Preface:
A recent study suggests that high temperatures affect the growth and pathogenicity of decapod iridescent virus 1 (DIV1), affect viral replication in tissues and affect the survival rate of P. albican. As a result, shrimp farmed at high water temperatures have a higher viral load and are more susceptible to DIV1 infection.
Decapod iridescent virus 1 (DIV1), belonging to the iridoviridae family, is a strange virus in the shrimp family that has caused great damage to shrimp farming in recent years. Iridoviridae can infect insects, amphibians and fish, and infected vannamei has a high mortality rate, up to 100% within 15 days. Shrimp infected with DIV1 has the following signs: the shrimp body is red, the shell is soft, sinks to the bottom of the pond, and after about two or three days, all the shrimp in the pond will be infected and die.
Temperature fluctuations affect the physiological activities of shrimp, such as growth rate, disease resistance and metabolic rate. In addition, studies have shown that temperature affects the replication rate of different viruses (Kan et al., 2013).
Several studies have shown that high water temperature can reduce the mortality of shrimp infected with WSSV virus (Vidal et al., 2001) and inhibit WSSV replication (Moser et al., 2012); Incidence of Taura TSV syndrome (Coté et al., 2010) and epithelial and hematopoietic necrosis virus IHHNV (Montgomery-Brock et al., 2007).
1. Research settings
1. Randomly sampled vannamei shrimp (average weight 3.58±1g) and PCR test to ensure that there is no infection with IHHNV, WSSV and DIV1. Shrimp naturally infected with DIV1 were obtained from a farm in Guangdong. The water temperature is adjusted according to the experimental temperature (26±1 °C and 32±1 °C). DIV1 virus samples were extracted from the tissues of naturally infected vannamei shrimp.
2. Penaeus vannamei was cultured at different temperatures (26±1 °C and 32±1 °C) and the same salinity, and then injected with DIV1 (shrimp injected with 50μL DIV1 extract. Contains 1.0 x 104 cfu/μL). The shrimp in the control group are kept at the same temperature and injected with 50 μL of PBS. The test was divided into 3 groups of 30 shrimp per pond each.
3. At 6, 12 and 24 hours (HPI) after infection, 3 shrimp were randomly sampled at each temperature. Collect the gills, hepatopancreas, abdominal legs, intestines, and muscles and place in a -80 °C freezer (Fouzi et al., 2010). Survival rates for each group are calculated after 6 h.
2. Research results
1. Survival rate
During the trial, the survival rate of P. vannamei continued to decline. At the same time, there was a large difference between the experimental group (infected group) and the control group (uninfected group) at the same water temperature. The survival rate of 96 hpi in the experimental group was significantly lower than that in the uninfected control group. At a temperature of 26±1°C, the survival rate of the infected group was 4.44%, while the survival rate of the control group was 78.89%. On the other hand, the survival rate was the lowest in the infected group at 32±1°C at 2.22%, compared with 86.67% in the control group. There was no significant difference in survival between the 26±1°C and 32±1°C infected groups, both below 5%. Shrimp at 32±1°C died earlier than 26±1°C, possibly related to the increased susceptibility of shrimp to high water temperatures, making them more susceptible to disease and more DIV1 infection.
2. Proliferation of DIV1 at different water temperatures
qPCR analysis showed that in the early stages of DIV1 infection, the viral copy number in most tissues in the 32±1 °C group was significantly higher than in the 26±1 °C group. This study is very similar to the correlation between temperature and pathogens in the 2005 study by Cheng et al., showing that P. vannamei are more sensitive to Vibrio algae in the first 24 hours after changing from 27-28°C to 32-34°C (Cheng W et al.005). At a temperature of 26±1 °C, there was a significant difference in the number of copies of DIV1 in each infected tissue at 6 and 24 hpi. DIV1 replicates fastest in the gut and is detected in the highest number (Figure 2). There were also significant differences between tissues sampled at 32±1°C at 6hpi and 24hpi. Of these tissues, muscle tissue has the highest viral load (Figure 3).
DIV1 replicates at different rates at two different water temperatures. In addition to the intestine, at 6 hpi, the DIV1 copy number in the sample tissue at 32 ± 1 °C was higher than the DIV1 copy number at 26±1 °C, especially the DIV1 copy number in the gills, gastropods, and muscles was significantly different. At 12 hpi, the copy number in the intestine and muscle at 32±1 °C was significantly higher compared to 26±1 °C, especially the copy number of DIV1 in muscle at 32±1 °C was much higher than at 26±1 °C. However, at 24 hpi, the DIV1 copy number in the gills, hepatopancreas, legs, and intestines was much higher than at 32±1°C at 26±1 °C, while the DIV1 copy number in the muscle at 32 °C was much higher than 26±1 °C.
3. Draw conclusions
At the two water temperatures tested, there were significant differences in survival and viral load between the DIV1 infection and control groups. At these two test temperatures, the survival rate of DIV1-infected shrimp dropped to 5% at 30 hpi. This study showed that transferring Pennamei vannamei to higher water temperatures had no beneficial effect on the damage caused by DIV1 infection. The survival rate of SIV1-infected vannamei shrimp dropped to 4.44% within 48 hours at 26±1°C and to 2.22% within 30 hours at 32±1°C. P. vannamei cultured at high water temperatures has a high viral load and is more susceptible to DIV1 infection. At 32±1 °C, almost all tissues reach the viral load threshold in the first stage of infection.