Effect of an immunopotentiator on Aeromonas salmonicida infection in rainbow trout (Salmo gairdneri)

The immunoactive peptide FK-565 (heptanoyl-y-D-glutamyl-(L)-mesodiaminopimelyl-(D)-alanine) was found to induce protection against intraperitoneal Aeromonas salmonicida infection in rainbow trout (Salmo gairdneri Richardson). The survival rate was as high as 60% when FK-565 was given intraperitoneally as a single dose (1mg/kg) one day before bacterial challenge. A non-specific stimulation of phagocytic cells by FK-565 at an early stage of the bacterial infection may contribute to the resistance observed. The phagocytic activity of peritoneal phagocytic cells as well as phagocytic cells of the pronephros were stimulated by FK-565 in vivo and in vitro, respectively, as compared to untreated control fish. Furthermore, decreased activity of phagocytic cells previously immunosuppressed with cyclophosphamide was rapidly restored by application of FK-565.     

The ontogeny of humoral immunity in rainbow trout, Salmo gairdneri

The ontogeny of the humoral immune response to a 'thymus dependent' and a 'thymus independent' antigen, human gamma globulin (HGG) and Aeromonas salmonicida (AS) respectively, was investigated in the fry of rainbow trout, Salmo gairdneri, by direct immersion vaccination in the antigens (dose 5 mg/L HGG; 10(8) cells/ml AS; 30 minutes) at known ages/weights from 7 days post hatch, and at 1, 2, 3, and 4 months post hatch. Half the fry in each group were tested for antibodies 4 weeks after vaccination, the remainder were reimmunised and tested again after a further 4 weeks. Appropriate controls to test for tolerance induction and memory responses were included. The results indicate that fry are capable of mounting a humoral immune response very early in ontogeny. There is a period of 'unresponsiveness' which persists for longer against HGG, than against AS, though it was not thought to be tolerance as such. Memory could be detected to HGG in fry given a first immunisation at 2 months. The results are compared with preliminary experiments in which fry were first thymectomised 4 weeks before the first immunisation. In fry thymectomised at 1 month post hatch, and tested for primary and secondary responses at 2 and 3 months, the primary response to HGG is unaltered, but the secondary response is reduced. Both the primary and secondary response to AS is unaltered. When thymectomy is performed later, the effect on the secondary response to HGG is no longer apparent, but the primary response to AS is slightly reduced.     

Effects of metals on the chemiluminescent response of rainbow trout (Salmo gairdneri) phagocytes

Quantification of an induced chemiluminescent (CL) response in phagocytes is currently being evaluated as an indicator system for determining those environmental pollutants that may predispose fish to disease. A CL assay was developed using phagocytes from the pronephros of rainbow trout (Salmo gairdneri). The CL response of phagocytes to phorbol myristate acetate, a chemical inducer of CL, was shown to be dose-dependent. The response to five species of bacteria was also evaluated. Staphylococcus aureus and Aeromonas hydrophila produced the most intense CL responses and the longest duration of response (100 min.) Yersinia ruckeri induced an immediate strong CL response of short duration (20 min.) whereas Vibrio anguillarum and Aerococcus viridans failed to stimulate CL under the test conditions employed. The effect of sub-toxic levels of Cu, Al, and Cd on the CL response of phagocytes to S. aureus was examined using phagocytes exposed to the metals immediately before assay or after 1 hr or 24 hr exposure times. Copper caused a significant decrease in CL to the baseline level under all treatment conditions upon stimulation with S. aureus. Similar results were obtained with Al except that the decrease in CL, although significant, was not to the baseline level. In contrast, Cd caused a significant increase in CL when added 1 hr prior to or immediately before the assay; but, following a 24 hr exposure, the results were variable, in that either no change or a decrease was observed. The addition of Cu to phagocytes already exhibiting a strong CL response to S. aureus caused an immediate decrease in CL to that seen with the negative controls.     

