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1.
Amoebic gill disease (AGD) of Atlantic salmon is treated commercially by bathing affected fish in freshwater. Recently, the efficacy of freshwater bathing has been questioned, and the aim of this study was to examine the potential for improving bathing efficacy using additives to the freshwater bath. AGD‐affected Atlantic salmon were bathed in 350 L tanks containing oxygenated freshwater to which chlorine dioxide (0–50 mg L?1), chloramine‐T (0–50 mg L?1) or hydrogen peroxide (0–100 μL L?1) was added. Before and following a 3‐h exposure to the freshwater and chemical additive, the gills were removed from a sub‐sample of fish and the number of live amoebae on the gills were counted and smears made for confirmation of the presence of Neoparamoeba pemaquidensis, the causative agent of AGD. Following a further 3‐h exposure, a sub‐sample of fish was bled from the caudal vein and the gills were removed for histological examination. Chlorine dioxide and chloramine‐T at 25–50 and 10–50 mg L?1, respectively, reduced the number of amoebae on the gills by approximately 50% compared with pre‐exposure numbers. The results from hydrogen peroxide treatment were equivocal and the toxicity of hydrogen peroxide was high. The toxicity of chlorine dioxide varied with freshwater hardness and/or suspended solid load, whereas chloramine‐T toxicity was low, with mortalities attributable only to elevated temperatures at the highest concentration tested. In conclusion, chlorine dioxide and chloramine‐T show promise as potential freshwater additives for the improved removal of N. pemaquidensis and possibly, other amoebae from the gills of commercially farmed Atlantic salmon.  相似文献   

2.
Amoebic gill disease (AGD) of cultured salmonids in Tasmania is caused by the amphizoic parasitic amoeba Neoparamoeba pemaquidensis. The freshwater tolerance of amoebae isolated from the gills of AGD-affected salmon (predominantly N. pemaquidensis) was tested in vitro using a trypan blue exclusion assay. Amoebae exposed to water containing high concentrations of Ca2+ or Mg2+ (200 mg l−1) showed high levels of survival up to 3 h of exposure. Exposure to water containing elevated Na+, choline chloride or water at different pH all had no significant survival of amoebae. Exposure of amoebae to different concentrations of chlorine dioxide, chloramine-T or hydrogen peroxide in artificially hard water demonstrated that chloramine-T and hydrogen peroxide were the most efficacious at killing amoebae in vitro. This work suggests that the hardness of freshwater may be an important factor for the survival of marine amoebae (predominantly N. pemaquidensis) on the gills of AGD-affected salmon and have significant implications with regard to the efficacy of freshwater bathing practices for the control of AGD on farms. Additionally, chloramine-T and hydrogen peroxide appear to be efficacious at killing marine gill amoebae in vitro and may be useful for the control of AGD in farmed Atlantic salmon.  相似文献   

3.
Previous work in our laboratory defined a method of inducing laboratory‐based amoebic gill disease (AGD) in Atlantic salmon, Salmo salar L. Gills of AGD‐affected fish were scraped and the debris placed into fish‐holding systems, eliciting AGD in naïve Atlantic salmon. While this method is consistently successful in inducing AGD, variability in the kinetics and severity of infections has been observed. It is believed that the infections are influenced by inherently variable viability of post‐harvest amoeba trophozoites. Here, a new method of experimental induction of AGD is presented that redefines the infection model including the minimum infective dose. Amoebae were partially purified from the gills of AGD‐affected Atlantic salmon. Trophozoites were characterized by light microscopy and immunocytochemistry and designated Neoparamoeba sp., possibly Neoparamoeba pemaquidensis. Cells were placed into experimental infection systems ranging in concentration from 0 to 500 cells L?1. AGD was detected by gross and histological examination in fish held in all systems inoculated with amoebae. The number of gross and histological AGD lesions per gill was proportional to the inoculating concentration of amoebae indicating that the severity of disease is a function of amoeba density in the water column. The implications of these observations are discussed in the context of the existing AGD literature base as well as Atlantic salmon farming in south‐eastern Tasmania.  相似文献   

