Red sea bream iridoviral disease (RSIVD) causes serious economic losses in the aquaculture industry. In this paper, we evaluated RSIV kinetics in rock bream under various rearing water temperatures and different RSIV inoculation concentrations. High viral copy numbers (approximately 103.7–106.7 RSIV genome copies/L/g) were observed during the period of active fish mortality after RSIV infection at all concentrations in the tanks (25 °C and 20 °C). In the group injected with 104 RSIV genome copies/fish, RSIV was not detected at 21–30 days post-infection (dpi) in the rearing seawater. In rock bream infected at 15 °C and subjected to increasing water temperature (1 °C/d until 25 °C) 3 days later, the virus replication rate and number of viral copies shed into the rearing seawater increased. With the decrease in temperature (1 °C/d) from 25 to 15 °C after the infection, the virus replicated rapidly and was released at high loads on the initial 3–5 dpi, whereas the number of viral copies in the fish and seawater decreased after 14 dpi. These results indicate that the number of viral copies shed into the rearing seawater varies depending on the RSIV infection level in rock bream.

Background: Olive flounder, Paralichthys olivaceus, is an economically important aquaculture species in Korea. Olive flounders have been heavily damaged by streptococcal infections every year and are treated with antibiotics. However, antibiotic abuse is causing the emergence of resistant strains, and to overcome this, research has shown that new antibiotics must be applied. Tylosin is a relatively safe antibiotic and has good activity against Grampositive bacteria and mycoplasma. We studied the therapeutic effects and side effects of tylosin on Streptococcus parauberis-infected olive flounder. Methods: After artificial infection of olive flounder with S. parauberis SPOF18J3, an appropriate dose of tylosin was confirmed by intramuscular injection (I.M.) at 2.5, 5, 10, and 15 mg/kg, and oral administration at 10 and 20 mg/kg. After I.M. and oral administration dosing of tylosin, side effects were confirmed by serological analysis, histopathological analysis, and median lethal dose (LD 50) analysis at both an appropriate concentration and a high concentration. Statistical analysis was performed using one-way analysis of variance (ANOVA) and Tukey's test (p < 0.05). Results: The appropriate I.M. and oral administration concentration of tylosin administered to olive flounder infected with S. parauberis SPOF18J3 was found to be 10 mg/kg. Alanine aminotransferase (ALT) and aspartate aminotransferase (AST) levels were showed not significantly different between the control group and the experimental groups. The histopathologic results showed mild inflammatory responses in muscle and tubular vacuolization and tubular atrophy appeared, but there were no significant differences between the groups. The LD 50 was confirmed to be 461 mg/kg. Conclusion: In this study, an effective treatment method was provided by verifying the treatment effects and side effects of tylosin in olive flounder infected with S. parauberis, which can be applied directly to aquaculture sites. In addition, these results may be used as a reference for evaluation required upon request to obtain approval for tylosin antibiotics as fishery antibiotics in Korea. After approval, it is possible that a fishery disease manager will be able to prescribe and sell the antibiotic tylosin.

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Antimicrobial peptides (AMPs) are known to play a role as a first line of defence against microbial invasion. Liver Expressed Antimicrobial Peptides-2 (LEAP-2) is one of the AMPs. LEAP-2 includes four highly conserved cysteine residues and belongs to a cysteine-rich peptides group. We identified and characterized the molecular properties of LEAP-2 in rock bream. The expression levels of rock bream LEAP-2 (RbLEAP-2) in the 12 different tissues of healthy fish and the RbLEAP-2 expression pattern after infections with Edwardsiella piscicida (E. piscicida), Streptococcus iniae (S. iniae) and red seabream iridovirus (RSIV) were examined. This data provide that RbLEAP-2 plays an important role in innate immunity when rock bream is infected with a pathogen.

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