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 monitor ed for 20 months in a commercial white sturgeon hatchery for occurrence of outbreaks of white sturgeon iridovirus (WSIV) and white sturgeon herpesviru s-2 (WSHV-2). Five WSIV but no WSHV-2 outbreaks occurred during the study p eriod. Signs of WSIV were restricted to tanks from a single spawn each time (except for one tank during the first outbreak). Temporal-spatial statisti cal analysis of outbreaks did not indicate that WSIV case tanks were cluste red in time and space. Furthermore, WSIV was isolated from progeny of all s ix spawns participating in the study, even though occurrence of outbreaks a nd clinical presentation varied greatly among fish from different spawns. D espite failure to identify virus in samples from broodstock, these observat ions support a hypothesis of vertical transmission of WSIV, with tank-to-ta nk transmission having a lesser or no role in the spread of the virus. Diff erences in the onset and severity of WSIV outbreaks in fish from the six pa rticipating spawns indicate a possible genetic resistance to the virus and/ or a role of stressors. All outbreaks, followed at least one major stressfu l event that occurred 9-32 days before the appearance of the first disease signs, and simulation modeling showed that the probability of this occurren ce being a chance event was 0.14%. We conclude that minimization of stresso rs (avoidance of pump failures, handling and transportation) of the fish, s hould be a priority for the hatchery managers. Furthermore, since differenc es in resistance to WSIV probably exist among spawns, exclusion from reprod uction of parents that yielded progeny susceptible to WSIV could improve su rvival of the young white sturgeon in hatcheries. (C) 2001 Elsevier Science B.V. All rights reserved.