Risk factors for outbreaks of disease attributable to white sturgeon iridovirus and white sturgeon herpesvirus-2 at a commercial sturgeon farm

Abstract Objective—To determine management, fish, and environmental risk factors for increased mortality and an increased proportion of runts for white sturgeon exposed to white sturgeon iridovirus (WSIV) and white sturgeon herpesvirus-2 (WSHV-2). Animals—White sturgeon in 57 tanks at 1 farm and observations made for fish at another farm. Procedure—A prospective cohort study was conducted. Data on mortality, proportion of runts, and potential risk factors were collected. Five fish from each tank were examined for WSIV and WSHV-2 via inoculation of susceptible cell lines and microscopic examination of stained tissue sections. An ANCOVA was used to evaluate effects of risk factors on mortality and proportion of runts. Results—Major determinants of number of dead fish (natural logarithm [ln]-transformed) were spawn, source (90% confidence interval [CI] for regression coefficient, 0.62 to 2.21), and stocking density (90% CI, 0.003 to 0.03). Main predictors of proportion of runts (ln-transformed) were spawn, mortality incidence density (90% CI, 0.004 to 0.03), age (90% CI, –0.012 to –0.004), and the difference in weight between the largest and smallest nonrunt fish (90% CI, 0.0002 to 1.24). Additional observations indicated a possible protective effect attributable to previous exposure to the viruses. Conclusions and Clinical Relevance—Mortality and proportion of runts for white sturgeon after exposure to WSIV and WSHV-2 may be reduced for a farm at which the viruses are endemic by selection of specific broodstock, stocking with fish that survived outbreaks of viral disease, using all-in, all-out production, and decreasing stocking densities. (Am J Vet Res 2000;61:1232–1240)