Rickettsia-, Chlamydia- and Mycoplasma-like organisms
Bacteria belonging to the orders Rickettsiales, Chlamydiales and Mycoplasmatales (class Mollicutes) infect a wide range of bivalves and crustaceans (Table 20.2). With certain exceptions, described below, Rickettsiaceae are Gram-negative, intracytoplasmic, membrane-bound, oval to rod-shaped bacilli, measuring 0.3-0.6 mm x 0.8-2.0 mm. Chlamydiales are also Gram-negative, but have a complex developmental cycle, multiplying within cytoplasmic vacuoles and producing small, rigid-walled, infectious stages (elementary bodies). Larger, non-infectious forms (reticulate or initial bodies) have flexible walls and divide by fission, producing intermediate or ellipsoid bodies. Chlamydias are coccoid or pleomorphic and generally measure 0.2-1.5 mm in diameter. Mycoplasma-like organisms (MLOs), are tentatively associated with the Mollicutes as small (0.3-0.8 mm), wall-less, prokaryotes, which vary in shape from spherical to filamentous.
Generally rickettsias, chlamydias and MLOs cause negligible tissue pathology (Harshbarger et al., 1977; Comps et al., 1979; Meyers, 1979b; Comps, 1982, 1983; Lauckner, 1983; Elston, 1986a) (Fig. 20.10). Some significant exceptions, however, have been reported (Elston, 1986b; Olson and Pierce, 1988; Leibovitz, 1989; Bower and Meyer, 1991). Gill rickettsias of adult giant sea scallop (Placopecten magellanicus) in New England were associated with myodegeneration and mass mortalities (Gulka et al., 1983; Gulka and Chang, 1984a,b); however, similar infections in other P. magellanicus have shown no association with overt pathology or mortalities (Morrison and Shum, 1983; McGladdery, 1990; Getchell, 1991; Karlsson, 1991; McGladdery et al,
1993). Rickettsia-like organisms (RLOs) in the gills of adult Saint-Jacques scallops (coquille Saint-Jacques) (Pecten maximus) in France were also linked to mortalities (Le Gall et al., 1988, 1991). Infected gill epithelia showed cytoplasmic degeneration, but no myodegeneration. In contrast, RLOs in the gills of Japanese scallops (Patinopecten yessoenesis) are not associated with pathology (Elston, 1986b). Likewise, infections of various clam species have seldom been linked to overt pathology. One exception is recent high mortalities of 6-9-month-old cultured Hippopus hippopus (giant clam) in the Philippines and Malaysia, which were associated with heavy gill infections by RLOs (Norton et al., 1993). The precise factors triggering pathogenic infections are unknown, but, as with other bacterial infections of shellfish, adverse environmental conditions (overcrowding, limited water exchange) and physiological stress (transportation and handling) are believed to play a significant role (Morrison and Shum, 1983; Leibovitz, 1989).
Nuclear inclusion X (NIX) causes sporadic late summer and early autumn mortalities in Pacific razor clams (Siliqua patula) from the north-western USA and British Columbia (Elston, 1986a; Olson and Pierce, 1988). Nuclear inclusion X is unusual in that, unlike most rickettsias, infections of the gill epithelia are intranucleic rather than cytoplasmic. The only other endonucleic bacterium in shellfish described to date occurs in the gill epithelium of Tapes decussatus (Azevedo, 1989). Also, NIX is abnormally large (16 mm x 25 mm) for an intracellular prokaryote, and the hypertrophied host cells are visible to the naked eye as nodules on the gill filaments.
Larval bivalves are particularly susceptible to lethal infections by intracellular prokaryotes (Leibovitz, 1989; Bower and Meyer, 1991). The first epizootic attributed to chlamydial infection was in larval bay scallop (Argopecten irradians) at two eastern US shellfish hatcheries (Leibovitz, 1989). Basophilic inclusion bodies were found in disrupted digestive duct and tubule epithelia. Similar inclusion bodies infect adult bay scallops, but are not associated with disease (Morrison and Shum, 1982, 1983; Getchell, 1991; Karlsson, 1991). High stocking densities and water temperatures (> 20°C) may enhance microbial proliferation when larval development and metamorphosis render them vulnerable to opportunistic infections.
'Intracellular bacterium disease' produces pinkish-orange pustules in the adductor muscle of young Japanese scallops (P. yessoensis) and is associated with poor growth and high mortalities at several grow-out sites in British Columbia (Bower and Meyer, 1991). Experimental inoculations with high concentrations of lesion homogenate caused mortalities 2-4 weeks postinjection. Lower concentrations resulted in a 2-month prepatent period and development of adductor muscle lesions. Intracellular prokaryotes, tentatively identified as Mycoplasma-like, were found in some of the haemocytes within the abscess lesions (Bower and Meyer, 1991; Bower et al., 1994a) (Fig. 20.11). Another MLO was described from cockles (Cerastoderma edule) in central Portugal (Azevedo, 1993), which were suffering high summer mortalities. Infected gill epithelia demonstrated pyknotic nuclei, reduced mitochondria and no ribosomes. The MLOs have a trilaminar membrane, measure 0.5-4.0 mm x 0.9 mm, lack a cell wall and are located in the cytoplasmic cisternae of the basal portion of infected cells. Attempts to transmit the infection under laboratory conditions failed and the pathogenic significance in wild cockle populations is unknown (Azevedo, 1993).
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