Experimental evidence for direct in situ binding of IgM and IgT to early trophonts of Ichthyophthirius multifiliis (Fouquet) in the gills of rainbow trout, Oncorhynchus mykiss (Walbaum)

Freshwater fish are able to mount a protective immune response against the parasite Ichthyophthirius multifiliis (Ich) following a non‐lethal exposure. Factors involved in immunity comprise cellular and humoral factors, but antibodies have been suggested to play a prominent role in protection. However, host antibodies have not yet been demonstrated to bind to the parasite in situ. By the use of immunohistochemical techniques, this study demonstrated that IgT and IgM bind to surface structures, including cilia, on the early feeding stage of the parasite in the gills of immune rainbow trout, Oncorhynchus mykiss, shortly (2 h) after invasion. No binding of IgT and no or only a weak binding of IgM was observed on the parasites in the gills of similarly exposed but naïve rainbow trout. This study indicates that antibodies play an important part in the protection of immune fish against Ich although additional humoral and cellular factors may contribute to this reaction.     

Rainbow trout (Oncorhynchus mykiss) immune response towards a recombinant vaccine targeting the parasitic ciliate Ichthyophthirius multifiliis

The protective effect in rainbow trout (Oncorhynchus mykiss) of an experimental subunit vaccine targeting antigens in the parasite Ichthyophthirius multifiliis has been evaluated and compared to effects elicited by a classical parasite homogenate vaccine. Three recombinant parasite proteins (two produced in E. coli and one in insect cells) were combined and injected i.p., and subsequently, protection and antibody responses were analysed. Both the experimental and the benchmark vaccine induced partial but significant protection against I. multifiliis when compared to control fish. Specific antibody responses of vaccinated trout (subunit vaccine) were raised against one neurohypophysial n‐terminal domain protein #10 of three recombinant proteins, whereas the benchmark vaccine group showed specific antibody production against all three recombinant proteins. The immunogenic parasite protein #10 may be a potential vaccine candidate supplementing the protective I‐antigen in future vaccine trials.     

Recruitment and effects of Discocotyle sagittata (Monogenea) infection on farmed trout

Farmed rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta were monitored over 3 years for infection with the blood-feeding gill fluke Discocotyle sagittata. Parasite transmission is seasonal: new infections take place during summer/autumn, and transmission is generally negligible during winter/spring. There are 2 sources of infection for naïve fish-of-the-year: limited invasion when fish are in the raceways by riverborne larvae originating external to the farm; and internally, within the farm, when 0+ fish are transferred to ponds previously occupied by older cohorts of infected fish. Thereafter, infection levels continue to increase in rainbow trout primarily through transmission within the farm. Prevalence rose to 100% in 1+ fish by the end of their second summer. In O. mykiss, mean abundance reached 194 worms/host for 1+ fish (up to 489 worms/host) and 160 worms/host for 2+ fish. By contrast, in S. trutta, parasite prevalence never exceeded 85% and, after the first year's invasions, infection levels decreased over time: in 1+ and 2+ brown trout, parasite mean abundance was < 4 (maximum 15) worms/host. We present evidence of the detrimental effects of D. sagittata on the host: high burdens are associated with pale gills, decreased body condition and host mortality. Parasite burdens become overdispersed during the warmer part of the year, as prevalence and mean abundance increase. However, the degree of parasite overdispersion decreases over winter; we cannot distinguish whether decreased aggregation is due to parasite losses from infected fish (including immune-mediated parasite mortality) or parasite-induced host mortality.     

