Raising temperature at lower heating rate is an effective way to prevent Micropterus salmoides rhabdovirus disease

Abstract

Micropterus salmoides rhabdovirus (MSRV), is a single-stranded RNA virus that results in mass morbidity and mortality in largemouth bass (Micropterus salmoides), leading to severe economic losses for the largemouth bass aquaculture. To control outbreaks of MSRV disease, we investigated the influence of water temperature on the infectivity of MSRV and the resistance of largemouth bass to MSRV. In addition, we explored whether higher water temperature is an effective way to prevent outbreaks of MSRV disease. The rhabdovirus was added to three sterile pond water samples and incubated at 10 °C, 20 °C, and 30 °C for 14 days. The viral load of each pond water sample was calculated by RT-qPCR, and the infectivity was evaluated by inoculating grass carp ovary cells with samples from each pond water. In addition, juvenile fish were challenged with the MSRV at different water temperatures (15 °C, 20 °C, 25 °C and 30 °C). After the challenged, these juveniles were transferred to ponds with a water temperature ranging from 25 °C to 30 °C with different heating rates. The results showed that the number of viral copies decreased as temperature increased, and the viral copies in the 10 °C, 20 °C, and 30 °C groups were lower than the detection threshold at 11, 7, and 3 dpi, respectively. The viral infectivity was significantly reduced in the 30 °C group, with infectivity lost at 5, 5, and 1 dpi in the 10 °C, 20 °C and 30 °C group, respectively. In addition, the fish mortality rates were 56.67 % ± 10.41 %, 73.33 % ± 5.77 %, 60 % ± 10 % and 5 % ± 5 % at 15 °C, 20 °C, 25 °C and 30 °C, respectively. The mortality rate of the juvenile fish decreased from 59.52 % ± 4.12 % to 14.28 % ± 7.14 %, and the viral copies decreased significantly in the liver, spleen, and kidney of the T2 group that was exposed to a temperature increase of 0.5 °C every 1 h from 25 °C to 30 °C. In conclusion, higher temperatures (30 °C) can reduce MSRV infectivity, and a low heating rate (0.5 °C every 1 h) exerts a synergistic antioxidant effect by enhancing SOD and CAT activity and improving host immunity, ultimately helping prevent outbreaks of MSRV disease in largemouth bass. These results can be applied to improve biosecurity practices in largemouth bass farms to control the outbreaks of MSRV disease.