Striped bass Morone saxatilis were infected intraperitoneally with approximately 10 5 Mycobacterium marinum, M. shottsii sp. nov., or M. gordonae. Infected fish were maintained in a flow-through freshwater system at 18 to 21°C, and were examined histologically and bacteriologically at 2, 4, 6, 8, 17, 26, 36 and 45 wk post-infection (p.i.). M. marinum caused acute peritonitis, followed by extensive granuloma development in the mesenteries, spleen and anterior kidney. Granulomas in these tissues underwent a temporal progression of distinct morphological stages, culminating in well-circumscribed lesions surrounded by normal or healing tissue. Mycobacteria were cultured in high numbers from splenic tissue at all times p.i. Standard Ziehl-Neelsen staining, however, did not demonstrate acid-fast rods in most early inflammatory foci and granulomas. Large numbers of acid-fast rods were present in granulomas beginning at 8 wk p.i. Between 26 and 45 wk p.i., reactivation of disease was observed in some fish, with disintegration of granulomas, renewed inflammation, and elevated splenic bacterial densities approaching 10 9 colony-forming units g -1 . Infection with M. shottsii or M. gordonae did not produce severe pathology. Mild peritonitis was followed by granuloma formation in the mesenteries, but, with 1 exception, granulomas were not observed in the spleen or anterior kidney. M. shottsii and M. gordonae both established persistent infections in the spleen, but were present at densities at least 2 orders of magnitude less than M. marinum at all time points observed. Granulomas in the mesenteries of M. shottsii-and M. gordonaeinfected fish resolved over time, and no reactivation of disease was observed. KEY WORDS: Mycobacteriosis · Striped bass · Morone saxatilis · Mycobacterium marinum · Mycobacterium gordonae · Mycobacterium shottsii · Granuloma Resale or republication not permitted without written consent of the publisherDis Aquat Org 54: [105][106][107][108][109][110][111][112][113][114][115][116][117] 2003 respectively (Sakanari et al. 1983, Hedrick et al. 1987. Mycobacterium marinum was cultured from fish in the latter study, and has also been isolated from wild Pacific striped bass (Landsell et al. 1993). Recently, visceral and dermal lesions in striped bass from Chesapeake Bay and its tributaries were shown to be associated with mycobacterial infection (Vogelbein et al. 1999). An epizootiological study of striped bass from Virginia tributaries of Chesapeake Bay found up to 62.7% prevalence of mycobacteriosis, based on histological presence of characteristic granulomas. This suggests the disease has significant effects on wild stocks (Cardinal 2001).In addition to Mycobacterium marinum and several other Mycobacterium spp., 2 recently described species of mycobacteria, 'M. chesapeaki ' (Heckert et al. 2001) and M. shottsii (Rhodes et al. 2001(Rhodes et al. , 2003 in press), have been isolated from Chesapeake Bay striped bass. Mycobacteria were cultured from splenic tissue of approximately 76% of stripe...

An emerging epizootic of mycobacteriosis currently threatens striped bass Morone saxatilis populations in Chesapeake Bay, USA. Several species of mycobacteria, including Mycobacterium marinum, species resembling M. avium, M. gordonae, M. peregrinum, M. scrofulaceum and M. terrae, and the new species M. shottsii have been isolated from diseased and healthy bass. In this study, we describe the ultrastructure of developing M. marinum granulomas in experimentally infected bass over a period of 45 wk. The primary host response to injected mycobacteria was formation of large macrophage aggregations containing phagocytosed bacilli. M. marinum were always contained within phagosomes. Close association of lysosomes with mycobacterial phagosomes, as well as the presence of electron-opaque material within phagosomes, suggested phagolysosomal fusion. Development of granulomas involved epithelioid transformation of macrophages, followed by appearance of central necrosis. Desmosomes were present between mature epithelioid cells. The necrotic core region of M. marinum granulomas was separated from overlying epithelioid cells by several layers of flattened, electron-opaque spindle-shaped cells. These cells appeared to be formed by compression of epithelioid cells and, aside from a flattened nucleus, did not possess recognizable organelles. Following the development of well-defined, paucibacillary granulomas, secondary disease was observed. Recrudescence was marked by bacterial replication followed by disruption of granuloma architecture, including loss of epithelioid and spindle cell layers. In advanced recrudescent lesions, normal tissue was replaced by macrophages, fibroblasts, and other inflammatory leukocytes. Large numbers of mycobacteria were observed, both intracellular and suspended in cellular debris. KEY WORDS: Mycobacteriosis · Striped bass · Morone saxatilis · Mycobacterium marinum · Granuloma · Ultrastructure Resale or republication not permitted without written consent of the publisherDis Aquat Org 62: [121][122][123][124][125][126][127][128][129][130][131][132] 2004 body granulomas are composed of a solid mass of epithelioid cells and giant cells, with the eliciting agent often visible in the center. The other major granuloma type, the immune granuloma, is formed in response to insoluble particles that stimulate a cell-mediated immune response, such as mycobacteria. Cytokines produced by T cells, such as IL-12 and interferon-γ, play an important role in formation and maintenance of immune granulomas (Cooper et al. 1993, Ehlers et al. 2000. A prominent feature of immune granulomas produced by mycobacteria such as Mycobacterium tuberculosis is caseous necrosis in the lesion core, which produces a highly acidic, anoxic environment that may serve to degrade otherwise refractory bacilli.The structure and development of mycobacterial granulomas in mammals, especially those produced in response to Mycobacterium tuberculosis and BCG (attenuated M. bovis), have been described (Papadimitriou & Spector 1972, Ad...