Factors influencing transmission, onset and severity of outbreaks due to white sturgeon iridovirus in a commercial hatchery

Progeny from six different spawns of white sturgeon broodstock were monitored for 20 months (January 1997 to August 1998) in a commercial white sturgeon hatchery in North Carolina, USA for occurrence of outbreaks of white sturgeon iridovirus (WSIV) and white sturgeon herpesvirus-2 (WSHV-2). Five WSIV but no WSHV-2 outbreaks occurred during the study period. Signs of WSIV were restricted to tanks from a single spawn each time (except for one tank during the first outbreak). Temporal-spatial statistical analysis of outbreaks did not indicate that WSIV case tanks were clustered in time and space. Furthermore, WSIV was isolated from progeny of all six spawns participating in the study, even though occurrence of outbreaks and clinical presentation varied greatly among fish from different spawns. Despite failure to identify virus in samples from broodstock, these observations support a hypothesis of vertical transmission of WSIV, with tank-to-tank transmission having a lesser or no role in the spread of the virus. Differences in the onset and severity of WSIV outbreaks in fish from the six participating spawns indicate a possible genetic resistance to the virus and/or a role of stressors. All outbreaks, followed at least one major stressful event that occurred 9-32 days before the appearance of the first disease signs, and simulation modelling showed that the probability of this occurrence being a chance event was 0.14%. We conclude that minimization of stressors (avoidance of pump failures, handling and transportation) of the fish, should be a priority for the hatchery managers. Furthermore, since differences in resistance to WSIV probably exist among spawns, exclusion from reproduction of parents that yielded progeny susceptible to WSIV could improve survival of the young white sturgeon in hatcheries.