Contribution à l'étude de virus de mollusques marins apparentés ...

HERPESVIRUSES ASSOCIATED WITH MORTALITIES AMONG PACIFIC OYSTER, IN FRANCE
Host species Size References
Capsids Enveloped virions
C. virginiea 70/90 nm 1 161
C. gigas 70±2.Dnm 90 ± 5.0 nm 1121
C. gigas 97 ± 4.0 nm 131 ± 9.0 nm 1101
O. edulis BOnm 160/1BOnm 141
C. gigas (lafVae) 75 ± 3.0 nm 122 ± 6.0 nm ln this report
C. gigas (young spats) 75 ± 3.0 nm 121 ± 7.0"m ln thi5 report
TABLE IV. - Morphological characleristics of herpcs-like viruses observed among bivalve molluscs.
Indeed, it seems that the herpes-Iike viruses detected
among bivalve molluscs are not a real danger for animaIs
cultured at low temperatures Iike the observations performed
by FARLEY el al. [10] suggest. Thus, since 1970, no
catastrophic mortalities associated with herpes-like virus
detection were reported among the species C. virginica.
However it seems necessary to perform regular controls
for a survey of the evolution of these virus infections
among C. gigas oysters.
The evolution of the disease in hatchery-reared larvae
with the flfst signs visible three to four days after the spond
seems to point out that contamination of larvae occurs
quite carly, and may result of a vertical transmission.
Among the batches of 3-7 month old young oysters, the
mortalities appcared suddenly and the course of the disease
was very brief in time, only a week. These reports suggest
a high pathogenicity of this herpes-Iike virus, with a great
infectivity in a same batch. But in the case of young spat
oysters, the origin of virus contamination is uncJear.
Moreover, the presence of two similar viruses among C.
gigas larvae and C. gigas young oyster spat may indicate
that some larvae, survivors of animais infected by herpes­
Iike viruses could be healthy latent carriers and express
subsequently the disease under stressed conditions. In
point of fact, four batches of herpes-Iike virus infected
C. gigas young oyster spat are originating from French
hatcheries and the larvae were produced in 1993. One
bateh of these infected young oysters came from a hatchery
in which mortalities in association witl! herpes-virus
detection were observed among several breedings during
the summer of 1993. The three other batehes were originating
from hateheries in which no larvae sam pie was
taken and no examination was made during their productions,
So it is impossible to know if the larvae were infected
or not by the herpes-Iike virus before their exit of these
hateheries.
Purification· and fine characterization seem to be necessary
to develop sensitive diagnosis methods for detection
of these pathogens described among C. gigas larvae and
C. gigas young oysters and to determine relationship between
these viruses.
84
Acknowledgements
This sludy could nOI been completed without the valuable help of sorne
oyster French farmers and priva te French hatcheries. The aUlhors ack.
nowledge André OIË.RARD for providing biological material. Thanks to
D' Henri GRIZEL for critical comments on the manuscript. This research
\Vas supported in part by the Conseil Général de la Charente-Maritime.
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