Vol. 101: 69–86, 2012 DISEASES OF AQUATIC ORGANISMS Published October 10 doi: 10.3354/dao02501 Dis Aquat Org REVIEW Viral diseases of wild and farmed European eel Anguilla anguilla with particular reference to the Netherlands Steven J. van Beurden1,2, Marc Y. Engelsma1, Ineke Roozenburg1, Michal A. Voorbergen-Laarman1, Peter W. van Tulden1, Sonja Kerkhoff1, Anton P. van Nieuwstadt1, Aart Davidse1, Olga L. M. Haenen1,* 1Central Veterinary Institute, Wageningen UR, PO Box 65, 8200 AB Lelystad, The Netherlands 2Department of Infectious Diseases and Immunology, Faculty of Veterinary Medicine, Utrecht University, PO Box 80.165, 3508 TD Utrecht, The Netherlands ABSTRACT: Diseases are an important cause of losses and decreased production rates in fresh - water eel farming, and have been suggested to play a contributory role in the worldwide decline in wild freshwater eel stocks. Three commonly detected pathogenic viruses of European eel Anguilla anguilla are the aquabirnavirus eel virus European (EVE), the rhabdovirus eel virus European X (EVEX), and the alloherpesvirus anguillid herpesvirus 1 (AngHV1). In general, all 3 viruses cause a nonspecific haemorrhagic disease with increased mortality rates. This review pro- vides an overview of the current knowledge on the aetiology, prevalence, clinical signs and gross pathology of these 3 viruses. Reported experimental infections showed the temperature depend- ency and potential pathogenicity of these viruses for eels and other fish species. In addition to the published literature, an overview of the isolation of pathogenic viruses from wild and farmed A. anguilla in the Netherlands during the past 2 decades is given. A total of 249 wild A. anguilla, 39 batches of glass eels intended for farming purposes, and 239 batches of farmed European eels were necropsied and examined virologically. AngHV1 was isolated from wild A. anguilla yellow and silver eels from the Netherlands from 1998 until the present, while EVEX was only found spo- radically, and EVE was never isolated. In farmed A. anguilla AngHV1 was also the most com- monly isolated virus, followed by EVE and EVEX. KEY WORDS: Anguillid herpesvirus 1 · Eel virus European · Eel virus European X Resale or republication not permitted without written consent of the publisher INTRODUCTION Stone 2003). The cause of the decline is unknown, but is probably multifactorial, with suggested factors Freshwater eels of the genus Anguilla have extra - including pollution, habitat loss, fisheries, migration ordinary catadromous lifecycles, with the spawning barriers, and diseases (Dekker 2004). Indeed, the grounds of some species located in the ocean sev- swimbladder nematode Anguillicoloides crassus eral thousands of kilometres away from their fresh- (Székely et al. 2009), several pathogenic bacteria water growth habitats in lakes and rivers of the (Esteve & Alcaide 2009) and certain viruses (Haenen mainland (van Ginneken & Maes 2005). The wild et al. 2009) have been suggested to play a contribu- freshwater eel stocks have shown strong declines tory role in the decline of the wild European eel worldwide since the 1980s (Dekker et al. 2003, Anguilla anguilla stock. *Corresponding author. Email: [email protected] © Inter-Research 2012 · www.int-res.com 70 Dis Aquat Org 101: 69–86, 2012 Anguilla anguilla and the Japanese eel A. japon- nen et al. 2002), as well as double infections with dif- ica are traditionally consumed in several European ferent viruses (Ahne & Thomas 1985, Haenen et al. countries and Japan, respectively (Heinsbroek 1991). 2002, van Ginneken et al. 2004, Varvarigos et al. Historically, eels for consumption were wild-caught, 2011), have been observed. Several diagnostic assays but European eel fisheries have recently been sub- have been developed for the detection of eel viruses, ject to limitations because of the population decline more recently with a focus on molecular assays. In (Council of the European Union 2007). In Japan, eel eel farming, identification of the causative agent can farming for consumption purposes started at the end be used to take adequate quarantine and water tem- of the 19th century. Eel farming has developed from perature regulation measures in order to reduce clin- non-intensive polyculture in outdoor ponds to inten- ical signs and losses. sive indoor culture in greenhouses since the 1970s. In this literature review, we give an overview of Eel farming in Europe has its origin in Italy, but the current knowledge on the aetiology, geographi- gradually moved to northwestern Europe, where it cal distribution, clinical signs, mortality and gross changed into an intensive form of aquaculture after pathology of pathogenic European eel viruses. In the Japanese example. addition, we present a retrospective analysis of diag- As artificial reproduction of freshwater eel is not nostic data from the Dutch National Reference Labo- yet possible on a commercial scale, production for ratory (NRL) for Fish Diseases over the period 1990 consumption is still