Branchial rickettsia-like infection associated with clam Venerupis rhomboides mortality

A histopathological survey was performed to search for the cause of high mortality of the clam Venerupis rhomboides (Pennant) in exploited beds of the Ensenada de Riveira (Ria de Arousa, Galicia, NW Spain). V. rhomboides from 2 beds affected by high mortality, Airos and Coroso, and a 3rd non-affected bed, Agudos, were sampled in spring and autumn of 1996. In addition, clams of the species Venerupis pullastra, with unnoticeable mortality, were taken from Airos during autumn sampling. According to prevalence, infection intensity and associated histopathological signs, a branchial rickettsia-like organism was the only pathogen that could be tentatively blamed for the mortality. Spherical to elongated intracytoplasmic rickettsia-like colonies up to 25 mu m in length were observed at the base of gill filaments of the clams. Transmission electron microscopy study permitted identification of the micro-organisms in the colonies as rickettsia-like. Individual prokaryotes measured about 0.5 to 0.8 mu m in diameter and up to 3 mu m in length. The infection process resulted in extreme hypertrophy and lysis of host epithelial cells. infection intensity was rated for each clam and comparison among high mortality-affected and non-affected populations indicated the branchial rickettsia-like infection as the probable cause of the high mortality.