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Concomitant sporadic high mortalities were reported in France in May 1994 among batches of hatchery-reared larval Pacific oysters Crassostrea gigas and European flat oysters Ostrea edulis in 2 hatcheries, and in June and July 1994 among batches of cultured spat of both species in a shellfish nursery. Histological observation showed the presence of cellular abnormalities in moribund animals. Transmission electron microscopy revealed the presence of herpes-like virus particles in infected larvae and spat of both oyster species. This is the first description of a herpes-like virus infection in larval O. edulis. Viruses observed in diseased larvae and spat of both species are similar with respect to ultrastructure and morphogenesis. They were detected simultaneously in C. gigas and O. edulis larvae and spat, indicating possible interspecific transmission. Moreover, these viruses are associated with high mortality rates in both oyster species. An electron microscopic examination revealed hemocytes with condensed chromatin and extensive perinuclear fragmentation of chromatin. These data suggest that herpes-like viruses infecting oysters may induce apoptosis in oyster hemocytes. KEY WORDS: Herpes-like virus · European flat oyster · Ostrea edulis · Pacific oyster · Crassostrea gigas · Oyster mortality · Concomitant infection · Virus replication · Apoptosis Resale or republication not permitted without written consent of the publisherDis Aquat Org 42: [173][174][175][176][177][178][179][180][181][182][183] 2000 viral particles reported by Hine et al. (1992) in New Zealand and the virus-like particles described by Comps & Cochennec (1993) among French cultured European flat oyster spat.We describe for the first time a herpes-like virus infecting European flat oyster larvae and concomitant herpes-like virus infections associated with high mortalities among hatchery-reared larvae and nurserycultured spat of both Crassostrea gigas and Ostrea edulis during the summer of 1994 in France. We performed an ultrastructural comparative study between both virus-like particles found respectively in Pacific oysters and European flat oysters and compared them to viruses belonging to the Herpesviridae family. MATERIALS AND METHODS Specimens.Larval Ostrea edulis and Crassostrea gigas, 6 to 10 d old, 80 to 150 µm in size, were collected in May 1994 from 2 hatcheries located in Bourgneuf Bay (Vendée, France). Nursery-cultured spat of C. gigas and O. edulis, 3 to 5 mo old, were also collected from Bourgneuf Bay in June and July 1994.Light microscopy. A total of 197 European flat oyster spat and 196 Pacific oyster spat were collected for light microscopical examination. Moribund cultured spat of both species, 3 to 5 mo old, were removed from the shell and, sagittally sectioned; then half of each specimen was fixed in Davidson's fixative for light microscopic examination and the other half in Carson's fixative for transmission electron microscopic analysis. After 48 h fixation in Davidson's fluid, samples were dehydrated using an ...

Two cases of haplosporidian infection occurred during 1993 in Pacific oystersCrassostrea gigas from the French Atlantic coast. The localization and ultrastructure of the plasmodia are described. In situ hybridization of infected tissue sections was conducted with DNA probes for oysterinfecting haplosporidians. The Haplosporidium nelsoni-specific DNA probe MSX1347 hybridized with the C. gigas parasite, and the H. costale-specific probe SSO1318 did not hybridize. Total genomic DNA was extracted from the infected tissue sections for polymerase chain reaction (PCR) amplification of the haplosporidian. PCR amplifications with H. nelsoni-specific primers and with 'universal' actin primers did not yield the expected products of 573 and 700 bp, respectively. A series of primers was designed to amplify short regions of small subunit ribosomal DNA (SSU rDNA) from most haplosporidians. The primers encompass a highly variable region of the SSU rDNA and did not amplify oyster DNA. PCR amplification of the infected C. gigas genomic DNA with these primers yielded the expected-sized product from the primer pair targeting the shortest region (94 bp). This PCR product was sequenced and it was identical to the corresponding SSU rDNA region of H. nelsoni. KEY WORDS: Pacific oyster · Crassostrea gigas · Haplosporidiosis · Haplosporidium nelsoniResale or republication not permitted without written consent of the publisher

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