Association of a specific major histocompatibility complex class IIβ single nucleotide polymorphism with resistance to lactococcosis in rainbow trout,Oncorhynchus mykiss (Walbaum)

Major histocompatibility complex (MHC) loci encode glycoproteins that bind to foreign peptides and initiate immune responses through their interaction with T cells. MHC class II molecules are heterodimers consisting of α and β chains encoded by extremely variable genes; variation in exon 2 is responsible for the majority of observed polymorphisms, mostly concentrated in the codons specifying the peptide‐binding region. Lactococcus garvieae is the causative agent of lactococcosis, a warm‐water bacterial infection pathogenic for cultured freshwater and marine fish. It causes considerable economic losses, limiting the profitability and development of fish industries in general and the intensive production of rainbow trout, Oncorhynchus mykiss (Walbaum), in particular. The disease is currently controlled with vaccines and antibiotics; however, vaccines have short‐term efficacy, and increasing concerns regarding antibiotic residues have called for alternative strategies. To explore the involvement of the MHC class II β‐1 domain as a candidate gene for resistance to lactococcosis, we exposed 400 rainbow trout to naturally contaminated water. One single nucleotide polymorphism (SNP) and one haplotype were associated with resistance (P < 0.01). These results are promising for using MHC class IIβ as a molecular marker in breeding rainbow trout resistant to lactococcosis.     

Ceratomyxa shasta genotypes cause differential mortality in their salmonid hosts

Ceratomyxa shasta is a myxozoan parasite of salmonid fish. In natural communities, distinct genotypes of the parasite are associated with different salmonid hosts. To test the hypothesis that genotypes of C. shasta cause differential mortality, the polychaete host was experimentally infected with different parasite genotypes. Genotype I was obtained from Chinook salmon, Oncorhynchus tshawytscha, and genotype II from either coho salmon, O. kisutch, or rainbow trout, O. mykiss, We then challenged four salmonid strains: Chinook and coho salmon that occur in sympatry with the parasite and allopatric Chinook salmon and rainbow trout. Parasite genotype I caused mortality only in Chinook strains, although mortality in the allopatric strain also occurred from exposure to genotype II. A second experiment demonstrated that genotype II could be separated into two biotypes based on differential mortality in rainbow trout and coho salmon. These differential patterns of mortality as a result of infection by certain genotypes of C. shasta support field observations and suggest a co‐evolutionary relationship between these parasites and their hosts.     

Dexamethasone treatment increases susceptibility of rainbow trout,Oncorhynchus mykiss (Walbaum), to infections with Gyrodactylus derjavini Mikailov

The parasite population dynamics of a Danish isolate of Gyrodactylus derjavini Mikailov 1975 on immunosuppressed fry of rainbow trout, Oncorhynchus mykiss (Walbaum), were investigated and compared with immunocompetent fry. Immunosuppression was induced 14 days prior to infection by intraperitoneal injections of dexamethasone (fluoro-methyl-prednisolone). Treated and untreated fish were infected by cohabitation with Gyrodactylus-infected donorfish, carrying 4-week-old infections. Initial infections did not differ between the two groups. However, parasite populations increased significantly faster and reached significantly higher levels in the dexamethasone-treated group from day 13 post-exposure and during the following 3 weeks when compared with the untreated group. Microhabitat distribution of parasites on the two host groups differed from day 20 post-exposure, which could indicate a different host response to infection. Changes in the density of superficial mucous cells in the caudal fins were investigated during the course of infection. No differences in mucous cell density between untreated and dexamethasone-treated groups were found.     

Are brown trout Salmo trutta fario and rainbow trout Oncorhynchus mykiss two of a kind? A comparative study of salmonids to temperature‐influenced Tetracapsuloides bryosalmonae infection

Proliferative kidney disease (PKD) of salmonids caused by Tetracapsuloides bryosalmonae causes high mortalities of wild brown trout (Salmo trutta fario) and farmed rainbow trout (Oncorhynchus mykiss) at elevated water temperatures. Here the aim was to compare the temperature‐dependent modulation of T. bryosalmonae in the two salmonid host species, which display different temperature optima. We used a novel experimental set‐up in which we exposed brown trout and rainbow trout to an identical quantified low concentration of T. bryosalmonae for a short time period (1 hr). We followed the development of the parasite in the fish hosts for 70 days. PKD prevalence and parasite kinetics were assessed using qPCR. Exposures were performed at temperatures (12°C and 15°C) that reflect an environmental scenario that may occur in the natural habitat of salmonids. T. bryosalmonae infection was confirmed earliest in brown trout kept at 15°C (day 7 post‐exposure) while, in all other groups, T. bryosalmonae was not confirmed until day 15 post‐exposure. Moreover, significantly greater infection prevalence and a faster increase of parasite intensity were observed in brown trout kept at 15°C than in all other groups. These results indicate that PKD is differentially modulated by water temperature in related host species.     

