Experimental Transmission and Pathogenicity of a Viral Erythrocytic Infection in Sea Bass Dicentrarchus labrax

Abstract

The experimental transmission of a viral erythrocytic infection (VEI) in sea bass Dicentrarchus (=Morone) labrax of the Mediterranean is reported. Transmission procedures included the intraperitoneal inoculation of healthy fishes with blood and hematopoietic tissue extracts from VEI-affected fishes, direct contact between VEI-affected and unaffected fishes, and rearing of fishes in unfiltered, nonirradiated seawater that had been used for holding VEI-affected fishes. Successful transmission of the infection was achieved in all experimental conditions, as ascertained by the presence of inclusion bodies in Giemsa-stained blood cells and subsequently confirmed by indirect immunofluorescent staining. Transmission of VEI by the inoculation of filtered cell-free extracts from affected tissues confirmed not only the infectious character of the disease but also its viral etiology. Two different patterns of effects on the hematological variables were observed: an erythroblastic polycythemia occurred 3 months after infection, whereas a moderate anemia appeared after 5–6 months. Fish growth was also negatively affected by VEI. Sea bass infected with VET were found to be more susceptible to bacterial challenge infection with Vibrio anguillarum isolate AO 18 than were control fish; the V. anguillarum dose that was lethal to 50% of the test fish was lower for VEI-affected fish, and their cumulative mortality in the bacterial challenge was higher. The injection of cyclophosphamide produced immunosuppression in unaffected sea bass and an increase in the level of infection in VEI-affected fishes; these results suggest the role of the immunological status of the host sea bass in the development of VEI.