4.
Currently, the only effective and commercially used treatment for amoebic gill disease (AGD) in farmed Tasmanian Atlantic salmon is freshwater bathing. Hydrogen peroxide (H2O2), commonly used throughout the aquaculture industry for a range of topical skin and gill infections, was trialled in vitro and in vivo to ascertain its potential as an alternative treatment against AGD. Under in vitro conditions, trophozoites of Neoparamoeba perurans were exposed to three concentrations of H2O2 in sea water (500, 1000 and 1500 mg L?1) over four durations (10, 20, 30 and 60 min) each at two temperatures (12 and 18 °C). Trophozoite viability was assessed immediately post‐exposure and after 24 h. A concentration/duration combination of 1000 mg L?1 for >10 min demonstrated potent amoebicidal activity. Subsequently, Atlantic salmon mildly affected with experimentally induced AGD were treated with H2O2 at 12 and 18 °C for 15 min at 1250 mg L?1 and their re‐infection rate was compared to freshwater‐treated fish over 21 days. Significant differences in the percentage of filaments affected with hyperplastic lesions (in association with amoebae) and plasma osmolality were noted between treatment groups immediately post‐bath. However, the results were largely equivocal in terms of disease resolution over a 3‐week period following treatment. These data suggest that H2O2 treatment in sea water successfully ameliorated a clinically light case of AGD under laboratory conditions.  相似文献   

5.
Currently, there are two methods of inducing laboratory‐based amoebic gill disease (AGD) in Atlantic salmon, Salmo salar L.: cohabitation with infected fish or exposure to a suspension of amoebae. Amoebic gill disease cannot be induced with cultured amoebae; therefore, the only source of the infective organism is salmon with the disease. For experimental purposes and to maintain pathogen supply, salmon are kept in an infection tank and amoebae are isolated from salmon once the disease establishes. In this way, discrete batches of amoebae are collected periodically. This study investigated the infective ability of different batches of amoebae. Furthermore, the effect of stocking density of salmon on the progression of AGD was also examined. The infective ability of different batches of amoebae isolated periodically from AGD‐affected salmon varied in terms of quantifiable pathology. Salmon stocking density had a significant impact on survival after amoebae challenge, with morbidity beginning 23 days post challenge in tanks stocked at 5.0 kg m?3 and 29 days for those stocked at 1.7 kg m?3. For uniform initiation of AGD in multiple tanks, amoebae batches should be equally divided and added to tanks until the required concentration is reached and to maintain a standard biomass between replicate tanks and treatments.  相似文献   

6.
The current treatment for amoebic gill disease (AGD)-affected Atlantic salmon involves bathing sea-caged fish in fresh water, often sourced from local dams, for 3-4 h. In both a small-scale laboratory and an on-farm field experiment, the effects of water hardness on the efficacy of freshwater bathing were assessed. Results showed that soft fresh water (19.3-37.4 mg L(-1) CaCO3), whether it be naturally soft city mains water or artificially softened dam water, was more efficacious at alleviating AGD in affected fish than hard fresh water (173-236.3 mg L(-1) CaCO3). Soft freshwater bathing significantly reduced viable gill amoebae numbers (from 73.9 to 40.9% of total count) and significantly alleviated gill pathology, both gross and histological. Following bathing, gross gill pathological scores of soft freshwater bathed fish lagged 2 weeks behind hard freshwater bathed fish. Significant gill lesion fragmentation, and shedding of lesion-associated hyperplastic tissue, was accompanied by a significant reduction in AGD-affected gill filaments in soft freshwater bathed fish. Furthermore, soft freshwater bathing alleviated the blood plasma electrolyte imbalance seen in control (sea water) and hard freshwater bathed fish. This study showed that the use of soft fresh water for bathing AGD-affected Atlantic salmon could be an improvement to the current method of treatment. Not only does it reduce gill amoeba numbers, but also, it is of a therapeutic advantage with the potential to reduce bathing frequency.  相似文献   

7.
Infections of gill amoebae that manifest as amoebic gill disease (AGD) occur in Atlantic salmon in Tasmania. The treatment of choice is freshwater bathing; however, the effectiveness of this treatment has declined over time. In this experiment, cage trials of chloramine‐T (Cl‐T) to treat AGD in Atlantic salmon were conducted over 3 months, and involved an initial bath in either freshwater or seawater with Cl‐T, followed by a second bath 6 weeks later. Amoeba densities were reduced to 50–80% of original values for both treatments. Neoparamoeba sp. density was not affected by bathing, and was not significantly different over the course of the experiment. Lesion prevalence was higher for Cl‐T‐treated fish than for freshwater‐treated fish, with overall prevalence levels of 14.30±1.00% and 8.03±0.57% respectively. This was also seen for gross gill scores. In the fortnight after each of the two baths, Cl‐T‐treated fish had significantly higher lesion levels, although this difference was then resolved by 4 weeks post bathing. The use of Cl‐T in seawater is at least as effective as freshwater at reducing amoebae density, and may be a more practical alternative when freshwater is in short supply.  相似文献   