The occurrence of proliferative kidney disease (PKD) in cultured and wild fish: further investigations

Abstract. In an investigation of the occurrence of proliferative kidney disease (PKD) in freshwater fish other than rainbow trout, 18 species of wild fish and seven species of fish raised in cultivation wore sampled from waters where the disease occurred annually in rainbow trout, Oncorhynchus mykiss (Richardson). Results revealed that certain wild stocks of brown trout. Salmo trutta L., grayling, Thymallus thymallus L., and pike, Esox lucius L., were infected with PKD, as were cultivated Atlantic salmon, Salmo salar L., parr, brown trout and char, Salvelinus alpinus (L.). Microscopical examination revealed the presence of the PKX cell in these species and also intraluminal protozoa possibly related to the PKX cell, which were not found in the rainbow trout. Other species of freshwater fish had myxosporidan infections but, unlike PKD infection, there was little host/parasite tissue response. The PKX cell as a myxosporidan stage is discussed and the presence of the disease in wild fish is reported.     

PARASITES AND PARASITOSIS IN FISH CULTURE IN PORTUGAL

This paper deals with research work covering the period from May 1979 up to May 1981 and presents results of parasites and parasitosis diagnosed in a total of 195 complete dissections of six fish species from cultured and feral populations in Portugal: European eel (Anguilla anguilla) (49); mirror carp (Cyprinus carpio specularis) (29); common grey mullet (Mugil cephalus) (18); European plaice (Pleuronectes platessa) (11); rainbow trout (Salmo gairdneri) (42); common sole (Solea solea) (46). A list of protozoans and helminths identified with respective incidence is given: A) Feral fish: 1) European eel (17)—Echinorhynchus clavula 11.8% and Rhabdochona anguilla 5.9%; 2) common grey mullet (18)—Haploporus benedenii 16.7%, Neoechinorhynchus sp. 5.6%, and Raphidascaris sp. 5.6%; 3) European plaice (11)—Bothriocephalus sp. larvae 9.1%, Contracaecum aduncum 9.1%, and Cryptocotyle lingua 9.1%; 4) common sole (46)—Cucullanellus minutus 8.7%, Gyrodactylus elegans 4.3%, and Cestodes larvae 2.2%. B) Cultured fish: 1) European eel (32)—Trichodina anguillae 3.1% and Ichthyophthirius multifiliis 18.8%; 2) rainbow trout (42)—Eimeria truttae 2.4%, Hexamita intestinalis 4.8%, Holophrya simplex 2.4%, and Trichodina truttae 4.8%. The results show that feral fish harbor predominantly indirect life cycle helminths, in contrast with cultured fish only affected with protozoans, due mainly to overcrowding and deficient management, which propitiate a significant background infection. It was in these specimens that some lesions characteristic of protozoosis were found.     

MH II‐DAB gene expression in grass carp Ctenopharyngodon idella (Valenciennes) after infection with the ciliate parasite,Ichthyophthirius multifiliis

