Marine Fish and Shellfish

Rapp. P.-v. Réun. Cons. int. Explor. Mer, 182: 106—110. 1983. Spring ulcer disease in eel (Anguilla anguilla) N. Juul Jensen1, J. L. Larsen, and N. O. Christensen1 Laboratory of Aquatic Pathobiology Ambulatory Clinic and Clinical Central Laboratory Royal Veterinary and Agricultural University Biilowsvej 13, DK-1870 Copenhagen V, Denmark In eels two types of diseases characterized by lesions of the skin have been described. Previously “red disease” proper is one type. This disease occurs in marine environ ments during the summer and is caused by Vibrio anguillarum. The other type is characterized by the occurrence of distinct ulcers in the skin and is most frequent at cold temperatures in both fresh water, brackish water, and to a certain degree, sea water. This latter has been studied closely by us in Danish fjords. The symptom complex of the disease, for which we have proposed the name “spring ulcer disease” of eel, seems to have a typical sequence of events, starting with small white spots in the region of the lateral line, which develop into larger haemorrhagic and necrotic ulcers. The etiology of the disease is complex. Three species of bacteria - Aeromonas, Alcaligenes, and Vibrio - have been isolated. A possible primary viral etiology has been postulated but virological studies have, till now, been without success. Healthy eels, transferred to an abnormal environment may develop the disease which seems also to be transmitted by contact. local fishery districts, and replies disclosed that the dis Introduction ease was rather widespread in Denmark and, further Infectious disease in eels was first described by Canes- more, another disease was observed locally, especially trini (1892-1893) and the symptoms mentioned were after long and cold winter periods. similar to those characteristic of “red disease”, caused Hansen and Bonde (1973) studied repeated out by Vibrio anguillarum. A corresponding disease was breaks of disease of the latter type, which they called observed in Denmark by Feddersen (1896 a, b, 1897 a, furunculosis. The disease was characterized by an initial b), but no reports on isolation of bacteria from these occurrence of small haemorrhages in the skin, pro eels were given. Bruun and Heiberg (1932) studied an gressing to large ulcers with whitish-yellow necrotic epizootic among eels with “red disease” . The symptoms tissue centrally and with a diameter of 3 to 4 cm. were as follows: The first external symptoms are usually Aeromonas hydrophila and Alkaligenes bookerii were a red coloration at the end of the tail or on the body isolated from the lesions, and the prevalence of A. along the dorsal and anal fins; along with this the eels hydrophila was high in the water, where diseased eels become less lively and swim, if stimulated, with stiff were caught. wriggling movements — not so powerful, however, that During the last few years Japanese research workers the fish cannot easily be taken in the hands again. If have published papers on two bacterial diseases in eels: undisturbed it rests quietly on the bottom for the most 1) “red spot disease” caused by Pseudomonas anguilli- part. The red coloration gradually increases along the septica (Muroga et al., 1973, 1975, 1977). The predo fins until it appears as dark red streaks over longer or minant symptoms of this disease were petechial hae shorter distances, and a diffuse reddening spreads over morrhages in the skin occurring when the water tem most of the body. The lymph heart in the tail works perature was approximately 14°C; 2) vibriosis, caused more and more slowly until at last, it ceases to beat. The by Vibrio anguillarum, with the same symptoms as lis eels often die before the red coloration is well de ted previously (Muroga et al., 1976 a, 1976 b; Nishibu- veloped. Bruun and Heiberg (1932) made inquiries at chi and Muroga, 1977). The present work is a continuation of the work star ted by Hansen and Bonde (1973) and was performed on ’Present address: Institute of Toxicology, National Food In the request of the Danish Ministry of Fisheries, as a high stitute, Mørkhøj Bygade 19, DK 2860 Søborg, Denmark. frequency of diseased eels was reported from Randers ^ Deceased 26 May 1983 Fjord in the early spring of 1978. 106 Transmission experiments in aquaria Hadsund Healthy eels from Mariager Fjord and diseased eels from Randers Fjord were placed in the same aquaria at various water temperatures. Hobr« Results Mariager Pathological investigations The primary lesions in the eels appear as pin-point light spots along the lateral line, which can hardly be pal pated. Hyperaemia occurs later on and a haemorrhagic ulcer develops, which extends circularly from the pro cesses in the lateral line. A chequered pattern may be observed in the haemorrhagic zone giving the ulcers an irregular appearance. As the haemorrhages diminish, this pattern becomes more pronounced. Randej Later the ulcers have a greyish centre surrounded by a haemorrhagic marginal zone, which gradually grows narrower and finally disappears. In this later stage the ulcers are either completely greyish or the colour