Experimental infections of Pacific oyster Crassostrea gigas using the Australian ostreid herpesvirus-1 (OsHV-1) mu Var strain

In Australia, the spread of the ostreid herpesvirus-1 microvariant (OsHV-1 mu Var) threatens the Pacific oyster industry. There is an urgent need to develop an experimental infection model in order to study the pathogenesis of the virus under controlled laboratory conditions. The present study constitutes the first attempt to use archived frozen oysters as a source of inoculum, based on the Australian OsHV-1 mu Var strain. Experiments were conducted to test (1) virus infectivity, (2) the dose-response relationship for OsHV-1, and (3) the best conditions in which to store infective viral inoculum. Intramuscular injection of a viral inoculum consistently led to an onset of mortality 48 h post-injection and a final cumulative mortality exceeding 90%, in association with high viral loads (1 x 105 to 3 x 10(7) copies of virus mg(-1)) in dead individuals. For the first time, an infective inoculum was produced from frozen oysters (tissues stored at -80 degrees C for 6 mo). Storage of purified viral inoculum at + 4 degrees C for 3 mo provided similar results to use of fresh inoculum, whereas storage at -20 degrees C, -80 degrees C and room temperature was detrimental to infectivity. A dose-response relationship for OsHV-1 was identified but further research is recommended to determine the most appropriate viral concentration for development of infection models that would be used for different purposes. Overall, this work highlights the best practices and potential issues that may occur in the development of a reproducible and transferable infection model for studying the pathogenicity of the Australian OsHV-1 strain in Crassostrea gigas under experimental conditions.