The grass carp, Ctenopharyngodon idella (Valenciennes), is one of the most extensively aquacultured freshwater fish in China. However, because of the lack of effective control measures and the high‐density culture environment, considerable economic losses are caused by infection of C. idella with the parasitic ciliate, Ichthyophthirius multifiliis. The major histocompatibility (MH) DAB gene belongs to antigen‐presented genes in the class II genomic region, which is associated with parasite resistance. To understand the relationship of the DAB gene with I. multifiliis infection in grass carp, the expression profiles of MH II‐DAB were studied in tissues using real‐time quantitative polymerase chain reaction. The results showed that expression of the MH II‐DAB gene was up‐regulated in head kidney after I. multifiliis infection, and the expression peak appeared earlier in the study (case) group than in the control group. The obvious up‐regulation peak of MH II‐DAB gene was found at days 2 and 4 in skin; at 12 h to day 4 in spleen; at 12 h and days 1 and 6 in gill; and at day 10 in blood, whereas the MH II‐DAB gene was down‐regulated in liver and intestines after I. multifiliis infection. These results have implications for better understanding C. idella resistance to I. multifiliis infection.     

Microhabitat selection of Gyrodactylus salaris Malmberg on different salmonids

The microhabitat selection of the ectoparasite Gyrodactylus salaris (Lærdalselva strain, Norway) was investigated concurrently with studies on the parasite population growth on five strains of Atlantic salmon, Salmo salar L., and a strain of Danish rainbow trout, Oncorhynchus mykiss (Walbaum). The salmon used were hatchery‐reared parr of East Atlantic strains [River Conon (Scotland), River Storå (western Denmark) and River Ätran (western Sweden)] and Baltic strains [Lule and Ume (eastern Sweden)]. The location and numbers of parasites were recorded on anaesthetized fish once a week from week 0 to week 8. The mean abundance of G. salaris steadily increased to high levels on the River Conon, Storå and Ätran strains until the end of the experiment. The mean abundance of G. salaris on the two Baltic strains (River Lule älv and River Ume älv) initially increased but after 4–7 weeks the growth of the parasite infrapopulations decreased markedly. The Danish rainbow trout strain showed the lowest abundances of all the fish species and strains. Gyrodactylus salaris preferentially selected the fins and head region when colonising the hosts (all species and strains). Increasing percentages of G. salaris on the tail fins of the East Atlantic strains and rainbow trout were found during the course of infection, whereas the two Baltic salmon strains experienced a decreasing percentage of parasites in this microhabitat.     

Protective Immunity of Goldfish Against Ichthyophthirius multifiliis Infection Induced by Different Trophont Vaccine Preparations

Ichthyophthirius multifiliis, commonly called “Ich,” is a protozoan parasite that infects the epidermis and gills of freshwater fish. Here, we used goldfish (Carassius auratus) to determine whether the four vaccine preparations (live trophonts, formalin‐fixed trophonts, freeze‐thawed trophont lysates, and trophont cilial and cell cortical fractions) of I. multifiliis (Fouquet) can elicit resistance of immunized fish against subsequent theront challenge, and whether a relationship exists between in vitro immobilization of theronts in sera or mucus from immunized fish and host protective immunity against the parasite. Experimental goldfish were randomly divided into five groups with five parallel controls, with each group or subgroup consisting of 20 individuals. Each test goldfish was immunized with one of the four Ich vaccine preparations administered by one of the three routes: live trophonts by gavage (Group 1), formalin‐fixed trophonts by injection (Group 2) or by immersion (Group 3), freeze‐thawed trophont lysates by injection (Group 4), and trophont cilial and cell cortical fractions by injection (Group 5). The fish were then challenged by a standard Ich theront challenge on Day 21 postimmunization. For every 10 d following immunization, we tested the in vitro immobilization of Ich theronts by sera or mucus from immunized and control fish. We found that although all control (nonimmunized) and Group 3‐immunized goldfish died following theront challenge, more than half (55–65%) of the immunized fish from Groups 1, 2, 4, and 5 survived. Furthermore, except for the fish from Groups 3 and 4, the number of mean days to death in each test group was significantly higher than that in the respective control. These results indicate that goldfish immunized by injection or by gavage with any of the four vaccine preparations gained resistance to Ich infection. Further analysis indicated that this protective immunity was closely associated with the in vitro immobilization of Ich theronts by fish sera or mucus.     

