Color profile: Disabled Composite Default screen 3 Infectious Diseases of Coldwater Fish in Marine and Brackish Water Michael L. Kent1,* and Trygve T. Poppe2 1Department of Fisheries and Oceans, Biological Sciences Branch, Pacific Biological Station, Nanaimo, British Columbia V9R 5K6, Canada; 2Department of Morphology, Genetics and Aquatic Biology, The Norwegian School of Veterinary Science, PO Box 8196 Dep., N-0033 Oslo, Norway Introduction transferred with them to sea cages. Brown and Bruno (Chapter 4) deal with these Salmonids are the primary fishes reared in freshwater diseases, and our emphasis is cold seawater netpens. This component of infectious diseases that are contracted after the industry produces approximately transfer to sea cages. − 500,000 t year 1 on a worldwide basis. The principle species reared in netpens are Atlantic salmon (Salmo salar), coho Viral Diseases salmon (Oncorhynchus kisutch), chinook salmon (Oncorhynchus tshawytscha) and Several viruses are important pathogens of rainbow trout (Oncorhynchus mykiss). salmonid fishes, particularly during their Additional species include minor produc- early development in fresh water (Wolf, tion of Arctic char (Salvelinus alpinus), 1988). Viral diseases of fishes have histori- Atlantic cod (Gadus morhua), haddock cally been of great concern to fish health (Melanogrammus aeglefinus), Atlantic managers because they can cause high mor- halibut (Hippoglossus hippoglossus) and tality. In addition, the presence of certain Atlantic wolffish (Anarhichas lupus). The viruses in a fish population causes eco- purpose of this chapter is to review the most nomic hardships to fish farmers due to important infectious diseases affecting fish restrictions on transfer or sale of these fish. reared in cold seawater netpens. At least six viral diseases are of concern for The problems in controlling water pen-reared salmon: these include infectious flow make it difficult, if not impossible, to haematopoietic necrosis (IHN), infectious exclude pathogens in the water column from pancreatic necrosis (IPN), salmon pancreas netpens. Therefore, marine pathogens are disease (SPD), infectious salmon anaemia among the most important causes of diseases (ISA), salmonid herpesvirus 2 infections in seawater netpens. As salmonids are and erythrocytic inclusion body syndrome reared in fresh water before they are held (EIBS). The erythrocytic necrosis virus has in seawater, freshwater pathogens may be the potential to infect salmon in seawater, * Current address: Center for Fish Disease Research, Department of Microbiology, Oregon State University, Corvallis, OR 97331, USA. ©CAB International 2002. Diseases and Disorders of Finfish in Cage Culture (eds P.T.K. Woo, D.W. Bruno and L.H.S. Lim) 61 71 Z:\Customer\CABI\A4337 - Woo\A4419 - Woo Vouchers.vp Tuesday, November 12, 2002 3:08:47 PM Color profile: Disabled Composite Default screen 62 M.L. Kent and T.T. Poppe but has yet to be recognized as a serious are much larger than their counterparts in problem. At least one form of plasmacytoid fresh water, IHN in Atlantic salmon reared in leukaemia is associated with a retrovirus, seawater is usually chronic. Infected salmon but this disease is described under the also often exhibit frank haemorrhages in the section on Nucleospora salmonis. Cardiac visceral cavity. As with the freshwater form myopathy syndrome (CMS) has recently of the disease, IHN is best managed by avoid- been associated with a nodavirus, and is ance. There is circumstantial evidence to thus covered in this section. IHN, IPN and suggest a marine reservoir is the primary EIBS are also important diseases in fresh source of the infection for outbreaks in sea- water, and are dealt with in more depth water netpens. If this were the case, then in Chapter 4. However, the manifestations avoidance of the infection in netpens would of IHN and IPN as they occur in seawater be very difficult. Marine-phase chinook are described. Furthermore, viruses have salmon may harbour the virus for several caused disease in farmed Atlantic halibut months with no signs of the disease, and the and turbot larvae, resulting in considerable virus has been found in healthy chinook losses (Bloch et al., 1991; Grotmol et al., reared at netpen farms that have experienced 1995; Wood et al., 1996). However, to date, IHN outbreaks in Atlantic salmon (St-Hilaire specific viral diseases of non-salmonids et al., 2001). Therefore, chinook salmon may reared in coldwater sea cages have not been act as a subclinical reservoir for the virus identified as serious problems. Vacuolating when they are reared with Atlantic salmon. encephalopathy and retinopathy (VER) is a disease primarily of larval or juvenile marine fishes, but may be carried into Infectious pancreatic necrosis cage-cultured flatfish. This nodavirus is dealt with in Chapter 9. This is a freshwater viral disease (see Chapter 4) that causes mortality in sea cage culture. The infection is prevalent in Infectious haematopoietic necrosis in