Furunculosis in rainbow trout (Salmo gairdneri) raised in sea water

Ulcerative skin lesions were encountered in rainbow trout raised in sea water by a commercial concern in the Western Cape, South Africa. Grossly, the lesions resembled furunculosis but, histopathologically, they differed from typical furunculosis in that bacterial colonies were rarely found in the organs, and also the kidneys and spleens were minimally involved. The causative organism was identified as an achromogenic Aeromonas salmonicida that shared characteristics with all 3 subspecies, salmonicida, masoucida and achromogenes. This is the first report of an outbreak of this disease in South Africa.     

Contribution of food deprivation to the immune response in rainbow trout (Oncorhynchus mykiss) vaccinated against Cryptobia salmositica and Aeromonas salmonicida

The aims of the present study were to determine (a) the effectiveness of an attenuated live Cryptobia salmositica vaccine; (b) the effects of food deprivation on the immune response and its duration in rainbow trout (Oncorhynchus mykiss) immunised with a live C. salmositica vaccine or with a killed Aeromonas salmonicida vaccine. The fish were divided into three groups (I, II and III; 14 fish per group), those in Groups I and II were under food deprivation (0.40% of body weight), while Group III fish were fed to satiety. The study showed that the attenuated strain of C. salmositica did not cause anaemia and disease, and the fish were protected from clinical disease when they were challenged with virulent parasites. Parasitaemia in all fish vaccinated and challenged with virulent C. salmositica fluctuated and was relatively low; however, fish in Group III had higher parasitaemia than those in Groups I and II between weeks 8 and 14. The numbers of activated neutrophils increased [nitroblue tetrazolium (NBT) assay] after immunisation with both Cryptobia and Aeromonas vaccines and they remained high throughout the experiment. Antibody production (ELISA values) increased after vaccination and were slightly higher in Group III. ELISA titres against A. salmonicida increased after vaccination and decreased after 5 weeks. The titres increased again after the vaccinated fish were given booster, and they were higher than those in the first vaccinated fish.     

Comparative pharmacokinetics of oxytetracycline in rainbow trout (Salmo gairdneri) and African catfish (Clarias gariepinus)

A comparative pharmacokinetic study was conducted in rainbow trout (Salmo gairdneri) and African catfish (Clarias gariepinus) following intravenous (i.v.) and intramuscular (i.m.) administration of oxytetracycline (OTC) at a dose rate of 60 mg/kg body weight. Trout and catfish were kept in aerated tap water in tanks at constant temperatures of 12 degrees C and 25 degrees C, respectively. The two- and three-compartment open models adequately described plasma drug disposition in African catfish and rainbow trout respectively, following i.v. OTC administration. Compared to catfish (COP = 86 +/- 10 micrograms/ml) an eightfold higher extrapolated zero time concentration was obtained in trout (COP = 753 +/- 290 micrograms/ml). A significant difference was observed with respect to the relatively large apparent distribution volumes (Vd(area] after i.v. OTC administration (trout, mean value: 2.1 l/kg; catfish, mean value: 1.3 l/kg). The mean final elimination half-lives of both fish species were greater than previously reported in mammals (trout, 89.5 h; catfish, 80.3 h). A mean maximum plasma concentration (Cmax = 56.9 micrograms/ml) was obtained in trout at 4 h after i.m. administration of OTC. In catfish a lower Cmax of 43.4 micrograms/ml was determined at about 7 h. No significant difference was observed with respect to bioavailability following i.m. administration of OTC (trout, 85%; catfish, 86%).     

The indirect fluorescent antibody technique for the rapid identification of streptococcosis of rainbow trout (Salmo gairdneri)

The indirect fluorescent antibody technique has been used successfully for the rapid serological identification of the Streptococcus sp. responsible for streptococcosis of rainbow trout. This technique has been used to identify the Streptococcus sp. in pure cultures and smears made from experimentally infected and diseased fish.     

Isolation and antimicrobial susceptibility of Aeromonas salmonicida in rainbow trout (Oncorhynchus mykiss) in turkey hatchery farms

Three Aeromonas salmonicida strains were isolated from the livers of 265 rainbow trouts sampled. The antibiotic susceptibility test results showed that A. salmonicida strains were susceptible to streptomycine and ciprofloxacin. However, they were resistant to amoxycilline + clavulanic acid, penicillin, erythromycine, oxytetracycline and cefuroxime sodium.     

The distribution of 35S-sulfadiazine and 14C-trimethoprim in rainbow trout, Salmo gairdneri.