8.
Amoebic gill disease (AGD) in farmed Atlantic salmon is caused by the amoeba Paramoeba perurans. The recent establishment of in vitro culture techniques for P. perurans has provided a valuable tool for studying the parasite in detail. In this study, flow cytometry was used to generate clonal cultures from single‐sorted amoeba, and these were used to successfully establish AGD in experimental Atlantic salmon. The clonal cultures displayed differences in virulence, based on gill scores. The P. perurans load on gills, determined by qPCR analysis, showed a positive relationship with gill score, and with clonal virulence, indicating that the ability of amoebae to proliferate and/or remain attached on gills may play a role in virulence. Gill scores based on gross signs and histopathological analysis were in agreement. No association between level of gill score and specific gill arch was observed. It was found that for fish with lower gill scores based on histopathological examination, gross examination and qPCR analysis of gills from the same fish were less successful in detecting lesions and amoebae, respectively.  相似文献   

9.
Amoebic gill disease (AGD) has been attributed to infection by Neoparamoeba sp. The causal mechanisms for AGD lesion development and the primary pathogenic role of Neoparamoeba sp. require elucidation. Three groups of Atlantic salmon were exposed to viable gill isolated amoebae, to sonicated amoebae, or to sea water containing viable amoebae without direct contact with gill epithelia. Fish were removed 8 days post-exposure and the gills assessed histologically for AGD. AGD occurred only when fish were exposed to viable trophozoites. Consequently, in an accompanying experiment, infection was evaluated histologically at 12, 24 and 48 h post-exposure in three groups of salmon, one group being mechanically injured 12 h prior to exposure. A progressive host response and significant increase (P < 0.001) in the numbers of attached amoebae was apparent over the 48-h duration in undamaged hemibranchs in both treatment groups. There were no significant differences to mucous cell populations. Attachment of Neoparamoeba sp. to damaged gill filaments was significantly reduced (P < 0.05) by 48 h post-exposure. These data further confirm and describe the primary pathogenic role of Neoparamoeba sp. and the early host response in AGD. Preliminary evidence suggests that lesions resulting from physical gill damage are not preferentially colonized by Neoparamoeba sp.  相似文献   

10.
Previous studies have indicated that Atlantic salmon, Salmo salar L., affected by amoebic gill disease (AGD) are resistant to re‐infection. These observations were based upon a comparison of gross gill lesion abundance between previously infected and naïve control fish. Anecdotal evidence from Atlantic salmon farms in southern Tasmania suggests that previous infection does not protect against AGD as indicated by a lack of temporal change in freshwater bathing intervals. Experiments were conducted to determine if previous infection of Atlantic salmon with Neoparamoeba sp. would provide protection against challenge and elucidate the immunological basis of any protection. Atlantic salmon were infected with Neoparamoeba sp. for 12 days then treated with a 4‐h freshwater bath. Fish were separated into two groups and maintained in either sea water or fresh water for 6 weeks. Fish were then transferred to one tank with a naïve control group and challenged with Neoparamoeba sp. Fish kept in sea water had lower mortality rates compared with first time exposed and freshwater maintained fish, however, these data are believed to be biased by ongoing mortalities during the seawater maintenance phase. Phagocyte function decreased over exposure time and freshwater maintained fish demonstrated an increased ability to mount a specific immune response. These results suggest that under the challenge conditions herein described, antigen exposure via infection does not induce protection to subsequent AGD.  相似文献   

11.
A 2-year study was carried out on amoebic gill disease (AGD) involving monthly samples of 1+ Atlantic salmon, Salmo salar L., smolts, histological assessment of the gills and analysis of environmental data. Gill pathology was seen before amoebae could be detected microscopically. These changes in gill integrity were associated with marine environmental conditions, particularly elevated ammonium, nitrite and chlorophyll levels. The results suggest that the environmental changes predispose salmon to colonization by amoebae and ciliates. High densities of histophagous scuticociliates were observed in the gills during periods of advanced gill pathology. A number of different amoebae were observed in close association with gill pathology. Neoparamoeba was not seen in high densities, nor was it associated with gill pathology, indicating that Neoparamoeba may not be the primary agent of the AGD in Irish salmonid culture.  相似文献   