may change to greyish white or Figure 40. Map of the sampling sites in Randers and Mariager porcelain white with a weak yellow tint. A further de Fjords. velopment is the occurrence of small perforations in the centre of the greyish area often spreading in a star- shape from the centre, uncovering the underlying red tissue. Independently or simultaneously haemorrhagic or greyish lesions may be observed in the tail of the eel. Materials and methods The occurrence of real healing stages is subject to doubt. However, some fishermen have reported prob Eels lems with the flaying of eels which might be due to an A comparative study of eels trapped in Randers and increase in connective tissue in and beneath the skin. Mariager Fjords (Fig. 40) was performed during 1979. Fishermen from another region of Denmark have re Eels were investigated on board fishing vessels, and ported the occurrence of flattened non-pigmented ir prevalence of the disease was calculated. Bacteriologi regular changes in the skin of eels, which also might cal examination was done immediately after landing ac represent healing stages. cording to methods of Larsen and Jensen (1979) and pathological and histopathological examinations were also carried out. Attempts were made to isolate a virus Bacteriological investigations (Table 23) after the methods of Jensen et al. (1979). Seventy-nine % of the eels had infected skin lesions and 52 % infected kidneys. An infection was diagnosed, Water and sediment Table 23. Bacterial infection in ulcers and kidneys of eel with Water and sediment samples from the locations listed in spring ulcer disease (Randers Fjord). Figure 40 were examined bacteriologically (Larsen and O/ Jensen, 1977) and the medium of Shotts and Rimler /o (1973) was used for specific isolation of A. hydrophila. No. of diseased e e l........................ 124 100 Infection in ulcers.......................... 98 79 Aeromonas hydrophila ............... 20 20 Vibrio sp........................................... 23 23 Transmission experiments 100 Alcaligenes sp................................... 52 53 Healthy eels were transferred in a tank from Mariager Miscellaneous ................................ 3 3 Fjord to Randers Fjord and were combined in a spe Infection in kidneys...................... 65 52 cially constructed 100-1 well box. This box was an Aeromonas hydrophila ............... 4 6 Vibrio sp........................................... 32 49 chored so near the bottom that it contained both sedi 100 Alcaligenes sp................................... 28 43 ment and water, simulating the conditions in the fjord Miscellaneous ................................ 1 2 and allowing the eels to come in contact with both. 107 when massive growth of a bacterium was found, and showed that only 10 had skin lesions. In eight of these when this bacterium occurred in pure culture or pre eels the lesions were found on the tail and only two had dominantly so. Three bacterial species seem involved in more disseminated ulcers. It was not possible to decide this disease: 1) A. hydrophila, 2) Alcaligenes spp., and if it was the same disease as that in Randers Fjord. 3) Vibrio spp. Transmission experiments Virological investigations of spring ulcer disease Nineteen apparently healthy eels were transferred from in eels Mariager Fjord to the tank in Randers Fjord. After one Due to lack of uniformity in the bacteriological findings month only 14 eels were left alive. Three had typical it was found appropriate to perform virological investi spring ulcer disease, while four had non-specific skin gations. Organ pools from diseased eels were filtered lesions. (145 m|x) and passed three times of five cell-lines (RTG2, FHM, EPC, GM, and PS). No cytopathogenic effect was observed. Therefore it is still undecided Aquaria experiments (Table 24) whether a virus is involved in the disease. Transferring eels from Randers to aquarium conditions resulted in a high mortality rate among these animals. In a mixed population of diseased eels from Randers Fjord Prevalence of spring ulcer disease among eels in and healthy ones from Mariager Fjord kept at 8°C, one Randers and Mariager Fjords eel from Mariager Fjord showed skin changes but it was In Randers Fjord an increase in percentages of diseased not possible to decide for sure that it was a case of eels from 69 % to 86 % was observed from March to spring ulcer disease. the beginning of May, simultaneously with a rise in wa ter temperature. At the end of May when the water temperature reached 10°C, the prevalence of spring Microbiological investigations of water and ulcer disease was 26%. In July the disease was not sediment from Randers Fjord observed. While the haemorrhagic ulcers dominated in March, the white stages were most frequent at the end The monthly prevalences listed in the figures (Figs. 42 of May (Fig. 41). and 43) show some characteristic features of the fre An investigation of 2387 eels in Mariager Fjord quency and distribution of Vibrio and Aeromonas in the Temperature 0°C 4°C lo 't 13 C 16 C 19 C T O O .

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