Viral gametocytic hypertrophy of Crassostrea gigas: an emerging disease in France?

Viral gametocytic hypertrophy has been reported for the first time in 2001 in Pacific oyster Crassostrea gigas in France. Male and female gametes were hypertrophied, and basophilic inclusions were observed in gamete nuclei. Electron microscopy revealed the presence of viral particles with characteristics similar to those of families Papillomaviridae and Polyomaviridae. Viruses were found in male, female and hermaphrodite oysters, and no significant difference in viral infection was observed between the number of infected males and females. The virus seems to be similar to those described in Crassostrea virginica in the U.S.A. and Canada and in Crassostrea gigas in Korea, but further studies are required to confirm their precise identity. The frequency of detection (3.3 to 13.3%) and the intensity of infection were low, and no host defence reaction was recognised, suggesting that the virus has little impact if any on Crassostrea gigas. Meanwhile, since 2001, the number of reported cases has increased every year as well as its distribution area; however, the disease was not associated with particular gross sign or increased mortality. Therefore, the question of viral gametocytic hypertrophy as emergent disease or cryptic endemic disease according to the data series could be discussed.