Nocardiosis in chinook salmon

Comparative Pathology of Nocardiosis in Marine Mammals

Nocardia spp. infections in mammals cause pyogranulomatous lesions in a variety of organs, most typically the lung. Members of the Nocardia asteroides complex are the most frequently recognized pathogens. Nine cases of nocardiosis in free-ranging pinnipeds and 10 cases of nocardiosis in cetaceans were evaluated. Host species included the hooded seal ( Cystophora cristata, n = 8), leopard seal ( Hydrurga leptonyx, n = 1), Atlantic bottlenose dolphin ( Tursiops truncatus, n = 4), beluga whale ( Delphinapterus leucas, n = 4), and killer whale ( Orcinus orca, n = 2). The most common presentation of nocardiosis in both pinnipeds and cetaceans was the systemic form, involving 2 or more organs. Organs most frequently affected were lung and thoracic lymph nodes in 7 of 9 cases in pinnipeds and 8 of 10 cases in cetaceans. Molecular identification and bacterial isolation demonstrated a variety of pathogenic species. N. asteroides, N. farcinica, N. brasiliensis, and N. otitisdiscaviarum are pathogenic for pinnipeds. In cetaceans N. asteroides, N. farcinica, N. brasiliensis, N. cyriacigeorgica, and N. levis are pathogenic. Hematoxylin and eosin and acid fast staining failed to reveal bacteria in every case, whereas modified acid fast and Grocott's methenamine silver consistently demonstrated the characteristic organisms. In both pinnipeds and cetaceans, juvenile animals were affected more often than adults. Hooded seals demonstrated more cases of nocardiosis than other pinnipeds.

Nocardia species: host-parasite relationships

The nocardiae are bacteria belonging to the aerobic actinomycetes. They are an important part of the normal soil microflora worldwide. The type species, Nocardia asteroides, and N. brasiliensis, N. farcinica, N. otitidiscaviarum, N. nova, and N. transvalensis cause a variety of diseases in both normal and immunocompromised humans and animals. The mechanisms of pathogenesis are complex, not fully understood, and include the capacity to evade or neutralize the myriad microbicidal activities of the host. The relative virulence of N. asteroides correlates with the ability to inhibit phagosome-lysosome fusion in phagocytes; to neutralize phagosomal acidification; to detoxify the microbicidal products of oxidative metabolism; to modify phagocyte function; to grow within phagocytic cells; and to attach to, penetrate, and grow within host cells. Both activated macrophages and immunologically specific T lymphocytes constitute the major mechanisms for host resistance to nocardial infection, whereas B lymphocytes and humoral immunity do not appear to be as important in protecting the host. Thus, the nocardiae are facultative intracellular pathogens that can persist within the host, probably in a cryptic form (L-form), for life. Silent invasion of brain cells by some Nocardia strains can induce neurodegeneration in experimental animals; however, the role of nocardiae in neurodegenerative diseases in humans needs to be investigated.

Nocardia in naturally acquired and experimental infections in animals

There are three commonly recognized species ofNocardiathat cause disease in a large variety of animals including humans. In the United States, pulmonary or systemic nocardiosis in humans caused byN. asteroidesis most frequently diagnosed. It should be noted thatN. brasiliensiscan cause nocardiosis also. In Central and South America mycetomas induced byN. brasiliensisappear to be more prevalent even thoughN. asteroidescan be seen in this type of infection. Sporadic cases of both mycetoma and nocardiosis caused byN. caviaehave been reported. These three species ofNocardiaappear to be present in the soils of most countries; butN. asteroidesis more frequently isolated in the temperate climates whereasN. brasiliensispredominates in the tropical and subtropical regions of the world. No specific geographic distribution has been noted withN. caviae. Infections involving these three species of bacteria are, therefore, worldwide in occurrence. Even though nocardial infections in both humans and animals were recognized as early as 1888, traditionally, they have been considered rare in frequency as compared to many other infectious diseases. This is a perception that probably is not accurate. During the past twenty years, greater diagnostic awareness combined with more intensive medical and veterinary surveillance have resulted in significantly increased recognition of disease caused by these aerobic actinomycetes.