Effects of Hyperthermia Treatment on Infectious Spleen and Kidney Necrosis Virus (ISKNV) Infection in Fry of Cultured Nile Tilapia (Oreochromis niloticus)

Abstract

Infectious Spleen and Kidney Necrosis Virus (ISKNV) disease presents a significant threat to freshwater fish culture, particularly affecting Nile tilapia. The virus is associated with high mortality rates during the juvenile stage of the production cycle, leading to substantial reductions in productivity. As a viral infection, effective chemotherapeutic treatments remain challenging, with interventions primarily limited to managing secondary bacterial infection when necessary. Although available vaccines can only be administered when fry reach a size of 3 grams, ISKNV infection can occur at sizes below 1 gram, making vaccination unfeasible at this stage. This necessitates alternative strategies to prevent infection and limit disease transmission among fry. Heat shock treatment (hyperthermia) has been shown to enhance fish immunity against various pathogens, including ISKNV. Moreover, hyperthermia strengthens the innate immune response across all developmental stages of the fish. This study assessed the effectiveness of different water temperatures: 38ºC, 40ºC and 42ºC in controlling ISKNV infection. A control group was maintained at ambient room temperature (25–26.4ºC). ISKNV infected fry were kept in separate tanks containing freshwater, gradually heated to the target temperatures, and maintained at those conditions for 30 minutes daily in a controlled environment for six consecutive days. The heat-treated fry were subsequently raised in hapas for 60 days to monitor growth and survival. In contrast to the control group, significant differences in survival rates were observed among the treatment groups (p<0.0001) at 38ºC, 40ºC and 42ºC. Exposure to 42ºC proved lethal, resulting in 100% mortality. The findings indicate that subjecting Nile tilapia fry to hyperthermia at 40ºC effectively improved survival rates, suggesting its potential for managing ISKNV infection. However, despite the increased survival, the viral infectionpersisted across all groups. This suggests that while hyperthermia can mitigate the impact of ISKNV, it does not facilitate complete viral clearance in Nile tilapia.