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Surveillance Vol.17 No.2 1990 Important diseases of salmonid fish and the risk they pose to New Zealand Thc economic well-bciiig and future developi n r i i t of Nezu Zraland's rt~crentioizal aiid co~iimrrcialfishcric.5and aqzinciilt U re depeiici 011 l i d t h y f i s h stocks. Interiiatioiial trade iii fish stocks nizdfish ~ ~ r o d iposes ~ t s a potential risk ill so far a s snch trade could introdiiix~ s r ~ i o i i sfish dismsrs iiito N e w Zmland's ri1i7tc~zc~ays.Once established, siich diseases conld he inipossihl~to control or eradicatr. 171 this article Colin Atiderson, vctcririarinii niid fish ptliuloSist, examiiics the iniportant dis~'asi'sujsalinoiiid fish and the risks thy pose. The major diseases of salmonid fish are listed in Table 1. New Zealand is in the fortunate position of having only two of these agents, MyXCJbCJlL4s c-iwhrnlis and a Birna virus of the I I " group. Table 1: Important diseases of salmonid fish: Viral diseases Infectious haematopoietic necrosis (IHN) Viral haemorrhagic septicaemia (VHS) Infectious pancreatic necrosis (IPN) Piscine erythrocytic necrosis (PEN) Herpesvirus salmonis Bacterial diseases Bacterial kidney disease (BKD - Rmehacrerium salmoninarumj Aeromonus salmonicida (Furunculosis) Vibriosis (Vibrio ordalii, V . an,quillarum) Protozoal diseases Mxoholus cerebralis (Myxosonia cerehrulis - whirling disease) Proliferative kidney disease Ceraiomy.ra shasta Henneguya sulmoniida Parvicapsula disease Fungal diseases Rosette disease M!yxoboliis c-cwhralis (whirlingdiseasc was first reported in New Zealand from a rainbow trout sport fish hatchery (Acclimatisation Society hatchery) near Dunedin in the South Island of New Zealand in 1971.' Clinically, disease has been observed once since then, again in rainbow trout reared at a government hatchery near Christchurch in the South Island in 1980. The parasite has also been identified in routine examination of 'healthy' wild salmonids in a number of locations in the Canterbury and Otago provinces of the South Island. The Birna virus, the AB serotype, was first isolated from healthy adult Quinnat salmon (Chinook salmon, Oncorhynclzus tsl~arcytsclia)in 1985.' Two further isolations were made i n 1989 from the same class of stock. N o clinical disease has been seen, nor has the virus been isolated fromany other classofstockon the twenty salmonid farms which have been regularly inspected and tested for export certification purposes over the last five years. The economic significance of salmon disease The most important disease for salmon farmers in New Zealand is vibriosis. This disease has a world-wide distribution and affects other marine species including shellfish and prawns. On New Zealand salmon farms, vibriosis appears in the warm water temperatures of midsummer and is not usually seen on an individual property every year. The disease is frequently restricted to salmon in their first year in seawater and these outbreaks are adequately controlled with vaccination and short treatments with antibiotic medicated feed. Vibriosis and whirling disease have little economic impact on New Zealand salmon farmers. Losses due to vibriosis involve usually only 5-10% of the least valuable year class on the property. The presence of Myxobolus cerebralis has resulted in mild constraints to the sales of live fish within New Zealand and the provision of well water supplies to the hatchery building on the three farms involved. In contrast, salmonid farmers and sport fish hatcheries overseas have to contend with a large number of serious diseases. In the USA, the IHN virus has devastating effects in both hatcheries and wild populations of salmon.'f4 The bacterial diseases, furunculosis and BKD, cause significant mortalities in fresh water salmon culture, but when affected fish are exposed to the stress of transfer into sea water, the smolts often suffer heavy mortalities which are difficult to control. The costs are considerable in terms of replacement smolts, wasted labour, delayed marketing and medicated feeds (Canadian $70,000 to treat a 200 tonne farm for the usual 10 days).s Reduced marine survival of BKD infected wild smoltshreducessalmon returns and effects sport fishermen and businesses involved in the angling industry. In the USA and other Northern Hemisphere countries, singificant national funding is being spent on researching means of controlling salmonid diseases (especially IHN and BKD in the USA). VHS is a salmonid viral disease of particular economic significance as it often causes high mortalities in valuable market-size fish. The desire of national salmon fishing industries to avoid the debilitating effects of these salmon diseases on their native fishand their commercial and sport fish industries has resulted in regulations to control internal and international movements of fish. These have their associated bureaucratic and technical costs for the respective governments and farming i n d ~ s t r i e s .Governments ~.