Background
Nocardiosis is an infectious disease caused by various aerobic soil saprophytes of the gram-positive bacilli genus Nocardia. [1, 2] It can present as acute, subacute, or chronic infections, often in cutaneous, pulmonary, or disseminated forms. The disease is characterized by suppurative or granulomatous infections, with pneumonia being the most typical manifestation. Skin and central nervous system infections are also common.
Nocardia species, such as N asteroides and N brasiliensis, are primarily responsible for human infections. N asteroides usually leads to pulmonary and disseminated infections, whereas N brasiliensis is more associated with skin infections, particularly in tropical climates. Infection occurs through inhalation of dust containing the bacteria or direct inoculation via cuts or scrapes when contaminated soil or water enters the body.
Nocardiosis can affect individuals of all ages, but the incidence is higher among older adults, especially men, and those who are immunocompromised. Person-to-person transmission is rare. Diagnosis typically is made through culture and special stains, and treatment usually involves sulfonamides.
Primary cutaneous nocardiosis may present as cellulitis, abscesses, or lymphocutaneous infections, whereas pleuropulmonary nocardiosis manifests as pneumonitis, primarily in immunocompromised hosts. Disseminated nocardiosis can affect any organ, with brain and meninges lesions being the most common. Nocardia commonly is found in standing water, decaying plants, and soil, contributing to its potential for infection.
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 since the early 2000s.
The taxonomy has been challenging and likely remains in evolution. [3, 4] 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Nocardia asteroides sensu stricto, and Nocardia cyriacigeorgica; however, use of the term "N asteroides complex" is outdated because of the heterogenous group of organisms it includes.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 delicate, branching, beaded, filamentous, gram-positive bacteria with a characteristic morphology to a trained observer.
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. [5] 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.
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. [5] Therefore, nocardiosis is most problematic in individuals with impaired T-cell–mediated immunity. [4]
Epidemiology
Frequency
United States
The estimated incidence of nocardiosis in the United States is 500-1000 cases per year. [2]
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. [6]
Cure rates with appropriate therapy are approximately 100% in skin and soft-tissue infections.
Up to 90% percent of pleuropulmonary infections can be cured with appropriate therapy.
The cure rate in disseminated nocardiosis falls to 63%, while only half of the patients with brain abscess can be cured with therapy. [7]
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 is likely 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.
Prognosis
The one-year rate for all-cause mortality from nocardiosis in both immunocompetent and immunocompromised patients has been found to be 19-22%. [8, 9] Poor prognostic factors in nocardiosis patients are Nocardia CNS infections, recipients of solid-organ transplants, HIV/AIDS with advanced immunocompromised state, delayed diagnosis, and premature discontinuation of antimicrobial therapy. [9, 10]
Patient Education
Patients with nocardiosis must be educated about the need for protracted antimicrobial therapy.
Patients with nocardiosis should be informed of the potential adverse effects of protracted antimicrobial therapy and which circumstances require reporting to their physician promptly.
-
High-power microscopic appearance of Nocardia. Image courtesy of CDC.
-
Photomicrograph of tissue biopsy stained with Gomori methenamine silver demonstrating acute inflammatory response and organisms compatible with Nocardia.
-
Plain chest radiograph in a patient with nocardiosis. Image courtesy of Applied Radiology, Anderson Publishing, LTD.
-
Chest CT scan in a patient with pleuropulmonary nocardiosis. Image courtesy of Applied Radiology, Anderson Publishing, LTD.
-
Brain CT scan in a patient with nocardial brain abscess. Image courtesy of Applied Radiology, Anderson Publishing, LTD.
