Journal of Aquatic Animal Health 13 4 340 346 Bethesda, USA: American Fisheries Society.

Abstract

The factors affecting the horizontal transmission of infectious salmon anaemia virus (ISAV) among naive Atlantic salmon Salmo salar maintained in freshwater were examined. The ratio of injected to naive cohabitants was varied and mortality was monitored. Whether ISAV transmission in freshwater required contact among salmon was also examined. Pathognomonic histological lesions in fishes infected by injection or cohabitation were compared. In trial 1, duplicate tanks containing 0+25, 2+23, 4+21, 6+19, or 8+17 fishes (injected+naive, respectively) were established. Mean days to onset of mortality and mean days to death (mdd) were significantly less among injected fish (16.9 and 18.8 days, respectively) than in fishes infected by cohabitation (33.4 and 43.8 days). Cumulative mortality in naive cohabitants in trial 1 was significantly greater in groups containing 6 injected fishes compared with those containing either 2 or 8 injected fishes. In trial 2, triplicate tanks of 0+25, 4+21, and 6+19 fish were established. Mean days to onset of mortality and mdd were similar to those in trial 1. Mean cumulative mortalities among naive cohabitants were 93.7 and 93.0% in groups containing 4 and 6 injected fishes, respectively. In trial 3, salmon exposed to water draining from tanks containing infected fishes suffered mortality, and ISAV was detected by reverse transcriptase polymerase chain reaction. Pathognomonic lesions in the gut (stomach, caeca, upper intestine) were significantly more frequent in fishes infected by cohabitation than in those infected by injection. Lesion scores of gut histology were significantly greater in cohabitation-infected fishes than in injected fishes, whereas those of the liver were significantly greater in injected fishes. This study confirmed that ISAV was transmissible among freshwater-reared salmon and that contact among fishes was not required for transmission. The reduced frequency and severity of microscopic gut lesions in fishes infected by injection may have been related to the more rapid pathogenesis associated with this route of infection.