Parasitism by the protozoan Perkinsus atlanticus favours the development of opportunistic infections

It has been suggested that opportunistic pathogens could contribute to the mortality of Perkinsus atlanticus-infected clams. Examination of Tapes semi decussatus clams from the northern Mediterranean coast of Spain revealed th at while 86% of the clams heavily infected with P. atlanticus were co-infec ted by bacteria and/or viruses, neither non-infected nor lightly P. atlanti cus-infected specimens had bacterial or viral infections, The bacteria, whi ch had a Gram-negative cell wall, were always located in the apical pole of gill epithelial cells and enclosed within membranous compartments. Bacteri a-containing cells were hypertrophied and showed dysplasia with loss of cil ia and microvilli. The viruses shared ultrastructural, morphologic and cyto pathic characteristics of a polyomavirus. Viral particles with icosahedral symmetry were found in both the cytoplasm and the nucleus of numerous cell types. Virus-infected cells showed severe alterations, including hypertroph y, reduction of the intracellular compartments and extrusion of the nuclear envelope. Moreover, gill epithelial cells showed disorganization and swell ing of the apical region, which affected the ciliary structure. Our finding s show that P. atlanticus parasitism favours the development of opportunist ic infections which have detrimental effects in this clam population.