The distribution of ³⁵S-labelled sulfadiazine and ¹⁴G-labelled trimethoprim was studied in rainbow trout by use of whole body autoradiography and liquid scintillation. As compared to mammals, gastrointestinal absorption and elimination were slow. Accumulation in the skin and the uveal tract of the eye was observed for both drugs tested. The results also indicated that the bile was an important route of excretion. Considerable radioactivity was still present in the skin at 144 hr. survival time.     

Modulation of juvenile brook trout (Salvelinus fontinalis) cellular immune system after Aeromonas salmonicida challenge

In fish, the first line of defence against infectious microorganisms is based on non-specific cellular immune mechanisms (innate immunity). In this study, we measured the non-specific immune parameters (natural cytotoxic cells (NCC) activity, lymphoproliferation, percentage of phagocytosis and phagocytic activity) in brook trout (Salvelinus fontinalis) infected by a virulent strain of Aeromonas salmonicida. Eight days post-infection, the mortality of infected fish reached 70%. A transient immunostimulation of the NCC activity was noticed 24h post-infection, but there was no significant difference at 48 h. Then, infection of brook trout with A. salmonicida induced a biphasic immune response. At 24h post-infection, lymphoproliferation was drastically depressed but returned to control level at 96 h. A slight increase in the percentage of phagocytosis and the phagocytic activity was noticed throughout the experiment. Conversely the cell mortality was significantly higher in infected fish compared to control. The modulation of immunological parameters might reveal important clues on how innate immunity might protect fish from bacterial infections.     

An outbreak of white spot disease (Ichthyophthirius multifiliis) in young fingerling rainbow trout (Salmo gairdneri Richardson)

An outbreak of white spot disease (‘Ich’) in fingerling rainbow trout is reported. Approximately 3,000 fish died or were killed in extremis in a period of 5 weeks. Post mortem examinations were carried out on 12 fish, all of which showed thickened whitish foci on the skin. One fish showed skin ulceration and another gill ulcerations. The gill infestation was more severe than the skin. Pathological results are described and discussed.     

A selective procedure for the field isolation of pathogenic Streptococcus spp. of rainbow trout (Salmo gairdneri)

A procedure established for the selective isolation of the species of Streptococcus responsible for rainbow trout streptococcosis in South Africa, consisted of the inoculation of samples into nutrient broth which had been supplemented with 100 micrograms/ml of nalidixic acid, 160 micrograms/ml of oxolinic acid or 200 micrograms/ml of sodium azide. After incubation, the sample was plated onto tetrazolium agar on which the rainbow trout pathogenic Streptococcus species grew as a red colony. The colonies were isolated from the tetrazolium agar and identified as rainbow trout pathogenic isolates by biochemical and serological tests. In the laboratory the selective procedure is capable of detecting about 2 bacteria per ml. This procedure was used in the field and biochemically identical Streptococcus species were found in the mud and a freshwater crab from the water source of a site with a history of streptococcosis.     

Chronic oral exposure of rainbow trout (Salmo gairdneri) to a polychlorinated biphenyl (Aroclor 1254): pathological effects.

Groups of eight rainbow trout (Salmo gairdneri) were fed rations containing 0 ppm, 1 ppm, 10 ppm and 100 ppm of the polychlorinated biphenyl, Aroclor 1254, over a period of up to 330 days. Growth rates were unaffected by these levels. Pathological changes were found in the kidneys of 13 fish on the polychlorinated biphenyl ration. Foci of nephrosis with cellular or granular cast formation were seen. The greatest number of cases occured in fish at the 10 ppm concentration. An increase in the number of hepatocytes per unit area was related to the greater concentration of polychlorinated biphenyls in the diet. A reduction in the amount of white pulp in the spleen was observed in 23 of 27 fish on the 10 ppm and 100 ppm diets. The fish with nephrosis had reduced splenic white pulp and lowered white cell counts. Fish fed the 1 ppm and 10 ppm levels had mean tissue residues of 1.4 and 2.3 ppm polychlorinated biphenyl respectively. Fish on the 100 ppm ration had a mean value of 80.1 ppm polychlorinated biphenyl content.