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Color profile: Disabled Composite Default screen 4 Infectious Diseases of Coldwater Fish in Fresh Water Laura L. Brown1 and David W. Bruno2 1National Research Council of Canada, Institute for Marine Biosciences, 1411 Oxford Street, Halifax, Nova Scotia B3H 3Z1, Canada; 2Fisheries Research Services, The Marine Laboratory, PO Box 101, Victoria Road, Torry, Aberdeen AB11 9DB, UK Introduction in flow-through or recirculation facilities. The book concerns diseases of finfish and Raising fish in fresh water is an ancient we shall examine those diseases that have practice and the earliest records of relevance to cage and tank culture. Diseases aquaculture date from 2000 BC in China, specific to channel or earthen pond culture although these relate to aquaculture in fresh will not be discussed. warm water (Brown, 1977). The rearing of To avoid excessive repetition of infor- animals in a cold freshwater environment mation given elsewhere, we have defined is a relatively recent phenomenon and dates infectious diseases of cold fresh water as from the 1930s when trout were first raised those that rarely, if ever, occur in water in ponds in Denmark (Shepherd, 1988). whose temperature exceeds 15°C. The Since then, coldwater aquaculture has majority of infectious diseases discussed grown exponentially and in 1996 the global are those that are normally associated with cold freshwater aquaculture production the dominant species cultured in cold fresh including trout, salmon, eels and sturgeon water: trout and juvenile salmonids. Many was in excess of 1.5 Mt (New, 1999). pathogens have been isolated in fish cul- In addition to fish that are cultured tured both in seawater and fresh water and exclusively in fresh water, juvenile for some diseases it was decided that most salmonids are raised in a freshwater cases are seen in fresh water and thus are environment prior to smoltification and in this chapter. Furunculosis, caused by transfer to seawater. Research was first con- Aeromonas salmonicida, is an example, as is ducted on salmonid rearing and salmonid viral haemorrhagic septicaemia. There are disease in enhancement hatcheries, and the other examples where the majority of cases technology and information accrued were are seen in seawater and we have merely later transferred to the commercial cage noted the importance of the disease in this culture industry. chapter. In these cases the reader is referred For the purposes of this chapter, fresh to the relevant seawater chapter. In cases water refers to zero salinity, and diseases where the disease may occur equally in fresh in estuarine environments are included or seawater, the pathogen is discussed in this in Chapter 3. Freshwater fish are reared in chapter when transmission of the agent in water that originates from streams or fresh water either horizontally or vertically rivers, or in groundwater, and maintained is the most important aspect. Bacterial ©CAB International 2002. Diseases and Disorders of Finfish in Cage Culture (eds P.T.K. Woo, D.W. Bruno and L.H.S. Lim) 107 117 Z:\Customer\CABI\A4337 - Woo\A4419 - Woo Vouchers.vp Tuesday, November 05, 2002 11:34:25 AM Color profile: Disabled Composite Default screen 108 L.L. Brown and D.W. Bruno kidney disease, caused by Renibacterium 4 represented by the strain 02-84. Both salmoninarum, is an example. type 3 and 4 were isolated from brown trout (Salmo trutta) (de Kinkelin and Le Berre, 1977; Le Berre et al., 1977; Castric et al., Diseases Caused by Viral Pathogens 1992). The North American strains of VHSV are genetically different from Egtved virus. Viral haemorrhagic septicaemia An attempt to define serogroups that avoided the overlap of strains within and Introduction between these serotypes was undertaken by Olesen et al. (1993). Viral haemorrhagic septicaemia (VHS) is the most serious, systemic, contagious dis- Impact ease of farmed rainbow trout (Oncorhyn- chus mykiss) in Europe. The aetiological VHS still remains a serious threat to fish agent, viral haemorrhagic septicaemia virus culture in Europe and a disease notifiable (VHSV), trout plague or Egtved virus is an to the Office International des Épizooties enveloped RNA rhabdovirus (Jensen, 1963; (OIE). In Denmark, for example, losses in Zwillenberg et al., 1965). Significant and the range of £5–7 million (Sterling) were recurring losses in infected fry are reported, estimated for 1992–1993. Currently, annual although surviving fish are usually immune losses are estimated to be £1.5–2.0 million to reinfection. Disease signs are variable, (N.J. Olesen, 1997, personal communica- but an acute to chronic profuse internal tion). In Italy, losses attributed to VHS in haemorrhaging and pale viscera are typical. coldwater areas have also declined over the last 10 years (G. Bovo, 1997, personal com- Characterization munication). In both countries eradication programmes and improved management are VHSV is a member of the newly accepted contributing factors to the decline of VHSV. genus Novirhabdovirus (cited in Olesen In general, rainbow trout are most et al., 1999) of