The immune response of rainbow trout, Salmo gairdneri Richardson, to the haemoflagellate, Cryptobia salmositica Katz, 1951

Abstract. Rainbow trout that recovered from experimental Cryptobia salmositica infection 6 and 10 weeks earlier were protected against multiple intraperitoneal challenges of 50 000 and 10 000 parasites isolated from infected fish. The immunity was non-sterile; low parasitaemias were detected following a larger challenge (112 000 parasites). The indirect haemagglutination test was used to detect C. salmositica -specific agglutimns. Antibody titers increased during the first 18 weeks of infection. The infectivity of cultured C. salmositica was neutralized by incubation in heat-inactivated immune plasma. Infectivity of C, salmositica from infected fish was not neutralized by similar treatment. Complement fixing antibody was detected using the in vitro immune lysis test. Immune lysis occurred when cultured C. salmositica were used. Adoptive transfer of both leucocytes and plasma from immune fish conferred partial protection against the parasite in naive recipients. Complement fixing antibody may be important during early acute infection while phagocytosis may be important during the later chronic phase.     

Cryptobia salmositica: an in vitro and in vivo study on the mechanism of anaemia in infected rainbow trout, Salmo gairdneri Richardson

Abstract. There are two basic antigenic components in Cryptobia that are responsible for the anaemia in infected rainbow trout. A 'lytic component', which is dosage-dependent, causes lysis of red blood cells independent of antibody or complement. The second, an 'immune-complex-forming component', attaches to red blood cells, forms immune complexes with specific antibody and activates complement resulting in haemolysis. These two antigenic components, from both live and lysed Cryptobia , were present in the serum of infected fish. When sonicated antigen or heat-inactivated antiserum (from infected fish) was incubated with red cells from uninfected fish, a portion of the red cells was lysed and a positive Coombs' reaction was observed with the remaining intact red cells. The positive Coombs' reaction was due to immune complexes adsorbed onto the red cells and these lysed when incubated with complement. Antibody by itself did not adsorb onto the red cells. From the fourth week post-infection, a positive Coombs' reaction was observed in all infected fish and haemolysis occurred with complement. The authors suggest that, in infected fish, one or more components of the complement cascade is depleted continually during infection and that the anaemia is due to the lytic action of the antigen and immune complex formation on red cells. These lead to intra-vascular haemolysis as well as erythrophagocytosis. In general, the mechanism of anaemia in cryptobiosis appears similar to that in African trypanosomiasis.     

Strawberry disease in rainbow trout, Salmo gairdneri Richardson

Abstract. Strawberry disease is a subchronic, non–debilitating, and non–fatal inflammatory skin disease of rainbow trout that has been recognized for the past 25 years. Morbidity ranges up to 80% and the disease affects fishes from 15cm in length to brood stock size. Epizootiological data indicate that strawberry disease usually appears, first in the autumn and reaches peak incidence during the following winter and spring. Data also indicate that the disease is found only in certain trout farms in the western and Pacific north–western regions of the United States. Although the cause of strawberry disease is unknown, the normal clinical course of 8 weeks can be reduced by oral treatment with oxytetracycline. Clinical signs include appearance of bright red, raised, circumscribed, ulcerated and indurated lesions in the skin that usually range up to 3 cm or more in diameter. Microscopically, the disease is characterized as a subchronic, focal, non–suppurative dermatitis with ulceration and extensive infiltration of mononuclear inflammatory cells.     

