Nocardia

Occurrence. The genus Nocardia includes species with morphology similar to that of the actinomycetes, differing from them in that the natural habitat of these obligate aerobes is the soil and damp biotopes. The pathogens known for involvement in nocardioses, a generally very rare type of infection, include N. asteroides, N. brasiliensis, N.farcinia, N. nova, and N. otitidiscaviarum.

Morphology and culture. Nocardia are Gram-positive, fine, pleomorphic rods that sometimes show branching. They can be cultured on standard nutrient mediums and proliferate particularly well at 30 °C. Nocardia are obligate aerobes.

Pathogenesis and clinical picture. Nocardia penetrate from the environment into the macroorganism via the respiratory tract or dermal wounds. An infection develops only in patients with predisposing primary diseases directly

affecting the immune defenses. Monoinfections are the rule. There are no typical clinical symptoms. Most cases of infection involve pyogenic inflammations with central necroses. The following types have been described: pulmonary nocardioses (bronchial pneumonia, pulmonary abscess), systemic nocardioses (sepsis, cerebral abscess, abscesses in the kidneys and musculature), and surface nocardioses (cutaneous and subcutaneous abscesses, lymphocutaneous syndrome).

Actinomycetomas are tumorlike processes affecting the extremities, including bone. An example of such an infection is Madura foot, caused by Nocardia species, the related species Actinomadura madurae, and Streptomyces soma- liensis. Fungi (p. 355) can also be a causal factor in this clinical picture.

Diagnosis. Detection of the pathogen by means of microscopy and culturing techniques is required in materials varying with the specific disease. Due to the long generation time of these species, cultures have to be incubated for at least one week. Precise identification to differentiate pathogenic and apatho- genic species is desirable, but difficult.

Therapy. The anti-infective agents of choice are sulfonamides and cotrimox- azole. Surgery may be required.

Epidemiology and prevention. Nocardioses are rare infections. Annual incidence levels range from about 0.5 to 1 case per 1 000 000 inhabitants. The pathogens, which are present in the natural environment, are carried by dust to susceptible patients. There are no practicable prophylactic measures.