an acute, subacute, or chronic infectious disease that occurs in cutaneous, pulmonary, and disseminated forms. Primary cutaneous nocardiosis manifests as cutaneous infection (cellulitis or abscess), lymphocutaneous infection (sporotrichoid nocardiosis), or subcutaneous infection (actinomycetoma). Pleuropulmonary nocardiosis manifests as an acute, subacute, or chronic pneumonitis, usually in immunocompromised hosts, although isolated cases have been reported in immunocompetent hosts. Disseminated nocardiosis may involve any organ; lesions in the brain or meninges are most common.

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Pathophysiology

Members of the genus Nocardia are aerobic actinomycetes that are ubiquitous saprophytes in soil, decaying organic matter, and fresh and salt water. Over 100 species of the genus Nocardia have been identified, more than half of which have been described during the last decade.

The taxonomy has been challenging and likely remains in evolution. [1, 2] Most human infections are due to members of the formerly called Nocardia asteroides complex. N asteroides complex was later separated into Nocardia abscessus, Nocardia brevicatena-paucivorans complex , Nocardia nova complex, Nocardia transvalensis complex, Nocardia farcinica, Nocardia asteroides sensu stricto, and Nocardia cyriacigeorgica; however, use of the term " N asteroides complex" is currently outdated because of the heterogenous group of organisms it includes. Nocardia species also cause infections in animals, including bovine mastitis and sporotrichoid nocardiosis in horses.

When observed microscopically, either in Gram-stained smears of clinical specimens or cultures or on histopathology in tissues, Nocardia organisms are branching, beaded, filamentous, gram-positive bacteria with a characteristic morphology to a trained observer. See the image below.

High-power microscopic appearance of Nocardia. Ima High-power microscopic appearance of Nocardia. Image courtesy of CDC.

Nocardia are typically weakly acid-fast after traditional staining and positive on modified acid-fast staining, but this is not invariable.

The cutaneous, lymphocutaneous, and subcutaneous forms of nocardiosis arise from local traumatic inoculation. These infections are not necessarily associated with immunocompromised host states, but dissemination from these sites of inoculation is more likely in immunocompromised hosts, particularly those with impaired cell-mediated immunity. Pleuropulmonary nocardiosis presumably arises from inhalation exposure. Disseminated nocardiosis results from hematogenous dissemination, usually from a pulmonary focus. Most persons with disseminated nocardiosis have underlying immunocompromising disease or are receiving immunosuppressive therapy.

Nocardiosis produces suppurative necrosis with frequent abscess formation at sites of infection. See the image below.

Photomicrograph of tissue biopsy stained with Gomo Photomicrograph of tissue biopsy stained with Gomori methenamine silver demonstrating acute inflammatory response and organisms compatible with Nocardia.

Disease manifestations of nocardiosis are determined by strain characteristics, inoculation site, tissue tropism, ability to survive initial neutrophilic leukocyte phagocytic attack, and the nature of the immune response. T-cell–mediated immunity is the principal protective immune response to nocardiosis. [3] Therefore, nocardiosis is most problematic in individuals with impaired T-cell–mediated immunity. [2]

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Epidemiology

Frequency

United States

The estimated incidence of nocardiosis in the United States is 500-1000 cases per year. [4]

Clusters of nocardiosis have been described in hospitalized patients, related to contaminated fomites from construction or contaminated hands of staff.

International

No reliable estimates on the international frequency of nocardiosis are available.

Mortality/Morbidity

Nocardiosis has a variable prognosis, depending on the site of infection, extent of infection, and underlying host factors. [5]

Cure rates with appropriate therapy are approximately 100% in skin and soft-tissue infections.

Ninety percent of pleuropulmonary infections can be cured with appropriate therapy.

The cure rate in disseminated nocardiosis falls to 63%, while only half of patients with brain abscess can be cured with therapy. [6]

Race

Nocardiosis has no apparent racial predilection.

Sex

Nocardiosis is more common in males than in females, with a male-to-female ratio of 3:1. This difference may be related to exposure frequency rather than a gender difference in susceptibility.

Age

All ages are susceptible to nocardiosis. The mean age at diagnosis is in the fourth decade of life.

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