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Nocardia species are saprophytic aerobic actinomycetes and are common worldwide in soil, where they contribute to the decay of organic matter. More than 50 species have been identified, mostly on the basis of 16S rRNA gene sequences. More than 30 species have been associated with human disease. Until recently, isolates from the majority of cases of pneumonia and systemic disease were identified as Nocardia asteroides, but human disease involving N. asteroides proper is actually rare. Nocardiae are relatively inactive in standard biochemical tests, and speciation is difficult or impossible without molecular phylogenetic techniques. Most clinical laboratories cannot speciate isolates accurately and may identify them simply as N. asteroides or Nocardia species.

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Nine species or species complexes are most commonly associated with human disease (Table 162-1). Most systemic disease involves N. cyriacigeorgica, N. farcinica, N. pseudobrasiliensis, and species in the N. transvalensis and N. nova complexes. N. brasiliensis is usually associated with disease limited to the skin. Actinomycetoma—an indolent, slowly progressive disease of skin and underlying tissues with nodular swellings and draining sinuses—is often associated with N. brasiliensis, N. otitidiscaviarum, N. transvalensis complex strains, or other actinomycetes.

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Table 162-1 Nocardia Species Most Commonly Associated with Human Disease and Their In Vitro Susceptibility Patterns
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Epidemiology

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Nocardiosis occurs worldwide. The annual incidence has been estimated on three continents (North America, Europe, and Australia) and is ∼0.375 cases per 100,000 persons. The disease is more common among adults than among children and among males than among females. Nearly all cases are sporadic, but outbreaks have been associated with contamination of the hospital environment, solutions, or drug injection ...

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