~ have also imposed restrictions on the types of products they will import. New Zealand and Australia only allow the entry of canned or 'hot smoked' salmon, because the required heat exposures are believed to kill fish pathogens in the salmonid flesh. The stipulated temperature-time requirements may not meet the marketer's ideal for a high quality product and could effect sales. It is essential for the well-being of the sport fish, native fish, and aquaculture industries of New Zealand that the wide ranging detrimental effects of these exotic fish diseases are avoided by preventing their introduction. The possible means by which fish disease could be introduced are many and varied. Live salmonids and their eggs pose the greatest risk and these would be required to undergo secure quarantine and rigorous disease testing in New Zealand before release. There is growing evidence that other fish species can carry, and be affected by, 'salmonid' diseases and vice versa. Cold water species are especially important. Ornamental gold fish have introduced a strain of Aerornonas salrnonicida which is highly virulent to Atlantic salmon (Salrno solar) into A ~ s t r a l i a . ~ Imported ornamental fish should be subjected to quarantine and disease examination appropriate to the risk they pose. Many fish diseases are preserved in fish tissue by chilling and freezing'"," and the disease agents could remain viable in table fish imported into New Zealand. Discarding uncooked imported salmon near waterways, or places where feral animals could mechanically transfer diseased material to waterways, feeding imported salmon flesh to aquarium gold fish or salmonids which are subsequently liberated into waterways (asoften occurs when they get 'sick') are some of the possible means of transferring exotic fish pathogens to New Zealand fish. It is important to note that fish diseases commonly thought of as salmonid diseases are not necessarily restricted to salmonids. VHS has been experimentally transmitted via a water borne route to turbot, a marine flatfish (Scophthalrnus and has caused disease outbreaks in a number of fish species including pike (Esox lucius).'3 PEN, a viral disease of the erythrocyte, causes mortalities in a number of fish species including salmon, herring and cod.'4 Visitors and imigrants also pose some risk. MAFs Border Protection Officers examine tourists and goods moving through our ports to prevent fish diseases gaining entry on equipment or in products. Ongoing surveillance, with the aim of early detection of introduced fish diseases, is also important. MAF officers routinely visit fish farms to look for clinical evidence of disease, and laboratory testing is performed for salmonid export certification and on diseased fish submitted from fish farms and by the public. Surveillance I7(2) 17 Surveillance Vol.17 No.2 1990 There would probably be a long lag time from the introduction of an exotic disease until it reached a high prevalence in the population. BKD was introduced by transfer of infected Pacific salmon to the Great Lakes, and it has taken 20 years for its effects to become evident.15 This, time lag together with the remote nature of our waterways, the extreme difficulties involved in completely depopulating an infected waterway by fish poisoning, and the impossibility of disinfection of even a small stream, means that preventive measures to exclude exotic fish diseases need to be stringently applied. The establishment of an exotic disease of salmonids could be devastating for New Zealand's freshwater fisheries References: Hewitt, G C, Little, R W, 1972: Whirling disease in New Zealand trout caused by My.rosonia cerebralis (Hofer 1903)(Protozoa : Myxosporida). Nenj Zealand Journal of Marine and Fresh Water Research 6 (1&2) : 1-10. Tisdall, D, Phipps, J, 1987: Isolation and characterisation of a marine bimavirus from retuming Quinnat salmon (Oncorhync,hustshowtsc.huj 18 Surveillance 1 7(2) 3 4 S 6 7 8 9 IO in the South Island of New Zealand. New riuai.y Journal 35 : 2 17-2 18. Mulcahy, D, Burke, J, Pascho, R, Jenes, C K, 1982: Pathogenesis of infectious hematopoietic necrosis virus in adult sockeye salmon (Oiicnrliyuchus nerka). Canadian fournu1 of Fish and Aquatic Scienw 39 : I 144-1 149. Pilcher, K S, Fryer, J L, 1980: The viral diseases of fish : a review through 1978, Part I : diseases ofproven viral etiology. CRC Critiwl Rei.ims in Mii,robiolocqy7 : 287-363. Brackett, J, 1989: Industry sets own guidelines for drug use in BC. Fish Farmer, November/ Dvwmher. issue : I I . Fryer, J L, Sanders, J E, 198 I :Bacterial kidney disease of salmonid fish. Annual R e ~ i e n , sin Mic.rohiology SS :273-298. Amos, K H, (editor), 198.5: Procedures for the detection and identification ofcertain fish pathogens, 3rd Edition, Fish Health Section, American Fisheries Society, Corvallis, Oregon. Department of Fisheries and Oceans, 1984: Fish Health Protection Regulations : nianual of.oniplianc,e. Fish Marketing Service Miscellaneous Special Publication, 31 (revised) 43 p. Carson, J, Hanlinger. J, 1988: Virulence of the aetiological agent of goldfish ulcer disease in Atlantic salmon, Sulnio solar L . Journul qfFish Diseases I I: 47 1-479. McCarthy, D H, 198.5:Some ecological aspects II 12 13 14 IS of the bacterial fish pathogen Acroniorras salnioiiicida. In Fish and Shrl/fish Pathology, editor A E Ellis, Academic Press, London, 299324. Pietsch, J P, Amend, D F, Miller. C M, 1977: Survival of infectious haematopoietic necrosis virus under various environmental conditions. Journal of the Fisheries Research Board of Canada 34 : 1360-1364. Castric, J, De Kinkelin, P. 1984: Experimental study of the susceptibility of two marine fish species, sea bass (Dicentrarchus /ohra.uj and niusinius) to viral haemturbot (Si~oiphthu1niu.s orrhagic septicaemia. Aqituc~ulture4 / : 203212. Meier, W, I98 1 :Viral haemorrhagic septicemia in non-salmonid fishes. Bulletin ofthe European A.tsoi,iatiori of'Fish Potho/o,~istsI : 1517. Reno. P W, Serrere, D V, Hellyer, S K, 198.5: Haematological and physiological effects of viral erythrocytic necrosis (VEN) in Atlantic cod and herring. Fish Puthdogy 2IJ :353.360. Kitchell, J, 1989: Chinook salmon declining in Lake Michigan. Aquaidtui-c, Magazine, S i p tenrher-Oc.tohei. : 12- 1.5. Colru Anderson Central Animal Health Laboratory