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Viral gametocytic hypertrophy (VGH) was detected during an investigation of mortalities in Pacific oysters Crassostrea gigas from 2 separate Irish production sites. The basophilic inclusions were observed in the gonad tissue of oysters sampled in August and October 2007. The oysters involved did not show any macroscopic disease signs. Transmission electron microscopy demonstrated the presence of viral particles in these intranuclear inclusions. The particles were small, nonenveloped, icosahedral and approximately 50 nm in diameter, and thus had characteristics similar to the Papillomaviridae and Polyomaviridae families. No host defence reaction was observed. The viral particles described here appear to be similar to those described in C. virginica from the USA and Canada and to those described in C. gigas from Korea and France.KEY WORDS: Crassostrea gigas · Viral gametocytic hypertrophy · Papillomaviridae · Polyomaviridae · Pacific oyster · Gonad Resale or republication not permitted without written consent of the publisherDis Aquat Org 83: [181][182][183][184][185] 2009 The actual impact of papilloma-like and polyomalike viruses on their hosts has not been fully assessed (Garcia et al. 2006). Neither is it clearly understood whether one or more viruses are involved in these gonad conditions. In 2007, we observed VGH in Crassostrea gigas gonad tissue sampled from 2 separate production sites in Ireland. We reprocessed the waxembedded oyster gonad tissue for electron microscopy and describe the ultrastructure of the viral particles observed in these infected oysters. MATERIALS AND METHODSFrom August to October 2007, following reports of increased levels of mortalities, a total of 77 marketsized Crassostrea gigas were collected from 2 separate production sites in Ireland: Site A (County Kerry) and Site B (County Donegal) (Fig. 1).Histological examination. Oyster tissue fixed in 10% v/v formalin solution was processed for routine histology. Sections were cut at 2 µm and stained with haematoxylin and eosin (H&E).Ultrastructural examination. When inclusion bodies were observed during light microscopy, the waxembedded oyster tissue containing the inclusion was reprocessed for transmission electron microscopy (TEM) as follows. With the H&E-stained section as a visual guide, the portion of wax-embedded tissue with the inclusion was removed with a scalpel from the wax block and dewaxed overnight in 2 changes of xylene with agitation. Following rehydration, the tissue was then placed in 3% glutaraldehyde in 0.1 M cacodylate buffer (pH 7.4) for 2 to 5 h, rinsed again in 0.1 M cacodylate buffer and finally post-fixed in 1% OsO 4 for 2 h. After dehydration through graded alcohols, the tissues were infiltrated with a 1:1 solution of Agar low viscosity resin and 50% ethanol with agitation for 1 h, followed by 100% resin for 2 h minimum. Tissues were embedded in resin and cured at 60°C for 2 to 3 d. Semithin sections were stained with 1% toluidine blue and ultra-thin sections were stained with uranyl acetate and lea...

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