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INTRODUCTION

Nocardia, a Gram-positive, filamentous bacteria, causes the rare infection known as nocardiosis and belongs to the Nocardia genus in the actinomycetes group. Ubiquitous in nature, Nocardia spp. are found worldwide in soil, water, and vegetation.1 Actinomycosis is an indolent infection caused by anaerobic or microaerophilic bacteria primarily belonging to the genus Actinomyces. Each is discussed in detail below.

NOCARDIOSIS

The clinical spectrum of nocardiosis ranges from localized to disseminated disease in both animal and human hosts. It is an opportunistic pathogen, primarily infecting immunocompromised individuals. New molecular techniques have improved Nocardia spp. identification. Clinical manifestations mainly include cutaneous, pulmonary, and disseminated disease involving the brain. Treatment often requires regimens of multiple antibiotics for a prolonged duration of therapy.

History and Classification

In 1888, Edmon Nocard first isolated an aerobic actinomycetes, later named Nocardia, in cattle afflicted with bovine farcy, a lymphatic disease.2 Two years later, Eppinger described the first human case of Nocardia infection in a man with both “pseudo tuberculous” pulmonary disease and central nervous system infection.3 In 1921, Henrici and Gardner reviewed the world literature for every case of aerobic acid-fast actinomycosis and identified only 26 cases.4 Despite the paucity of cases, they succeeded in describing aspects of nocardiosis that have influenced our current understanding; infection occurs via inhalation resulting in pulmonary disease, the occurrence of disseminated disease (i.e., the brain), and a disease course of approximately 6 months.

Molecular diagnostic techniques identifying key housekeeping genes such as 16S rRNA, gyrB, secA, hsp65, and/or rpoB have allowed a more comprehensive characterization of the taxonomy.5 Consequently, Nocardia spp. have undergone significant taxonomic changes and species reclassification. The Nocardia genus consists of more than 80 species.6 More than 40 species are associated with infections in humans.6 Elucidation of further Nocardia species is ongoing. Commonly identified species responsible for infections in humans are N. asteroides, N. nova, N. farcinica, N. otitidiscaviarum, N. transvalensis, N. brasiliensis, N. brevicatena, N. abscessus, N. cyriacigeorgica, N. veterana, N. paucivorans, N. wallacei, N. blacklockiae, N. elegans, N. concava, and N. cassostreae.7 Pulmonary infection is frequently caused by N. cyriacigeorgica, N. nova complex, and N. farcinica, while Nocardia brasiliensis is responsible for skin and soft tissue infections.8

Environmental Epidemiology

Nocardia spp. are not human commensals but are environmental saprophytes found widely in aquatic, vegetative, and soil-based environments.1 Human infection occurs primarily from inhalation, direct skin/soft tissue inoculation, and rarely ingestion of bacteria. Nocardiosis occurs in men more than women. In the United States the primary route of Nocardia infection is secondary to inhalation. Some authors suggest a higher incidence of pulmonary nocardiosis in geographic regions such as the Southwest United States, ...

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