Ostreid herpesvirus-1 (OsHV-1) is a pathogen of Pacific oysters (Crassostrea gigas), a high value food product and essential component of estuarine ecosystems. Since 2008, there has been a global emergence of a genotype of OsHV-1 referred to as the microvariant (μVar) that causes very high mortality disease and has seriously impacted Pacific oyster production in many countries. A bioassay is required to study this pathogen and devise disease control strategies because a permissive cell line for culture of OsHV-1 is not available.
Different formats of a bioassay were evaluated whereby naïve Pacific oysters were challenged with a virus preparation with control of the environment and nutrition within a physical containment laboratory. A positive bioassay result was defined by mortality of some oysters within 7 days with replication of the OsHV-1 genome demonstrated by quantitative PCR assay. Intramuscular injection of a virus preparation into juvenile oysters provided a more sensitive test for viable OsHV-1 compared to immersion challenge of oyster spat. Challenge by immersion and cohabitation was considered more suitable for studies of transmission and pathogenesis. The bioassay was used in these different formats to determine if the age and size of oysters were independent factors affecting susceptibility to infection and disease.
The bioassay demonstrated that OsHV-1 remained infectious in seawater for 48 hours and in both moist and dry oyster tissue for 7 days at 20°C. Several disinfection methods commonly used in aquatic disease control were effective for inactivation of OsHV-1 including commercial disinfectants applied according to directions (for example, Virkon-S, Dupont; Quaternary ammonium disinfectant preparation, Livingston) and empirical doses of chemicals including sodium hydroxide, iodine and formalin. Chlorine (used as sodium hypochlorite) at 50 parts per million available chlorine inactivated OsHV-1 in clean seawater but not in seawater after addition of protein. Chlorine at 200 ppm did not inactivate OsHV-1 in the presence of oyster tissue. This study highlighted the potential for OsHV-1 to spread on fomites and provided information that will assist in devising effective disease control strategies.