In the past, several reports have described oyster herpesvirus infections in the American oyster (Crassostrea virginica), the European flat oyster (Ostrea edulis), the Angasi oyster (O. angasi), and the Manila clam (Ruditapes philippinarum).
Consequently, such infections have not always remained limited to Japanese oysters (C. gigas) cultured in France. In fact, oyster herpesvirus has arisen as an important current topic in oyster research and industry circles because this virus is suspected as an agent causing high mortality of C. gigas larvae and 3-12 month juveniles cultured in different locations in France during each summer since 1991. This virus has been designated as oyster herpesvirus-1 (OsHV-1), but it might be said that there has been no positive recognition that this virus is the main causative agent of the C. gigas mass mortality observed in France.
During the summer of 2008, high mortalities of 40-100%, primarily affecting spat and juveniles (12-18 mo old), were observed in all major C. gigas culture areas of France, except in one area at Arcachon in the southwest. According to France’s main marine research institute, Ifremer, the oysters were particularly well fed and spent a lot of energy developing their sexual organs to the detriment of their natural reserves, leaving them vulnerable to OsHV-1. However, scientists do not know why Arcachon oysters have been spared exceptionally.
Regarding the “Summer mortality” in 2008 mentioned above, which shook the French oyster industry, Ifremer scientists characterized the presence of a genotype of OsHV-1 that was not already reported. They designated it as OsHV-1 μVar. This new genomic variant was detected only in C. gigas and was identified as a most probable cause of the summer mortality, suggesting that its virulence is specific for C. gigas. The current outbreak of C. gigas mortality was initially limited to France. However, this species began dying in Ireland in 2009 and in England in 2010. All affected areas had received oyster seed from France. In fact, OsHV-1 μVar was identified in at least in 40% of collected spat batches in 2008 and in 100% of samples in 2009. Ultimately, the culture of C. gigas in France received a crushing blow in 2010.
At least the infection’s expansion to England might have been avoided at least if the research result of Ifremer had been used effectively. Unfortunately, it was not avoided. It is said that the trial of tightening controls over the imports of oyster seed by England was not carried out because of the strong protest of France that such a trial constituted unfair interference in legal business transactions between these two countries. It is arrogance to make a profit simply at the expense of another country. The headquarters of the World Organization for Animal Health (OIE) is located in Paris; nevertheless, OsHV-1 μVar has not been listed as a mollusc disease by OIE, meaning that it is not an issue of concern in the oyster trade. The international trade of oysters infected with it would be prevented if the name of this virulent virus were on the OIE list.
A more severe problem is that the OsHV-1 μVar infection associated with C. gigas summer mortality has now spread throughout Europe to the Southern Hemisphere. This new variant was detected in moribund oysters collected from the North Island, New Zealand during November-December (early summer) 2010, and what is worse, also from Southeast, Australia in January (mid-summer) 2011. For the present, affected areas are localized. However, it is noteworthy that the oyster mortality rate is very high and that the damage extends to C. gigas of market size as well as juveniles. Marketable oysters produced in Australia are widely exported to Asia. Many live and fresh C. gigas imported from Australia arrive at oyster bars and restaurants in various parts of Japan. Until they are brought into these places after their importation, they are often stocked in tanks at the seaside. What do these facts suggest? The probable expansion of OsHV-1 μVar.
The C. gigas that are threatened at present are descendants of oyster seeds exported from Miyagi Prefecture, Japan to France in large quantities. Therefore, brand-new research themes have been presented for oyster researchers. One such theme is related to the susceptibility of C. gigas that are currently cultured in Japan to OsHV-1 μVar, which has emerged in France, where its original seed was first transplanted from Japan.
To protect the oyster industry in Japan from devastating mortality caused by OsHV-1 μVar necessitates preventive measures against its invasion into Japan. I strongly urge and expect that the responsible authorities such as the Fisheries Agency of Japan will begin to take necessary actions to stop its invasion as soon as possible.
(February, 2011)