Immunoglobulin heterogeneity in the rainbow trout, Salmo gairdneri Richardson

Abstract. Hetereogeneity of rainbow trout immunoglobulins was demonstrated by using monoclonal antibody 1A6 and polyacrylamide-gradient gel electrophoresis. Immunoglobulins defined by elisa using monoclonal antibody 1A6 were about 30% of the total immunoglobulins, detected by elisa using polyclonal antibodies, in healthy rainbow trout. In trout obtained from farms with a previous history of infectious viral diseases, 1A6-immunoglobulins were only about 14% of the total. Several serum pools from infected trout could be totally depleted of 1A6-immunoglobulins (about 12% of total immunoglobulins) by affinity chromatography over Sepharose immobilized monoclonal antibody 1A6. Polyacrylamide-gradient gel electrophoresis under denaturing conditions of total immunoglobulins, 1A6 immunoglobulins and no-1A6 immunoglobulins purified by affinity chromatography, showed a majority heavy chain of 70 KDa and a minority heavy chain of about 60 KDa, two light chains of 24 and 26 KDa, and a 11–14 KDa polypeptide.     

Cryptobia salmositica: cortisol increases the susceptibility of Salmo gairdneri Richardson to experimental cryptobiosis

Abstract. Intraperitoneal implants of cortisol (cortisol suspended in hydrogenated coconut oil) were used to induce a graded hypercortisolism in rainbow trout, Salmo gairdneri Richardson. There was no obvious reduction in circulating lymphocytes in cortisol-implanted rainbow trout (70, 140 or 210μg/g body weight). Cortisol-implanted fish infected with Cryptobia salmositica had significantly higher parasitaemia and lower antibody litres compared with controls infected with haemonagellate but given coconut oil implants. These confirm the immunodepressive effects of the steroid. The parasite was also more readily detected at the early stage of the infection (shorter prepatent period, more infected fish and higher parasitaemia) in cortisol-implanted fish (140 and 210 μg/g body weight) than in controls. The mortality of the infected cortisol-implanted fish was higher than that of the infected fish implanted with only coconut oil, or the cortisol-implanted but non-infected fish. This in vivo study suggests that protective immunity against C. salmositica is, in part, due to a humoral response.     

Proliferative kidney disease in rainbow trout Salmo gairdneri Richardson

Abstract. Two entirely separate outbreaks of PKD are described in an attempt to properly define the condition and enable it to be differentiated from other diseases of a similar nature. Losses of up to 75% of fingerling rainbow trout were recorded in one outbreak, illustrating the potentially serious nature of the disease. Although the kidneys showed the most obvious pathological changes, other organs were also affected. The host response was granulomatous in type, the foci of each cellular reaction being 5-20 μm eosinophilic cells considered to be parasitic. The cytoplasm of these cells contained numerous electron dense bodies some with a tail, but each with an electron lucent bar always orientated at right angles to the plasmalemma whenever they were near it. The most obvious cytoplasmic inclusion of these 'parasites' were other cells numbering up to five or six, each with its own nucleus and nucleolus, but devoid of any of the small electron dense bodies. The significance of either of these inclusions is unknown.
A limited differential diagnosis is given, and the argument presented that the condition PKD is the result of infection by a parasite, possibly an amoeba, in combination with poor quality water.     

Phagocytic cells in the epidermis of rainbow trout, Salmo gairdneri Richardson

Abstract. Electron microscopical observations of skin of rainbow trout revealed the presence of several types of phagocytic cells in the epidermis, most notably macrophages, some of which were seen crossing the basal lamina. Occasionally, filament-containing cells showed either melanin granules or phagosomes, suggesting these cells were also capable of phagocytosis. No cells with the characteristics of the Langerhans cells were observed, which does not necessarily preclude the possibility of macrophages being able to play an identical role as antigen-trapping cells.     

Impact of a pathogenic haemoflagellate, Cryptobia salmositica Katz, on the metabolism and swimming performance of rainbow trout, Oncorhynchus mykiss (Walbaum)

Abstract. Oxygen consumption of juvenile rainbow trout (5 g at 13°C) at moderate swimming speeds did not change significantly when infected with Cryptobia salmositica. However, significant reductions of as much as 44% of the maximum aerobic scope for activity and 24% of the critical swimming speed were observed when the parasitaemia reached a maximum of 57.6 × 10⁶ ml⁻¹ fish blood at 3 weeks post- infection. Blood haematocrit was significantly reduced from the initial 34.1 to 19.7% at 4 weeks post- infection, probably as a result of haemolysis by the parasite. The destruction of red blood cells clearly led to lower oxygen carrying capacity, and reduced respiratory and swimming performance.     

