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

For these reasons, tarvat rearing at 22-23°C is potentially hazardous because acute infection is
avoided, limiting larval mortalities, but latent, asymptomatic carrier animais could be produced and
disseminated. Then, animais held at low temperature (22-23°C), even negative for viral particles
detection by transmisson electron microscopy, could be infected by the herpes-like virus and these
carriers may represent reservoir of the virus which could transmit this agent. But also, oysters
produced by such larval breedings could be unsafe from activation of the latent state to a productive
infection at a later time.
ln a conclusion, it seems that both temperature and origin of parents, when breeding temperatures are
the sa me, apparently influence herpes-like virus expression in C. gigas larvae. Higher temperatures
promote earty production of viral particles in association with high mortalities. The result is a fast
spread of the disease to the whole breeding and high sudden mortalities. Moreover, larvae originating
from some oyster broodstocks, particularly trom Brest, grew weil at both rearing temperatures. Since it
was not possible to detect herpes-like virus nor nuclear alterations in association with these larvae,
parental origin seemed to be important for the appearance of the disease among larvae. As ail the
larval breedings were performed at the same time and in the sa me conditions at both temperatures,
this result suggests that one mode of transmission of this virus cou Id be vertical, from parents to
larvae. Moreover, with the gonad strip dissection procedure, this possible vertical transmission could
occur either via the gamete cells or via the many extraneous, non-gamete cells.
Although no mortalities occurred among the different broodstocks of C. gigas used in this study, a
further hypothesis is that the virus is present in a latent phase in some of the parents and is activated
by stress in these hosts (Garcia-Blanco and Cu lien, 1991), such as a modification of rearing
temperature. Adults are usually held at 22°C, which is generally higher than in the natural environ ment,
to accelerate the gonadal maturation process. Other stressors include manipulation or transport of
animais, crowding or a modification of feeding. The virus induced by this stress would be transmitled to
larvae. Consequently, in larvae reared at high temperature (25-26°C) the viral productive cycle is
faster, the virus spread quickly to the whole breeding, leading to high and sud den mortalities of larvae.
ln contrast, in larvae reared at lower temperature (22-23°C), and presenting nuclear alterations, the
viral repli cation may rcmain in an abortive viral cycle (Girard and Hirth, 1989).
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