Efficacy of High-concentration Ascorbic Acid Supplementation against Infectious Hematopoietic Necrosis in Salmonid Fish Influenced by Viral Strain and Fish Size

Although dietary supplementation with high-concentration ascorbic acid (AsA) can be a promising approach for reducing fish losses due to infectious hematopoietic necrosis (IHN), ineffective cases of AsA supplementation have occasionally been reported in fish farming operations. In the present study, we investigated the influence of viral strains and fish sizes on the efficacy of AsA supplementation in rainbow trout Oncorhynchus mykiss and kokanee O. nerka. Fish with different body weights were fed commercial diets supplemented with 5,000 mg of AsA per kg of diet for 7 days, and then challenged by bath exposure with two IHNV isolates (TK8901 and TV0026) belonging to different genogroups. AsA supplementation in the diet did not significantly reduce the mortality of 1-g rainbow trout as a result of infection with either isolate. On the other hand, the mortality of 2-g kokanee and 6-g rainbow trout fed AsA decreased significantly against TV0026 infection. The efficacy of AsA against TK8901 infection was, however, not confirmed with these fish sizes. These results revealed that differences in IHNV strain or fish size can affect the efficacy of high-concentration AsA administration in IHN of farmed salmonid fish.