Characterization of intracytoplasmic prokaryote infections in Dreissena sp(Bivalvia : Dreissenidae)

This study characterizes intracytoplasmic infections with prokaryote microo rganisms in Dreissena sp. (near Dreissena polymorpha) from northeastern Gre ece and represents the first report of such infections in freshwater bivalv es. Light microscope observations of stained tissues revealed basophilic, c ytoplasmic inclusion bodies in 87.5% (28/32) of the mussels sectioned. Incl usions in epithelial cells and connective tissues were noted, respectively, in 34.4 and 71.9% of the sample, with 5 mussels (15.6%) having both tissue types infected. Epithelial cell infections were observed in histological s ections only in digestive gland tubules and ducts; within tubules, inclusio ns were present more often in secretory than digestive cells. Connective ti ssue infections, however, were systemic; among the 32 mussels sectioned, in clusions were found in the gills (65.6%), foot (12.5%), mantle (9.4%), labi al palps (6.3%), digestive gland (6.3%), stomach (6.3%), and gonads (3.1%). Cytoplasmic inclusions (maximum dimension, 138 mum) were prominent enough in the gills to be visible in 17.0% of the 247 mussels dissected. Ultrastru cturally, prokaryote cells in gill connective tissues were clearly characte ristic of Chlamydiales-like organisms, with each intracytoplasmic inclusion containing a loosely packed mixture of elementary, reticulate, intermediat e bodies, and blebs. Prokaryote colonies in digestive gland epithelial cell s exclusively contained 1 of 4 morphological cell types and were considered Rickettsiales-like. Hexagonal, virus-like particles were present in the cy toplasm of the largest of these Rickettsiales-like prokaryotes. Although ho st stress was evident from localized cell necrosis and dense hemocyte infil tration, overall infection was fairly benign, with no major, adverse impact on body condition evident among sectioned or dissected mussels. A possible negative effect was partial constriction of gill water tubes, but at the i nfection intensity observed (typical range 1 to 7 inclusion bodies per sect ion), significant interference with respiration and other metabolic functio ns of the gills was highly unlikely.