Susceptibility of Ezo Abalone Haliotis discus hannai to Abalone Asfa-Like Virus (AbALV) Infection

Abalone asfa-like virus (AbALV) is a pathogen responsible for the mass mortality of juvenile abalones in hatcheries in Japan. However, the susceptibility of the Ezo abalone, H. discus hannai, to the virus has not been investigated, unlike the other commercially important Haliotis species in Japan. In this study, H. discus hannai at 6 months of age were exposed to effluent from an aquarium containing AbALV-infected H. discus discus and monitored for 49 days. The mortality rate began to increase at approximately 40 days post-infection (dpi). The cumulative mortality was significantly higher in the infected group (45% - 55%) than in the uninfected control group (14% - 15%) at the end of the experiment. The mean viral load peaked at 10^{5.4 ± 0.4} copies/μg total DNA at 29 dpi. Histological analysis detected many degenerated cells in the gills at 29 dpi, and these cells were immunohistochemically positive for AbALV. Although the number of degenerated cells decreased thereafter, oedema became remarkable in the gills at 42 and 49 dpi, which may have contributed to high mortality in juveniles. Our study suggests that infection control against AbALV is essential for stable juvenile production of H. discus hannai, similar to that of other susceptible Haliotis species/subspecies.