Are native brook charr and introduced rainbow trout differentially adapted to upstream and downstream reaches?

Abstract— Due to species introductions, brook charr (Salvelinus fontinalis) and rainbow trout (Oncorhynchus mykiss) occur together in many North American streams and typically exhibit a pattern of distribution in which brook charr numerically dominate headwaters and rainbow trout dominate downstream reaches. It has been suggested that 1) the two species compete or 2) the two species do not compete because they are differentially adapted to environmental conditions found in upstream and downstream zones. We assessed whether there were differences in growth and macrohabitat (pool, run and riffle) selection of brook charr and rainbow trout in upper, middle and lower stream zones of a small Pennsylvania stream. Brook charr and rainbow trout placed in replicate paired enclosures set in upstream and downstream reaches showed no significant differences in growth and survival rates upstream, but brook charr had significantly greater growth rates than rainbow trout downstream. Enclosed fish and free-ranging fish both had negative growth rates during the summer. Enclosed fish lost significantly less weight than free-ranging fish. Instantaneous growth rates of free-ranging adult brook charr and rainbow trout from May to August were negative for both species in all stream zones. Underwater observations of adult brook charr and rainbow trout showed both species occurred significantly more often in pool macrohabitats than expected on the basis of macrohabitat availability, except for rainbow trout in the upstream zone. The proportion of pool macrohabitat was not significantly different among stream zones. Brook charr do not appear to be better adapted to upstream environments in Powdermill Run based on growth, survival and macrohabitat selection during summer. Negative biotic interactions acting along with differential environmental adaptations may explain the pattern of distribution of brook charr and rainbow trout in streams, but long-term transplant experiments with additional life stages will be necessary to examine this hypothesis.     

Stress response of juvenile rainbow trout (<Emphasis Type="Italic">Oncorhynchus mykiss</Emphasis>)to chemical cues released from stressed conspecifics

The objective of this study was to determine whether exposure of rainbow trout (Oncorhynchus mykiss) to water containing a stressed trout or skin extract from stressed and non-stressed trout would elicit a stress response in conspecifics. Juvenile rainbow trout were exposed for 1 hour to water containing a stressed fish, homogenized skin extracts from a non-stressed fish, skin extract from a stressed fish and water with none of these factors. The stress response was measured over a 24-h period (1, 6, 12, 24 h after exposure). Plasma cortisol levels increased at 12 h in fish exposed to water from a stressed fish and skin extract from a stressed fish. Plasma glucose and hepatic hsp70 levels were not affected by treatments. The results suggest that rainbow trout elicit a stress response when exposed to stress-related alarm cues released from conspecifics.     

Comparative effects of four feed types on white spot disease susceptibility and skin immune parameters in rainbow trout, Oncorhynchus mykiss (Walbaum)

The effects on rainbow trout, Oncorhynchus mykiss (Walbaum), immune parameters by differently formulated fish feed types containing immunostimulants have been tested in a double-blind, duplicated and controlled study performed over 50 days. A total of 800 rainbow trout (10-12 g) were kept in eight duplicate fish tanks (each containing 100 fish) and fed at a daily feeding rate of 1.5% of the biomass. The feed types were (1) control feed (C) without additives, (2) feed containing beta-glucan, nucleotides, manno-oligosaccharides (MOS), vitamins C and E (GNMCE), (3) feed containing probiotic bacteria and plant extracts (PP) and (4) feed with nucleotides, manno-oligosaccharides, vitamins C and E (NMCE). Plasma lysozyme activity was increased in fish fed two feed types (GNMCE and NMCE) but slightly depressed in fish fed PP. A non-significant trend for a higher mucous cell density at days 30 and 50 was shown in all fish receiving feeds with additives compared to the control group. All fish became infected with Ichthyophthirius multifiliis when exposed, but fish fed GNMCE showed a significantly lower infection both at days 30 and 50. Expression of genes encoding C3 and MHCII was significantly up-regulated in fish fed GNMCE for 50 days, and the expression of genes coding Hepcidin was significantly down-regulated in fish fed NMCE for 50 days. Beta-glucan was the single component, when used in combination with other feed ingredients, which was found associated with increased parasite resistance, increased lysozyme and immune gene up-regulation.     

