Spinal deformities in triploid all-female rainbow trout (Oncorhynchus mykiss)

A batch of experimental rainbow trout was found to have a high level of spinal deformities. An equal deformity level was found in fish from the same batch, but reared at the fish farm from where the fry originated, suggesting that the all-female triploid status of the rainbow trout might account for the high level of deformity. Many factors being of physical, chemical, environmental or genetic origin, but also malnutrition and exposure to infectious organisms may give rise to abnormal skeletal structure in fish (McCrimmon & Bidgood, 1965; Aulstad & Kittelsen, 1971; Poynton, 1987; Sandnes, 1991; Brown & Núñez, 1998; Vågsholm & Djupvik, 1998; Bæverfjord & Lein, 2000). More than one factor is often involved (Brown & Núñez, 1998), and cultured fish are particularly susceptible to developmental deformities, through exposure of developing fish to a variety of suboptimal environmental conditions (Brown & Núñez, 1998). A man-made application associated with abnormal skeletal structure is the antibiotic oxytetracycline (OTC), which has been linked to fracture of the spinal column (Toften & Jobling, 1996; Høie, 1999). Rainbow trout fry weighing 0.5 g were purchased from a Danish fish farm and acclimatised at our laboratory. At the weight of 1 g the fry were used in an oxytetracycline treatment experiment. Fish were kept in 200 l tanks, each of which had individual recirculation of water (13-18∞C) and an airsupply. Stocking densities as well as treatments of the groups are shown in Table I. The fish were fed dry commercial pellets (Ecostart 17, BioMar A/S) according to appetite. The medicated feed comprised the same pellets coated with oxytetracycline. The experiment ran for 8 months and was terminated by euthanizing the fish with an overdose of MS222 (Sigma). The trout were radiographed according to Madsen & Dalsgaard (1999). The location of spinal deformities was recorded. Deformity percentages were found to be high (Table I). Replicates were not statistically different from each other (P>0.05, Chi-square test), and when results in the different groups were compared statistically, significance was Bull. Eur. Ass. Fish Pathol., 20(5) 2000, 207 only found between OTC medicated fish and control fish (control fish had the highest deformity percentage). The severity of deformities (i.e. more than 10 affected vertebrae) was very low (0 % to 6 %) in all groups apart from one control group where the level was 19 %. More than 75 % of the deformities in each group were located in the cranial region of the vertebral column (data not shown). A radiograph of two fish, one with no deformities and one with deformities located in the cranial region of the spinal column, is shown in Fig. 1. The comparable levels of deformities in controls and in treated fish in our study indicated that the investigated medication quantities of oxytetracycline did not affect the spinal column in rainbow trout. The high levels of deformities found among all groups in this study were consistent with findings among fish from the same batch reared at the fish farm from where the fry originated. At the farm, 20-25 % of the fish had been discarded because of externally visFIG. 1. Radiograph of two fish from the group medicated with double OTC dose. Upper fish: no deformities of the spinal column. Lower fish: vertebrae 11-13 are fused. TABLE I. Prevalence of vertebral column deformities in rainbow trout treated with oxytetracycline (OTC). OTC =100 mg OTC/kg fish/day for 10 days. 2 x OTC = two treatments with the above dosage, 10 days between treatments. Double OTC = 200 mg OTC/kg fish/day for 10 days t n e m t a e r T h s i f f o . o N p u o r g r e p t a t h g i e W n a e M ) g ( y a R X h t i w h s i f f o o N s e i t i m r o f e d l a r b e t r e v C T O 3 0 2 7 8 1 2 6 8 4 ) % 3 . 5 1 ( 1 3 ) % 4 . 3 1 ( 5 2 C T O x 2 3 5 1 0 6 1 3 6 1 4 ) % 9 . 0 2 ( 2 3 ) % 1 . 3 1 ( 1 2 C T O e l b u o D 9 9 1 9 9 1 4 5 3 3 ) % 1 . 5 1 ( 0 3 ) % 1 . 8 1 ( 6 3 l o r t n o C 1 8 1 1 0 2 7 5 8 4 ) % 5 . 1 2 ( 9 3 ) % 4 . 1 2 ( 3 4 Bull. Eur. Ass. Fish Pathol., 20(5) 2000, 208 ible deformities, which is within the range shown in Table I regardless of treatment. A dietary cause (i.e. deficiency in vitamin C in the first feeding diet) resulting in high levels of deformities was rejected, as other offspring than the batch of experimental fish were fed the same diet at the farm in the same period, and those fish did not show spinal deformities. A genetic cause for the deformities could not be excluded, but when the experiment was terminated it came to our knowledge that the fish originated from a batch of eggs that had been treated to develop all-female triploids, and it was suggested that a wrong step in this process had resulted in the higher levels of vertebral/spinal deformities. The results from our study also suggested that such deformities were primarily located in the cranial region of the spinal column. A higher incidence of deformities in triploid rainbow trout compared to diploids has been found in other studies (J. Forster, personal communication). The technical assistance of Kirsten Kaas, FishDisease Laboratory, Danish Institute for Fish-eries Research and Bent Rye Nilson, Radiol-ogy, Department of Clinical Studies, TheRoyal Veterinary and Agricultural University,is greatly appreciated. ReferencesAulstad, D. & Kittelsen, A. (1971). Abnormalbody curvatures of rainbow trout (Salmogairdneri) inbred fry. J. Fish. 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