Monitoring for Anguillicoloides crassus, Anguillid herpesvirus 1, aquabirnavirus EVE and rhabdovirus EVEX in the European eel population of southern Spain.

European eel is critically endangered in Europe. Among other stressors, pathogens are well-known to harm eels' fitness. One hundred and eighty-two eels were captured in three Eel Management Units in Andalucía (SE Spain) and analysed for Anguillicoloides crassus, Anguillid herpesvirus 1 (AngHV1), the rhabdovirus Eel Virus European X (EVEX) and the aquabirnavirus Eel Virus European (EVE). A. crassus adults and preadults were isolated and morphometrically identified, and the eel swimbladders were artificially digested to count A. crassus larvae. Also, eel tissues were examined by PCRs for the presence of viruses. EVEX and EVE were not detected in any of the eels. The estimated prevalence (95% confidence limits) was 71 (64-78)% for A. crassus and 35 (28-42)% for AngHV-1, varying these prevalences significantly between and within EMUs. Moreover, A. crassus prevalence was highest in smaller eels, in sites closest to the sea and eels sampled in the autumn. By contrast, AngHV-1 prevalence was highest in biggest eels, in sites far from the sea and sampled in the summer or winter. However, in mixed effects logistic models including site as a random variable, the risk of infection was associated with distance to the sea in both A. crassus and AngHV-1 infections and also to winter sampling in the case of AngHV-1 and not to other variables. These results are evidence that both pathogens are highly endemic in eels from Andalusian habitats. Further studies are needed to better understand the risk factors associated with these pathogens on eel populations.

Identification of virus infections of European eels intended for stocking measures.

The spread of viral diseases in eels is suggested to severely affect the European eel (Anguilla anguilla) panmictic population. The European Commission has initiated the Eel Recovery Plan (Council Regulation No. 1100/2007) to try to return the European eel stock to more sustainable levels within that measures eel restocking. However, scientific evidence evaluating the efficacy of stocking remains scarce. In addition, knowledge about the impact and contribution of eel stocking on the distribution of infectious diseases is insufficient. In this study, we aimed to investigate virus infections in batches of eels intended for restocking. We analysed samples of glass eels from certified fisheries and farmed European eels from different aquaculture farms. All analysed eels were purchased within a North Rhine Westphalian conservation program. Via a combination of cell culture and qPCR-based techniques, we detected infections of glass eels with the rhabdovirus Eel Virus European X and anguillid herpesvirus 1 infections in farmed eels (10-15 cm).

Is the Illegal Trade of Glass Eels (Anguilla anguilla) Increasing the Spread of Disease? A Case of EVEX.

The European eel (Anguilla anguilla) is a catadromous species that inhabits the rivers of the Adriatic watershed in Croatia. It is a critically endangered fish species, according to the IUCN, due to its declining abundance in European rivers caused by overfishing and trafficking and by diseases caused by nematodes, pathogenic bacteria and viruses. An illegal parcel of glass eels was confiscated at the Zagreb Airport and was intended to re-populate Croatian rivers. Barcoding was employed to determine species affiliation, and a thorough health check was carried out. This study reports the evaluation of gross lesions, histological findings, and EVEX virus isolation and identification. Since the confiscated glass eels were of unknown origin and given the serological and genetic similarities of EVA and EVEX, we designed primers and probes for almost whole genome sequencing to elucidate the origin of glass eels based on viral phylogeny. Bayesian phylogeny showed that the isolated strain had the most common ancestor with a Danish isolate and likely evolved from the French isolate of EVEX. These findings are discussed in light of the divergence of recently isolated strains and their possible contribution to the decrease of the abundance of the European eel in European waters.

Glass eels and viruses - a lesson learnt from stocking the eastern German Baltic Sea coast.

Concerns about Anguillid herpesvirus 1 (AngHV-1) in European eels, especially due to stocking measures, is increasingly coming into focus and raises questions regarding disease monitoring and prevention. In the past, stocking of AngHV-1-positive eels into waters assumed AngHV-1-free has led to a rapid increase of infected eels in the wild. For this study, a glass eel stocking experiment was conducted in the eastern German Baltic coast from 2014 - 2016. Retrospective analysis of stocked glass eels shows, that virus prevalence varied from 5.3 - 37.4 % during stocking years and appeared to be influenced by a prolonged holding period in the catching region. On average, 16 % glass eels were AngHV-1 positive. Given that stocked eels were alizarin red s marked, it was possible to monitor the local eel stock between 2018 and 2019 for AngHV-1 infection and additionally two commonly coinfections, namely European Virus Eel (EVE) and Europe eel virus X (EVEX), of marked and not marked eels. In recaptured eels, mortality was significantly higher in the glass eel cohort which was found to be heavily AngHV-1 infected (stocked in 2014) compared to the less infected cohorts. Furthermore, virus-positive stocked eels turned out to be disadvantaged compared to non-infected conspecifics in terms of total length and fat content. However, co-infections with EVE and EVEX could not be detected in this study. In conclusion, eel conservation and maintaining an economically successful eel fishery is probably best achieved by stocking eels that have been shown to be free of AngHV-1. This article is protected by copyright. All rights reserved.