Abstract :
[en] The European eel (Anguilla Anguilla) is a culturally and commercially important fish species and along with other members of the genus Anguilla, they are flagship species for aquatic conservation efforts. Anguillid eels have a fascinating and complex life cycle involving different life stages and extended migration through both marine and freshwater environments. The lifecycle of the European eel begins in the Atlantic Ocean. After hatching, leptocephali larvae drift thousands of kilometres across the ocean to reach continental coastal waters where they metamorphose into glass eels. These juvenile eels then migrate upstream towards freshwater habitats where they grow into yellow eels and remain there for several years. Finally, they fully mature into silver eels and migrate back to their place of birth, where they reproduce before dying. Within the last 50 years, the European eel population has dramatically decline and the species is now critically endangered. The reason for this steep decline in population is likely to be multifactorial, with one potential major factor being infectious diseases. In terms of viral diseases, Anguillid herpesvirus 1 (AngHV 1) is the causative agent of a haemorrhagic disease affecting freshwater eels (Anguilla spp.), and is one of the most frequently detected viruses from European eels, especially in fish farms. This herpesvirus is a member of the genus cyprinivirus of the family Alloherpesviridae.
As a consequence of this decline, the European Commission has established the “Eel Recovery Plan” within member states with the aim of protecting and restoring the population of European eel. One of the main measures of this plan relies on restocking. This process involves the active transfer of eels to inland areas where eel populations have declined. Although this restocking is widely practiced in Europe, additional studies are still needed to evaluate and improve its effectiveness.
The broad objective of the present thesis was to contribute to the conservation of the critically endangered European eel, through focusing on infectious diseases as one of the potential causative factors of its decline. Firstly, we investigated the sanitary status of wild caught glass eels entering estuaries and aimed to develop quarantine stations that would facilitate the implementation of therapeutic and prophylactic measures prior to their reintroduction into the wild. Our results showed that glass eels were negative for AngHV-1 before their reintroduction into Belgian rivers. We also established that under certain conditions, a two-week pre-release quarantine period could be implemented without major impact on important metrics such as eel survival rate, sanitary status, health and morphology.
One of the major goals of this PhD project was to gain insights into AngHV-1 pathogenesis using recombinant viral strains combined with in vivo bioluminescent imaging (IVIS). As a prerequisite for the study of AngHV 1 pathogenesis, we sequenced the entire genome of seven strains of AngHV-1 from different geographic origins and compared their biological properties in vitro, which also provided insights into the evolution of this virus. Based on this, we selected 4 strains representative of the viral species, compared their properties in vivo and produced recombinants expressing luciferase and a copepod green fluorescent protein (here after named LucGFP). Characterization of these recombinants revealed that they are appropriate for the study of AngHV-1 with no detectable negative effect associated with the insertion of the transgene. As all strains expressed comparable properties in vivo, the UK LucGFP strain was selected for the next steps of the project.
Using the UK LucGFP strain and the IVIS, we investigated major questions related to the pathogenesis of AngHV-1 infection in its natural host: These experiments led to the following observations: (i) Glass eels are not susceptible to AngHV-1 infection through the natural route tested. (ii) Inoculation of elvers led to few positive subjects expressing bioluminescence on their cephalic part, mainly gills and jaws, with no spreading of the infection between subjects according to time. (iii) Inoculation of yellow eels led to infection of all subjects expressing bioluminescence signal associated with strong clinical signs. Morbidity and cumulated mortality reached 100% and 20% respectively. (iv) The gills and the periodontal mucosa represented major portals of entry of the virus into naïve subjects. (v) Two modes of transmission of AngHV-1 were highlighted in this study, i.e. direct transmission through interspecific agonistic interactions (e.g. biting) between fish, and indirect transmission through environmental water. Our results indicate that AngHV 1 transmission via contaminated water requires a high density of subjects and virions, and is thus probably restricted to intensive aquaculture conditions. On the other hand, AngHV-1 infection mediated by biting appears highly efficient and is likely to be facilitated in environments with either high or low host density. We hypothesize that viral transmission mediated by interspecific agonistic interactions may partially explain the high prevalence of AngHV 1 in the wild despite the low infectivity of AngHV-1 in water, the low and declining host population density, the solitary behaviour of the host, and the strong ability of fish skin mucus to neutralize pathogens such as AngHV 1.
In conclusion, this project contributed to recommendations for optimization of glass eel restocking practices in Belgium and highlighted major aspects of AngHV 1 pathogenesis. The findings create ample scope for future projects in relation to eel conservation, AngHV 1 evolution, host-pathogen dynamics in aquatic environments as well as setting the groundwork for AngHV 1 vaccine development.
