Clinical screening for cataracts in rainbow trout and non-salmonid fish in traditional fresh water pond farming

Routine screening of rainbow trout and other non-salmonid fish species from traditional pond farms in Lower Saxony, Germany, indicated a low incidence of cataract and other optical changes. The cataract problem and more severe eye changes, although present in freshwater fish farming, seem to be less prevalent than in marine production of Atlantic salmon. Cataracts have been a subject of increasing concern since 1992 in farmed Atlantic salmon in Norway, Ireland and Scotland with significant economic & ethical impact (Wall & Richards,1992; Bjerkås et al.,1996; Waagbo et al.,1996; Wall,1998; Midtlyng et al.,1999; Bjerkås and Bjørnestad, 1999). The term cataract summarizes all types of opacifications of the lens, which might involve also the lens capsule (Hagis, 1991). Cataracts do have, however, a diverse morphology and location in the lens, both being indicative of causative factors and period of development. The diversity of fish cataract types, their location and light scattering density in farmed Atlantic salmon recently has been demonstrated by Wegener et al. (2001, in press). In fish, cataracts are considered as a living (farming) condition related disease of explicitly multifactorial origin (Elliott and Hurley,1997; Koskela et al.,1997; Bjerkås et al.,1996; Waagbø et al.,1996; Wall,1998; Bjerkås and Bjørnestad,1999). This assumption is strongly supported by knowledge gathered in studies on the factors and mechanisms of cataract in mammals and humans (Müller-Breitenkamp and Hockwin, 1991). In view of the increasing concern about cataracts in Atlantic salmon, the present authors wish to address the question of similar findings in traditional fresh water pond farming of rainbow trout, brown trout and in some nonsalmonid fish. The investigations summarized here were performed within the frame of a multidisciplinary and multinational initiative (FAIR CT 97-3963) focused on the elucidation of causes for fish cataract development and possible solutions to this problem (Bucke,1998). Bull. Eur. Ass. Fish Pathol., 20(6) 2000, 253 Rainbow trout of three EU-approved pond farms (in non-approved zones, regarding to freedom of list II diseases(VHS-IHN) Directive 91/76 EEC) in Lower Saxony were examined on the spot with a slit-Iamp in Spring 1999. The pond water temperature in Lower Saxony in May fluctuates between 13 and 18°C. Fish were caught by netting them directly from the ponds and immobilised for a moment as it is done routinely for stripping. In the hands of an expert ophthalmologist the slit-Iamp technique needs only a few seconds to evaluate the eyes on both sides of the fish. Thus the stress was kept at a minimum. The results of this slitIamp supported screening are summarised in Table 1. Additionally, during 1998 and 1999 special attention was paid to possible eye changes within the daily casuistic of fish samples received by the Fish Health Service of Lower Saxony. With an average of 600-700 cases (fish groups)/year of many species of fish this casuistic is considerable and varied (Schlotfeldt et al.,1991). In this case the observation was exclusively clinical and not slitIamp supported. Thus these observations were focused on lens and corneal opacities, The results are summarised in Table 2. The lenses of the cases shown in Table 1 were frozen, those of Table 2 fixed in glutaraldehyde/paraformaldehyde (1.5% / 1.5%) in phosphate buffered saline (PBS) and sent for further histochemical examination and crystalline pattern determination. These results will be published elsewhere. Only two cases of genuine cataracts in 2 out of 90 fish and this in rather older > 1000 g rainbow trout were found using the slit lamp technique (Table 1). There was a predominance of cornea opacities and chamber bleedings, which may be the consequence of fish handling, grading, etc. All three facilities are wire-covered and that because of parasitic problems (diplostomiasis) were not recorded. Table 1. Clinical Findings (eye changes: unilateral or both-sided) rainbow trout m r a F / h s i f f o . o N ) g ( t h g i e w n a e m s t c a r a t a C y t i c a p o s n e L a e n r o C y t i c a p o n o i s o r E a e n r o C r e b m a h C g n i d e e l B I 1 . 1 0 0 0 2 / 0 1 0 0 0 0 0 2 . 1 0 5 3 / 0 1 0 0 ) l a r e t a l i n u ( 1 ) l a r e t a l i n u ( 1 , l a r e t a l i n u ( 1 ) r e b m a h c r o i r e t n a 3 . 