Crassostrea gigas mortality in France: the usual suspect, a herpes virus, may not be the killer in this polymicrobial opportunistic disease

Adele James

Abstract:

Successive disease outbreaks in oyster (Crassostrea gigas) beds in France have
resulted in dramatic losses in production, and subsequent decline in the oyster-farming
industry. Deaths of juvenile oysters have been associated with the presence of a
herpes virus (OsHV-1 μvar) and bacterial populations of the genus Vibrio. Although the
pathogenicity of OsHV-1 μvar, as well as several strains of Vibrio has been demonstrated
by experimental infections, our understanding of the complexity of infections occurring

in the natural environment remains limited. In the present study, we use specific-
pathogen-free (SPF) oysters infected in an estuarine environment to study the diversity

and dynamics of cultured microbial populations during disease expression. We observe
that rapid Vibrio colonization followed by viral replication precedes oyster death. No
correlation was found between the vibrio concentration and viral load in co-infected
animals. We show that the quantity of viral DNA is a predictor of mortality, however, in
the absence of bacteria, a high load of herpes virus is not sufficient to induce the full
expression of the disease. In addition, we demonstrate that juvenile mortalities can occur
in the absence of herpes virus, indicating that the herpes virus appears neither essential
nor sufficient to cause juvenile deaths; whereas bacteria are necessary for the disease.
Finally, we demonstrate that oysters are a reservoir of putative pathogens, and that the
geographic origin, age, and cultivation method of oysters influence disease expression.