Stability of Effective Edwardsiella tarda Vaccine Developed for Japanese Eel (Anguilla japonica)

Anguilla sp. diseases diagnoses and treatments: The ideal methods at the crossroads of conservation and aquaculture purposes

Anguilla anguilla, A. japonica and A. rostrata are the most fished and consumed eel species. However, these species are Critically Endangered, Endangered and Endangered, respectively. A combination of factors is thought to be responsible for their decline including fisheries, climate change, habitat destruction, barriers to migration, pollution and pathogens. Among them, viruses, bacteria and parasites are causing weakening of wild eels and serious economic losses for fishermen and eel farmers. Early detection of pathogens is essential to provide appropriate responses both for conservation reasons and to limit economic losses. Classic diagnosis approaches are time consuming and invasive and usual treatments, for example, antipathogenic substances are becoming obsolete because of pathogen resistance and environmental impact problems. The need for early and non-invasive diagnostic methods as well as effective and environmentally friendly treatments has increased. Vaccine development and diet supplementation have known a growing interest since their use could allow prevention of diseases. In this review, we summarize the main pathogens-viruses, bacteria and parasites-of the three northern temperate eel species, the methods used to detect these pathogens and the different treatments used. We discussed and highlighted the need for non-invasive, rapid and efficient detection methods, as well as effective and environmentally friendly treatments for both conservation and aquaculture purposes.

Immunostimulation by starch hydrogel-based oral vaccine using formalin-killed cells against edwardsiellosis in Japanese eel, Anguilla japonica

Edwardsiellosis outbreaks cause significant losses in Japanese eel aquaculture. The causative agent, Edwardsiella tarda, is an intracellular pathogen, and the use of antibiotics has a limited effectiveness. As Japanese eels are sensitive to stress, injection vaccines are not recommended for treatment; immersion methods are less stressful, but not cost-effective. Alternatively, oral vaccination methods are more promising. The aim of this study was to develop a starch hydrogel-based oral (SHO) vaccine against edwardsiellosis in Japanese eel, using formalin-killed cells. To assess the protective effect, we compared SHO vaccine with the conventional formalin-killed cell (FKC) vaccine. A bacterial agglutination test showed that agglutination titers in SHO-vaccinated group were higher than in the FKC-vaccinated group. Japanese eel survival rate (%) was monitored after challenge by E. tarda at four weeks post-vaccination. Survival rates in the FKC group (60%, first trial; 70%, second trial) were lower than in SHO groups. Percentage survival rates in three SHO groups (first and second trials, respectively) were as follows: 70% and 80% in the group vaccinated once per day for one day; and 80% and 90% in both groups vaccinated for four and eight days. Additionally, a boost SHO vaccination at 46 days prompted a similar or even higher protection against edwardsiellosis than after the initial vaccination. Both FKC and SHO vaccination upregulated levels of pro-inflammatory cytokines (interleukin (IL)-6, tumor necrosis factor (TNF)-α), and host defense cytokine (interferon (IFN)-α) in all immunized groups of fish when compared with the control. These results reveal the immunostimulation effect of SHO vaccine in Japanese eel, emphasizing its potential as an oral vaccine in aquaculture.

Specific and rapid diagnosis of Edwardsiella tarda by a novel loop-mediated isothermal amplification targeting the upstream region of hlyb gene

Edwardsiella tarda has become one of the most severe pathogens in aquaculture industries throughout the world; therefore, a specific and rapid identification method for this bacterium is urgently needed. In the present study, a novel loop-mediated isothermal amplification (LAMP) was developed by targeting the upstream region of the hlyb gene of E. tarda, which was then named as UH-LAMP. The Mg(2+) concentrations, the reaction temperature, and the reaction time of UH-LAMP were optimized to 10 mM, 65°C, and 45 min, respectively. The detection limit of the UH-LAMP was 100-times higher than that of conventional polymerase chain reaction (10 versus 1000 CFU/test). Furthermore, the new UH-LAMP assay showed no cross-reactivity to the E. ictaluri belonging to the other species in the genus Edwardsiella. The high specificity of the assay was also confirmed by testing the nine strains of E. tarda collected from different geographical locations and the other 20 bacteria species. The assay can be performed in a simple water bath or a heat block and the detection result can be visualized by adding a fluorescent reagent to the reaction mixture. Taken together, our preliminary results indicate that this UH-LAMP assay provided a rapid, sensitive, and species-specific diagnostic tool for E. tarda and can easily be applied for the diagnosis under clinical or onsite conditions.