1 0 8 1 / 0 1 0 0 ) l a r e t a l i b ( 1 0 , l a r e t a l i n u ( 1 ) r e b m a h c r o i r e t n a I I 1 . 2 n w o r b ( 0 0 3 1 / 2 ) t u o r t 0 0 0 0 0 2 . 2 0 5 5 / 4 0 0 ) l a r e t a l i n u ( 1 0 , l a r e t a l i n u ( 1 ) r e b m a h c r o i r e t n a 3 . 2 0 0 0 1 / 2 0 0 0 0 0 4 . 2 0 0 0 4 0 0 4 / 5 2 ) l a r e t a l i n u ( 1 l a r e t a l i n u ( 1 ) r a l o p r o i r e t n a h s i f g K 5 r y 3 1 , l a r e t a l i n u ( 3 t n e m g i p h t i w ) t i s o p e d 0 , l a r e t a l i n u ( 2 ) r e b m a h c r o i r e t n a I I I 1 . 3 0 5 3 / 2 1 0 ) l a r e t a l i n u ( 1 0 h t i w , l a r e t a l i n u ( 1 ) t i s o p e d t n e m g i p 0 2 . 3 0 0 1 1 / 5 1 , l a r e t a l i n u ( 1 ) l a c i t r o c 0 0 0 0 l a t o T h s i f 0 9 2 2 7 2 5 Bull. Eur. Ass. Fish Pathol., 20(6) 2000, 254 In other species examied (Koi, Rudd, goldentench, smelt and Arcytic charr) of very hetero-geneous origin, diplostomiasis was a commonfinding and the main cause of lens opacities(Table 2). Corneal opacity was a prevalent find-ing in Koi, a genuine pet animal which is of-ten overhandled. The results of the histochemical examinationas well as crystalline pattern determinationwill be of great interest to evidence differencesbetween the simple clinical findings presentedhere and the details shown by histochemistryeven up to intra-species differences. In conclu-sion, in fresh water fish farming, the cataractproblem and/or heavier eye changes seem tobe far from the importance and the impactwhich they have in Atlantic salmon in sea pro-duction facilities.ReferencesBjerkaas, E., Waagbo, R., Sveier, H., Breck, 0.,Bjerkaas, I., Bjornestad, E. and Maage, A.,(1996). Cataract development in Atlanticsalmon (Salmo salar) in freshwater. Acta Vet.Scand. 37,351-360 Bjerkås, E. and E. Bjørnestad (1999). Is there aconnection between rapid fluctuation in wa-ter temperature and cataract development inthe Atlantic salmon (Salmo salar L)? Bull. Eur.Ass. Fish Pathol. 19 (4),166 Elliott, J.M. and Hurley, M.A (1997). A func-tional model for maximum growth of Atlanticsalmon parr, Salmo salar, from two populationsin north-west England. Functional Ecology 11,592-603 Bucke, D. (1998). Cataracts in farmed fish AMulti-disciplinary initiative for scientificprogress: Histological techniques for Teleosteyes. Bull. Eur. Ass. Fish Pathol. 18 (4),121-123seicepshsiFthgiew/hsif.oNyticaposnelyticapoaenroc ioKg52/5)laretalinu(1 ioKg022/1)laretalinu(1 ioKg006/2)laretalinu(1 ioKg002/3)laretalinu(1 ioKg055/4/dedishtob(2)sisaimotsolpiDhcnetnedloGg52/5)laretalinu(1 dduRg07/4/dedishtob(2)sisaimotsolpiDtlemSg51/01)laretalinu(1)dedishtob(2rrahccitcrAg53/52)dedishtob(1 latoThsif9458 Table 2 Clinical Findings (eye changes: unilateral or both-sided) in fish other than Rainbow trout 1998/1999 Bull. Eur. Ass. Fish Pathol., 20(6) 2000, 255 Hargis, W.J. (1991) Disorders of the eye infinfish. Ann. Rev. Fish Dis. 95-117 Koskela, J., Pirhonen, J. and Jobling, M. (1997)Feed intake, growth rate and body composi-tion of juvenile Baltic salmon exposed to dif-ferent constant temperatures. Aquaculture In-ternational 5, 351-360 Midtlyng PJ, Ahrend, Bjerkås E, Waagbø R andWall T (1999). Current research on cataracts infish. Bull. Eur. Ass. Fish Pathol. 19 (6),299-301 Müller-Breitenkamp U, Hockwin O (1991) Riskfactors in cataract development – A review. In:Sasaki K, Hockwin O (eds): Distribution ofCataracts in the Population and InfluencingFactors. Dev. Ophthalmol., Basel, Karger, 1991,vol. 21, pp 60-65 Schlotfeldt, H.-J., Herbst, J., Rosenthal, H.,Alvarado, V., Stanislawski, D. and Böhm, K.H. (1991) Freshwater fish production and fishdiseases in Lower Saxony (Federal republic ofGermany): Fish Health Service experience. J.Appl. Ichthyol. 7 (1991) 26-35Waagbø R., Bjerkås E, Sveier, H., Breck, O.,Bjerkås, I., Bjørnestadt, E. and Maage, A. (1996).Nutritional status assessed in groups of smelt-ing Atlantic salmon, Salmo salar L., develop-ing cataracts. J. Fish Dis. 19, 365-373 Wall A.E. and Richards R.H. (1992). Occurrenceof cataracts in triploid Atlantic salmon (Salmosalar) on Four Farms in Scotland. Vet.Record131, 553-557 Wall A.E. (1998). Cataracts in farmed Atlanticsalmon (Salmo salar) in Ireland, Norway andScotland from 1995 to 1997. Vet.Record 142,626-631 Wegener A, Laser H, Ahrend M. H. J, Breck O,Bjerkås E, Glöckner C, Midtlyng PJ, BreipohlW (2001). Light scattering in normal and cata-ractous lenses of farmed atlantic salmon(Salmo salar): A slitlamp and Scheimpflugphotographic study. Ophthalmic Res. (in press)