Transmission of Ostreid herpesvirus-1 in Crassostrea gigas by cohabitation: effects of food and number of infected donor oysters

Ostreid herpesvirus-1 (OsHV-1) causes severe mortalities in commercially produced Crassostrea gigas, with oysters <1 yr of age being the most susceptible. It presents a significant threat to C. gigas aquaculture worldwide. Understanding the mechanisms behind the complex interactions of the pathogen, host and environment and the mode of transmission of the virus has been difficult. The aims of this study were to (1) investigate the role that food plays in OsHV-1 transmission and mortality and (2) assess the dose-response effect represented by the number of OsHV-1-infected donor oysters cohabitated with healthy naive oysters. Both addition of food and the number of donor oysters were important risk factors for OsHV-1 transmission and clinical disease in an aquarium model. Cumulative mortality was higher and incubation period (median time to death) was lower in fed oysters compared with non-fed oysters. Depending on the number of infected donors, fed oysters had a hazard of death 3.55 to 13.70 times that of non-fed oysters in the first 7 d post-exposure. Mortality rates were doubled when 6 donor oysters were included per tank compared with 2 donors in the first experiment. The severity of disease was lower in a second experiment conducted on a different batch of oysters. However, there was a consistent trend for increased hazard of death associated with food. OsHV-1 DNA was detected in aquarium water in both experiments but at low levels compared with prior studies, probably due to larger water volume, biofiltration and UV treatment. Results are consistent with the OsHV-1 particle attachment hypothesis; however, the adsorption of OsHV-1 to algal food particles and the effects of their ingestion are yet to be confirmed.