Nocardia are obligate aerobic, partially acid-fast, beaded, branching, gram-positive bacilli. The genus Nocardia has many species and an increasing number are recognized as causes of human disease.
N. asteroides usually causes pulmonary and disseminated infection.
N. brasiliensis most commonly causes skin infection, particularly in tropical climates. Infection is via inhalation or by direct inoculation of the skin.
Other Nocardia species sometimes cause localized or, occasionally, systemic infections.
Nocardiosis occurs worldwide in all age groups, but incidence is higher in older adults, especially men, and immunocompromised patients. Person-to-person spread is rare.
Nocardia is found in standing water, decaying plants, and soil. Infection is via inhalation of dust that contains the bacteria or by direct inoculation of the skin when contaminated soil or water enters through a cut or scrape.
Predisposing risk factors for nocardiosis include
However, about 20 to 30% of patients with nocardiosis have no identified preexisting disease or condition. Primary cutaneous nocardiosis usually occurs in immunocompetent patients 1 to 3 weeks after some type of local trauma.
Nocardiosis is also an opportunistic infection in patients with advanced HIV infection.
Nocardiosis usually begins as a subacute pulmonary infection that resembles actinomycosis, but Nocardia are more likely to disseminate locally or hematogenously. Dissemination with abscess formation may involve any organ but most commonly affects the brain, skin, kidneys, bone, or muscle.
The most common symptoms of pulmonary involvement—cough, fever, chills, chest pain, weakness, anorexia, and weight loss—are nonspecific and may resemble those of tuberculosis or suppurative pneumonia. Pleural effusion may also occur. Metastatic brain abscesses, occurring in 30 to 50% of cases, usually cause severe headaches and focal neurologic abnormalities. Infection may be acute, subacute, or chronic.
Skin or subcutaneous abscesses occur frequently, sometimes as a primary local inoculation. They may appear as
The lymphocutaneous syndrome consists of a primary pyoderma lesion and lymphatic nodules resembling sporotrichosis.
An actinomycetoma due to nocardiosis begins as a nodule, suppurates, spreads along fascial planes, and drains through chronic fistulas.
Diagnosis of nocardiosis is by identification of Nocardia species in tissue or in culture of samples from localized lesions identified by physical examination, x-ray, or other imaging studies. Clumps of beaded, branching filaments of gram-positive bacteria (which may be weakly acid-fast) are often seen.
Without treatment, pulmonary nocardiosis and disseminated nocardiosis are usually fatal. Among patients who are treated with appropriate antibiotics, the mortality rate is highest (> 50%) in immunocompromised patients with disseminated infections and is about 10% in immunocompetent patients with lesions restricted to the lungs.
Cure rates for patients with skin infection are usually > 95%.
TMP/SMX 15 mg/kg/day (of the TMP component) orally every 6 to 12 hours or high doses of a sulfonamide alone (eg, sulfadiazine 1 g orally every 4 to 6 hours) are used. Because most cases respond slowly, a dose that maintains a sulfonamide blood concentration of 12 to 15 mg/dL 2 hours after the last dose must be continued for ≥ 6 months. In immunocompromised patients and patients with disseminated disease, TMP/SMX should be used with amikacin, imipenem, or meropenem pending species identification and susceptibility testing results.
When sulfonamide hypersensitivity or refractory infection is present, amikacin, a tetracycline (particularly minocycline), imipenem/cilastatin, meropenem, ceftriaxone, cefotaxime, extended-spectrum fluoroquinolones (eg, moxifloxacin), or dapsone can be used. Tigecycline may be an effective alternative. In vitro susceptibility data should guide the choice of alternative drugs.
Localized abscesses typically require incision, drainage, and surgical debridement.
Immunosuppression and chronic pulmonary disease are predisposing factors, but up to 30% of patients have no preexisting disease.
Pneumonia is typical, but skin and central nervous system infections are common; hematogenous spread can involve almost any organ.
Treat with trimethoprim/sulfamethoxazole (or one of the numerous alternatives) for several months.