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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.1 Members
of the family Nocardiaceae reproduce by fragmenting into bacillary
and coccoid elements but are distinguished by filamentous growth
with true branching.2 These organisms appear as
gram-positive branching filaments and are weakly acid-fast.2Nocardia species
grow readily on simple media as pigmented colonies due to rudimentary
aerial mycelia.
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Epidemiology
and Pathophysiology
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The genus Nocardia contains at least 50 species
that were characterized by phenotypic and molecular methods.1Nocardia
asteroides is the most common species that causes human disease.
Other species that are associated with human disease include N
brasiliensis, N farcinica, N otitidiscaviarum, N nova, and N transvalensis.3Nocardia species
can be skin contaminants and respiratory tract saprophytes. They are
ubiquitous in the environment and can be found worldwide as saprophytes in
soil, water, and decaying organic material, including vegetation
and animal deposits.4 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.5N
brasiliensis is most frequently found in tropical and subtropical
areas. In the United States, N brasiliensis is
prevalent in the southeastern and southwestern states.6 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.7
Human beings acquire pulmonary infections by inhaling contaminated
dust particles from environmental sources, whereas traumatic inoculation
through the skin is responsible for subcutaneous disease.5 There
is no evidence for airborne, animal-to-person or person-to-person
spread. Nosocomial outbreaks have been linked to environmental sources.8 Dust
contaminating air systems in the operating rooms or the transplant
units has been suggested as the source.9 About
15% of all patients with nocardiosis are children.
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The immune response to Nocardia is both humoral
and cellular.3,10 The neutrophils inhibit the spread
of infection; however, the cell-mediated immune response is vital
in preventing dissemination.11Nocardia may persist
in neutrophils and macrophages by production of enzymes that inactivate
the myeloperoxidase system.12 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.
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Clinical Manifestations
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Nocardia can cause both localized and disseminated
infections in humans. Although typically considered as an opportunistic
infection, Nocardia...