Branchial rickettsia-like infection associated with clam Venerupis rhomboides mortality

A histopathological survey was performed to search for the cause of high mo rtality 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-affecte d bed, Agudos, were sampled in spring and autumn of 1996. In addition, clam s of the species Venerupis pullastra, with unnoticeable mortality, were tak en from Airos during autumn sampling. According to prevalence, infection in tensity and associated histopathological signs, a branchial rickettsia-like organism was the only pathogen that could be tentatively blamed for the mo rtality. Spherical to elongated intracytoplasmic rickettsia-like colonies u p to 25 mu m in length were observed at the base of gill filaments of the c lams. Transmission electron microscopy study permitted identification of th e micro-organisms in the colonies as rickettsia-like. Individual prokaryote s measured about 0.5 to 0.8 mu m in diameter and up to 3 mu m in length. Th e infection process resulted in extreme hypertrophy and lysis of host epith elial cells. infection intensity was rated for each clam and comparison amo ng high mortality-affected and non-affected populations indicated the branc hial rickettsia-like infection as the probable cause of the high mortality.