based entirely on catches of wild to 2011, which provides a historical overview on the glass eels or elvers. This leads to the potential intro- viruses isolated from wild and farmed A. anguilla in duction of eel disease agents in aquaculture pro - the Netherlands in the past 2 decades. The 3 viral duction systems. Anguilla anguilla is nowadays pro- agents that are observed most commonly in A. ang - duced generally in intensive recirculation systems at uilla are the aquabirnavirus eel virus European a regulated water temperature. With an annual pro- (EVE), the rhabdovirus eel virus European X (EVEX) duction of about 4000 t in the previous decade, the and the alloherpesvirus anguillid herpesvirus 1 Netherlands is the most important eel-producing (AngHV1). country in Europe (FAO 2012). High stocking densi- ties make detection and control of diseases vital for sustainable farming. Prevention and treatment of eel EEL VIRUS EUROPEAN viral diseases is particularly difficult, as commercial vaccines are not available. Aetiology When high-density Japanese eel pond culture exponentially grew in the late 1960s and early 1970s Since 1969, serious outbreaks of a new disease, in Japan, Anguilla anguilla and American glass eels called branchionephritis or viral kidney disease of and elvers A. rostrata were imported and stocked, Anguilla japonica, occurred every winter when and catastrophic viral disease outbreaks occurred water temperatures were below 20°C in eel culture frequently (Heinsbroek 1991). Permanently growing ponds in Japan (T. Sano et al. 1981). The aetiological cell lines were developed from A. japonica kidney agent was isolated for the first time from imported A. and ovary cells, and used for virus isolation (Chen & anguilla using the rainbow trout gonad cell line RTG- Kou 1981, Chen et al. 1982). During the many out- 2 (Wolf & Quimby 1962) in 1973, and tentatively breaks, new viruses were isolated and shown to be named eel virus European (EVE) (T. Sano 1976). EVE the causative agent (T. Sano 1976, T. Sano et al. 1977, was subsequently also isolated from A. japonica, and M. Sano et al. 1990). Although initial descriptions River’s postulates were fulfilled. The type of cyto- were usually thorough and detailed, nomenclature pathic effect (CPE) caused by EVE resembled the was ambiguous. Hence, several virus isolates were type of CPE caused by the aquabirnavirus infectious initially presented as a new virus, and later demon- pancreatic necrosis virus (IPNV) (T. Sano 1976, T. strated to be highly similar to an already described Sano et al. 1981). Electron microscopy (EM) revealed virus. that EVE virions were non-enveloped polyhedrons Identification of pathogenic eel viruses is further with a diameter of 68 to 77 nm, only present in the complicated by the non-pathognomonic clinical signs cytoplasm of infected cells. EVE also resembled and gross pathology of these eel viral diseases. In IPNV in terms of biological properties, such as addition, virus isolation from clinically healthy eels polypeptide composition and the bisegmented dou- (Castric & Chastel 1980, Bucke 1981, Castric et al. ble-stranded RNA genome (Nishimura et al. 1981a, 1984, Chen et al. 1985, Shchelkunov et al. 1989, Hae- T. Sano et al. 1981, Hedrick et al. 1983a). Hence, EVE van Beurden et al.: Viral diseases of European eel 71 is a tentative member of the genus Aquabirnavirus in Sano et al. 1981). It was therefore suggested — and the family Birnaviridae. Other names for EVE or allegedly proven— that EVE entered Japan with the IPNV of eel include eel virus (EV) [Berlin] (Schwanz- import of A. anguilla elvers (T. Sano et al. 1981, Pfitzner et al. 1984) and pillar cell necrosis virus Hedrick et al. 1983a). EVE/IPNV type Ab and IPNV (PCNV; Lee et al. 1999a, 2001). type VR-299 were later also isolated from various A. Aquabirnaviruses form an antigenically diverse japonica farms in Taiwan (Chen et al. 1985, Hsu et al. group of viruses, with the type species IPNV being 1989, 1993). EVE/PCNV from diseased A. japonica in the aetiological agent of an acute contagious systemic Japan was serologically most similar to IPNV sero - disease of several species of freshwater and marine type Sp (Lee et al. 1999a), but genetically closer fish, molluscs and crustaceans (Rodriguez Saint-Jean related to strain Ab (Lee et al. 2001). et al. 2003). Mortality caused by IPNV in salmonids is In 1977, Castric & Chastel (1980) isolated an IPNV- high in fry and fingerlings, but rare in older fish. Sur- like agent called B6 from Anguilla anguilla elvers vivors of the epizootic disease may become lifelong along the French Atlantic coast, and showed its relat- carriers. Host specificity and cell tropism are deter- edness to IPNV serotype Sp by serum neutralisation mined by viral proteins encoded by the larger RNA tests. EVE related to IPNV serotype Ab was repeat- segment A (M. Sano et al. 1992), and the occurrence edly isolated from an eel farm in the UK (Bucke 1981, of natural reassortment has recently been shown Hudson et al.