netpens pen-reared Atlantic salmon in Norway (Krogsrud et al., 1989). For many years, The first confirmed report of IHN in pen- the virus was extremely widespread in reared Atlantic salmon occurred in 1992 in Norwegian sea sites (Melby et al., 1991) British Columbia (Armstrong et al., 1993; without causing clinical disease. However, Traxler et al., 1993). Since this initial find- in recent years clinical IPN has impacted ing, IHN has been reported at many netpen on sea-farmed Atlantic salmon culture and farm sites and has become a major disease is considered as an emerging problem in concern on Atlantic salmon farms in the Chapter 9. Most outbreaks occur from a Quadra Island region of British Columbia. few weeks up to a couple of months after The existence or establishment of marine transfer to seawater (Jarp et al., 1994), but hosts or reservoirs of IHN virus (IHNV) that outbreaks up to 1 year after transfer to sea- may serve as sources of the virus at sea water may also occur (Smail et al., 1992, cage sites, and the reports of virus in 1995). IPNV may be the most important non-salmonids around netpens during an infectious disease of farmed fish in Norway, outbreak is of concern (Traxler and Richard, accounting for losses of approximately − 1996). In addition, IHNV has been found in Nkr 400 million year 1 (Christie, 1996). Pacific herring (Clupea herningus pallasi) In Scotland, significant mortality has been collected well away from infected farms associated with the infection, particularly (Kent et al., 1998). in combination with SPD. Clinical signs of IHN in Atlantic salmon Clinical signs of IPN in post-smolt in netpens are often similar to those seen Atlantic salmon may be minimal, but in infected salmonids in fresh water (see some fish stop feeding and show nervous Chapter 4). However, because affected fish distress. The most significant losses may 72 Z:\Customer\CABI\A4337 - Woo\A4419 - Woo Vouchers.vp Tuesday, November 05, 2002 11:34:04 AM Color profile: Disabled Composite Default screen Infectious Diseases of Coldwater Fish in Salt Water 63 sometimes be attributed to the long-term Elston, 1987), and rarely in British effects of reduced or completely ceased feed- Columbia. ing. Fish show hyperaemia and petechial A toga-like virus has been isolated from haemorrhage in the visceral fat and in the fish with SPD (Nelson et al., 1995), and pyloric caeca. McLoughlin et al. (1996) experimentally An epizootiological study of IPN in reproduced the disease with the virus. post-smolts has shown that the risk of Therefore, the evidence is essentially con- clinical disease was related to the mixing clusive that the cause of pancreas disease is of smolts from several suppliers at the same this virus, referred to as salmon pancreas sea site (Jarp et al., 1994). A way to avoid disease virus (SPDV). SPD is contracted after this would therefore be to buy smolts from smolts are transferred to seawater. Although as few producers as possible. Smolts with not specifically associated with fresh water no history of IPN in fresh water, but with (McVicar, 1987), a similar toga-like virus has specific humoral immunity against IPNV been isolated from rainbow trout (Boucher prior to smoltification, were protected et al., 1994). against clinical IPN for up to 4 months after Fish usually exhibit clinical signs of transfer to seawater (Jarp et al., 1996). SPD about 6–12 weeks after introduction to Because IPNV can be vertically trans- netpens, but fish that have been in pens for mitted and infected fish can excrete virus as long as 2 years may be affected (McVicar, for the rest of their life, the only effective 1987). Mortality associated with the disease control method is avoidance. The use of is low. Nevertheless, surviving fish often IPNV-free broodstock, rearing progeny in show poor growth and are more susceptible virus-free water and restricting the move- to other diseases (McVicar and Munro, ment of fish are measures that can reduce 1987). the spread of IPNV. A multivalent vaccine, which includes Escherichia coli-expressed Clinical and gross pathological changes. IPNV proteins, protects pre-smolt Atlantic Affected fish are anorexic, dark in colour, salmon against natural exposure to IPN lethargic, and usually at the sides of cages (Christie, 1996). This vaccine is now and near the surface. Fish with SPD are licensed in Norway, and results from the usually emaciated (Fig. 3.1). Internal signs 1996 season were promising, as mortalities include haemorrhage in the pancreatic due to IPN were reduced considerably. tissue and fat between the pyloric caeca, or the tissue between the pyloric caeca may be severely atrophied. Salmon pancreas disease Diagnosis. Haemorrhage in tissues associ- SPD of pen-reared Atlantic salmon is an ated with the pyloric caeca in emaciated important disease in Scotland, Ireland and Atlantic salmon smolts, along with the Norway (Munro et al., 1984; Ferguson et al., absence of other infectious agents (e.g. IHN 1986; McVicar, 1987; Menzies et al., 1996).