Nocardiosis is an uncommon gram-positive infection with protean clinical manifestations. It is caused by a soilborne aerobic actinomycete of the family Nocardiaceae and is acquired mostly through the respiratory tract. Members of the family Nocardiaceae reproduce by fragmenting into bacillary and coccoid elements but are distinguished by filamentous growth with true branching. These organisms appear as gram-positive branching filaments and are weakly acid-fast. Nocardia species grow readily on simple media as pigmented colonies due to rudimentary aerial mycelia.
PATHOGENESIS AND EPIDEMIOLOGY
The immune response to Nocardia is both humoral and cellular. The neutrophils inhibit the spread of infection; however, the cell-mediated immune response is vital in preventing dissemination. Killing of the organism occurs via cytotoxic T lymphocytes and activated macrophages. Nocardia may persist in neutrophils and macrophages by production of enzymes that inactivate the myeloperoxidase system. Nocardiosis produces suppurative necrosis and abscess formation typical of pyogenic infection. In contrast to the pronounced tissue fibrosis seen in actinomycosis, nocardiosis seldom provokes more than a loose wall of granulation tissue. This absence of encapsulation accounts for the tendency of this organism to disseminate from its initial pulmonary focus. Sulfur granules are not formed by this organism except in the skin in the lymphocutaneous or mycetoma syndromes.
The genus Nocardia contains at least 80 species that were characterized by molecular methods. Species from the Nocardia asteroides complex (N cyriacigeorgica, N farcinica, N abscessus, and N nova) are the most common cause of human disease in the United States. Other species that are associated with human disease that belong to the Nocardia genus but are not identified as part of the N asteroides complex include N brasiliensis and N otitidiscaviarum complex. Nocardia species can be skin contaminants and respiratory tract saprophytes. They are ubiquitous in the environment and can be found worldwide as saprophytes in soil, fresh and salt water, and decaying organic material, including vegetation and animal deposits. Some species may be more prevalent than others in certain geographic areas with specific climates. Infections with Nocardia appear to be more prevalent in the southwestern United States. Possible explanations include the facilitation and dispersal of Nocardia by the dusty, dry, and windy conditions in these areas. N brasiliensis is most frequently found in tropical and subtropical areas. In the United States, N brasiliensis is prevalent in the southeastern and southwestern states. Most systemic diseases in humans are caused by N asteroides. N farcinica has been reported to cause more severe disease and disseminated infections than other species. N farcinica and N otitidiscaviarum are increasingly reported in invasive infections and may have clinical implication due to their antibiotic resistance. Human beings acquire pulmonary infections by inhaling contaminated dust particles from environmental sources, whereas traumatic inoculation through the skin is responsible for subcutaneous disease. There is no evidence for airborne, animal-to-person, or person-to-person spread. Nosocomial outbreaks have been linked to environmental sources. Dust ...