the family Rhabdoviridae.In susceptible, followed by northern pike Europe five serotypes of VHSV have been (Esox lucius) (Meier and Vestergård- described using infectivity neutralization Jørgensen, 1980). Natural outbreaks occur assays (de Kinkelin, 1988). Type 1 (strain in grayling (Thymallus thymallus) and F1 from Denmark and the most common), whitefish (Coregonus spp.). Freshwater type 2 (Heddedam strain, also from Den- species reported as susceptible to VHSV mark), type 3 (French strain 23/75) and type are summarized in Table 4.1. The most Table 4.1. Susceptible freshwater fish hosts of viral haemorrhagic septicaemia virus (VHSV). Host/species Specific name Reference Atlantic salmon Salmo salar Rasmussen (1965) Brook trout Salvelinus fontinalis Rasmussen (1965) Brown trout Salmo trutta Ghittino (1968) Chinook salmon Oncorhynchus tshawytscha Winton et al. (1989) Coho salmon Oncorhynchus kisutch Winton et al. (1989) Golden trout Salmo aguabonita Ahne et al. (1976) Grayling Thymallus thymallus Wizigmann et al. (1980) Hybrid rainbow O. mykiss × O. kisutch Chevassus and Dorson (1990) trout × coho salmon Lake trout Salvelinus namaycush Ghittino (1973) Pike Esox lucius Meier and Vestergård-Jørgensen (1980) Rainbow trout Oncorhychus mykiss Jensen (1963) Whitefish Coregonus sp. Ahne and Thomsen (1985) 118 Z:\Customer\CABI\A4337 - Woo\A4419 - Woo Vouchers.vp Tuesday, November 05, 2002 11:34:26 AM Color profile: Disabled Composite Default screen Infectious Diseases of Coldwater Fish in Fresh Water 109 pathogenic strain of VHSV is type 1, with necrosis and increased pyknotic and karyo- outbreaks in freshwater fish occurring at lytic nuclei. Ascites is present and food is 14°C or below (Meier and Vestergård- absent from the gastrointestinal tract. Clini- Jørgensen, 1979; Meier et al., 1986). cal signs in pike fry are generally similar to those described in trout, with the exception Geographical distribution of extravasation or bloody swelling, deposi- tion of blood in the muscle and pancreatic VHS was first recorded in the 1930s and is necrosis (Meier and Vestergård-Jørgensen, now present in farmed trout throughout 1980). It is possible that these signs repre- most of Western Europe (Wolf, 1988). Out- sent a later stage of the infection. breaks of VHS have been reported in farmed The chronic phase is correlated with turbot in France and Germany (Schlotfeldt lethargy, dark skin colour and bilateral et al., 1991), Scotland (Ross et al., 1994) and exophthalmia. A haemorrhagic anaemia Ireland (J. McArdle, 1997, personal com- is reported with a markedly distended munication). The isolation of VHSV from abdomen due to oedema in the liver, spleen returning adult chinook salmon (Onco- and kidney. In chronically infected fish the rhynchus tshawytscha) and coho salmon (O. liver sinusoids remain congested, with some kisutch) in the Puget Sound area and Gulf of hyperplasia of the haematopoietic tissue. Alaska, North America (Eaton and Hulett, During the latent infection, or nervous 1990), was the first indication there was a phase, there is low mortality and some fish marine source of VHSV, possibly involving are hyperactive, but otherwise they appear one or more fish species. Currently VHSV normal. has been isolated from an increasing num- ber of marine fish species (Meyers et al., Diagnostic techniques 1992; Ross et al., 1994; Meyers and Winton, 1995; Dixon et al., 1997). The use of molec- Cell culture. A number of serological tech- ular techniques including T1 nuclease niques are used to tentatively identify fingerprinting and sequencing analyses VHSV; however, definitive diagnosis (Oshima et al., 1993; Stone et al., 1997; requires culture of the virus in an Mortensen et al., 1999; Smail, 1995; 2000) established susceptible cell line. A variety of has confirmed that these North American established fish cell lines are susceptible to isolates are distinct from those found in VHSV and the most sensitive for freshwater Europe. The significance of these findings isolates from rainbow trout are bluegill fry to farmed salmonids and European Union (BF-2) (Olesen and Vestergård-Jørgensen, (EU) fish health legislation (Directive 1992). Other cell lines suitable for VHSV 91/67/EEC) is reviewed in Chapter 3. detection include chinook salmon embryo (CHSE-214), epithelioma papulosum Clinical signs cyprini (EPC), fathead minnow (FHM), pike gonad (PG) and rainbow trout gonad (RTG-2) VHS occurs as an acute to chronic disease. (Wolf, 1988; Olesen and Vestergård- The acute phase is associated with a rapid Jørgensen, 1992). Biopsy samples or sex onset of heavy mortality. Fish are lethargic, products are homogenized, diluted and cen- anaemic, show dark body colour and trifuged to remove cell debris, filtered and exophthalmos, and occasionally an inter- the material added to a growth medium in mittent period of erratic spiralling. Haemor- flasks containing a tissue culture cell mono- rhaging may be evident in the ocular tis- layer. The cytopathic effect (CPE) results in sues, skin and the viscera, including the pronounced cell shrinkage with some cell skeletal muscle and intestinal submucosa.