Behaviour of rainbow trout, Salmo gairdneri Richardson, in marine cages

Abstract. Underwater television was used to observe the distribution and swimming behaviour of groups of farmed rainbow trout, Salmo gairdneri Richardson, in marine net cages. Undisturbed rainbow trout (mean total length 240 mm) aggregated near the surface, remaining motionless in slack water and formed polarized shoals at higher current velocities. Larger fish (mean total length 317 mm) occupied deeper parts of the cage and were more active during periods of slack water. The results are compared with previous work on farmed Atlantic salmon, Salmo salar L., and discussed in relation to the biology of wild rainbow trout and recent work on exercise, growth and condition in salmonids.     

A lymphosarcoma of thymic origin in the rainbow trout, Salmo gairdneri Richardson

Abstract. A rainbow trout with a malignant lymphoma of probable thymic origin showed direct spread of the tumour to the gills, metastases to the liver and head kidney, and the presence of abnormal circulating lymphocytes. The cells of the tumour possessed plasma membrane immunoglobulin detectable by immuno–fluorescence. Examination of the tumour cells by transmission electron microscopy revealed no evidence of virus associated with the cells. Infectious viruses could not be detected in the tumour tissue, and the tumour was not transmissible by injection of live tumour cells into young rainbow trout. Attempts to establish the tumour in long–term culture were unsuccessful.     

Dietary modulation of humoral immune response and anaemia in rainbow trout, Oncorhynchus mykiss (Walbaum), infected with Cryptobia salmositica Katz, 1951

Abstract. The humoral immune response in Cryptobia-salmositica -infected rainbow trout on pantothenic-acid-deficient/low protein (19%) diets was depressed. There was no significant difference in parasitaemia between fish on the pantothenic-acid-deficient diet and those on the supplemented diet; however, the parasitaemia in fish on 19% protein diet was significantly lower than that in fish on 38% protein diet. The low humoral responders, in contrast to the high responders (on 38 and 29% protein diets), had less severe anaemia and the packed cell volume at the end of the experiment was not significantly different from that at 2 weeks post-infection. Red cells from the low responders were Coombs' negative while those from the high responders were Coombs' positive from 4 to 6 weeks post-infection. The authors suggest that, in the low responders, red cell destruction is essentially due to the'lytic component' of the parasite antigen in contrast to the involvement of both the ^llytic' and the 'immune complex-forming' components in the high responders. The present study indicates that certain deficient diets are useful in modulating the parasitaemia and also in decreasing red cell destruction. Therefore, dietary modification may help in altering the course of such infections and the progression of the disease.     

Identification of lymphocytes in the epidermis of the rainbow trout, Salmo gairdneri Richardson

Abstract The presence of lymphocytes in the epidermis of the rainbow trout, Salmo gairdneri Richardson, was confirmed by light and electron microscopical observation and by the use of an indirect immunocytochemical technique utilizing peroxidase-labelled antibody. These results are compared with those of other workers and the source and function of these cells in the epidermis of teleosts is discussed.     

The pathology of chronic ammonia toxicity in rainbow trout, Salmo gairdneri Richardson

Abstract. Two groups of rainbow trout fingerlings were exposed for 90 days to 0.2 and 0.4 mg/1 of un-ionized ammonia, respectively. Several fish exposed to the higher concentration soon developed clinical signs suggestive of a neurological dysfunction but subsequently recovered. No lesion attributable to ammonia was seen in the gills of any of the fish. This raises questions about the precise role of ammonia in the production of gill diseases in intensively cultured trout.