Hexamitiasis leads to lower metabolic rates in rainbow trout Oncorhynchus mykiss (Walbaum) juveniles

This study assessed the effects of Hexamita salmonis (Moore) on metabolism of rainbow trout Oncorhynchus mykiss (Walbaum) and its effect on the host's susceptibility to infectious pancreatic necrosis virus (IPNV) after antiparasitic treatment. Rainbow trout naturally infected with H. salmonis were treated with 10 mg metronidazole kg fish^?1 per day, and their physiological recovery was assessed through measuring resting metabolism on the 7th, 14th, 21st and 28th day after treatment. In addition, we exposed the naïve fish to H. salmonis and measured the resting metabolism (oxygen consumption as mg O₂ kg^?1 per hour) on the 10th, 20th and 30th day after the exposure to assess the variation in metabolic rates after infection. Significantly lower rates of metabolic activity (P < 0.05) were anticipated 20 days after infection with H. salmonis compared with the fish infected with H. salmonis for 10 days or with the parasite‐free fish. Similarly, the treated fish needed about 20 days to fully recover from hexamitiasis. The susceptibility of rainbow trout to IPNV remained unchanged in the presence of H. salmonis. Weight loss was significantly higher (P < 0.05) in infected than that in the parasite‐free fish. Fish should be examined regularly for H. salmonis and treated immediately whether found to prevent economic losses and excessive size variation.     

Evaluation of 100% Fish Meal Substitution with Chicken Concentrate,Protein Poultry By‐Product Blend,and Chicken and Egg Concentrate on Growth and Disease Resistance of Juvenile Rainbow Trout,Oncorhynchus mykiss

The ability of poultry products to replace fish meal in diets for rainbow trout, Oncorhynchus mykiss, depends on their nutrient composition, cost, and consistency. The aim of this study was to assess the ability of three commercially available poultry products (chicken concentrate, CC, poultry by‐product blend, PBB; or chicken and egg concentrate, CE) to maintain growth and disease resistance when substituted for fish meal in a rainbow trout starter diet. A control diet was formulated to contain 48% crude protein and 18% crude lipid; 100% of the fish meal in test diets was replaced with CC, PBB, or CE. At stocking, fry were counted into groups (50 fish/tank) with six replicate tanks for each diet and fed their respective diets four times daily for 8 wk. All the poultry‐based diets supported growth (over 1600% increase over initial weight), nutrient retention, and feed conversion ratios of rainbow trout fry equal to or greater than those observed for fry fed with the fish meal‐based diet. No effect of diet on survival was observed following subcutaneous injection challenge with Flavobacterium psychrophilum. These data suggest that the examined products can be used in place of fish meal for rainbow trout fry without lowering growth and disease resistance.     

Timing and duration of constant light affects rainbow trout (Oncorhynchus mykiss) growth during autumn–spring grow-out in freshwater

Photoperiod enhancement of growth is becoming an area of increasing interest as a means of enhancing rainbow trout production efficiency in commercial practice. This paper examines the possible implications of shortening periods of constant light (LL) exposure on rainbow trout growth during autumn–spring grow-out under ambient water temperatures in freshwater to portion size. Triplicate groups of juvenile all-female rainbow trout were permanently exposed to LL in October, November, December or January. Growth was monitored and compared with those maintained under a simulated natural photoperiod (SNP) until the following May. Permanent exposure to LL (all treatments) resulted in significantly greater weight gain of rainbow trout than those under SNP. Furthermore, greatest growth was achieved when fish were left permanently exposed to LL from October. These findings suggest there may be implications for fish farmers if the period of photoperiod exposure is reduced, or timing of application is not considered with regards to ambient water temperatures.