12.
A relationship between increasing water temperature and amoebic gill disease (AGD) prevalence in Atlantic salmon (Salmo salar) has been noted at fish farms in numerous countries. In Scotland (UK), temperatures above 12°C are considered to be an important risk factor for AGD outbreaks. Thus, the purpose of this study was to test for the presence of an association between temperature and variation in the severity of AGD in Atlantic salmon at 10 and 15°C. The results showed an association between temperature and variation in AGD severity in salmon from analysis of histopathology and Paramoeba perurans load, reflecting an earlier and stronger infection post‐amoebae exposure at the higher temperature. While no significant difference between the two temperature treatment groups was found in plasma cortisol levels, both glucose and lactate levels increased when gill pathology was evident at both temperatures. Expression analysis of immune‐ and stress‐related genes showed more modulation in gills than in head kidney, revealing an organ‐specific response and an interplay between temperature and infection. In conclusion, temperature may not only affect the host response, but perhaps also favour higher attachment/growth capacity of the amoebae as seen with the earlier and stronger P. perurans infection at 15°C.  相似文献   

13.
Previous studies have indicated that when Atlantic salmon, Salmo salar L., are exposed to Neoparamoeba sp. the fish produce anti-Neoparamoeba sp. antibodies. It appears unlikely that these antibodies elicit any specific protection against amoebic gill disease (AGD) as fish with demonstrable activities have been affected by AGD. Experiments were conducted on Atlantic salmon cultured throughout Tasmania to assess the natural production of antibodies towards Neoparamoeba sp. Fish were sampled from areas where AGD was prevalent and from areas where there had been no reported cases. An enzyme-linked immunosorbent assay (ELISA) was used to measure anti-Neoparamoeba sp. antibody activities in serum. All fish from sea water had antibody activities greater than the negative control fish, including fish from areas with no reported cases of AGD. Time trial samples indicated that time after transfer to sea water did not appear to be a significant (P > 0.05) factor in antibody activity, however location was (P < 0.05). There was no agreement (corrected kappa value, 0.16) between the ELISA result and the isolation of Neoparamoeba sp. from the gills of the same fish. The results suggest that Atlantic salmon in seawater culture in Tasmania produce anti-Neoparamoeba sp. antibodies regardless of infection history, suggesting the presence of Neoparamoeba sp. in the environment.  相似文献   

14.
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15.
Four experiments were conducted to evaluate the effects of calcium and magnesium hardness on the acute toxicity of copper sulphate to Indian major carp, rohu (Labeo rohita, Hamilton) fingerlings and juvenile catfish (Channa punctatus, Bloch) in medium alkalinity experiments. A preliminary bioassay determined the 96 h LC50 of copper sulphate to be 0.56 mg L?1 for L. rohita fingerlings and 11.78 mg L?1 for juvenile C. punctatus placed in water with calcium hardness and total alkalinity set at 100 mg L?1 CaCO3. In the first experiment, rohu were exposed to 0.56 mg L?1 copper sulphate in environments where calcium hardness was varied from 50 to 350 mg L?1 CaCO3 and total alkalinity was 100 mg L?1 CaCO3. As calcium hardness increased, copper‐induced rohu mortalities decreased significantly from 90% at 50 mg L?1 CaCO3 to 7% at 350 mg L?1 CaCO3. In the second experiment, rohu were exposed to 0.56 mg L?1 copper sulphate in environments where magnesium hardness was varied from 50 to 350 mg L?1 CaCO3 with total alkalinity set at 100 mg L?1 CaCO3. Hundred percent mortality was observed in magnesium‐based hardness treatments. In the third experiment, catfish were exposed to 11.78 mg L?1 copper sulphate in environments where calcium hardness was varied from 50 to 400 mg L?1 and total alkalinity was 100 mg L?1 CaCO3. As calcium hardness increased, copper‐induced catfish mortalities decreased significantly from 90% at 50 mg L?1 CaCO3 to 4% at 400 mg L?1 CaCO3. In the fourth experiment, catfish were exposed to 11.78 mg L?1 copper sulphate in environments where magnesium hardness was varied from 50 to 400 mg L?1 CaCO3, with total alkalinity set at 100 mg L?1 CaCO3. In this case, 100% mortality was also observed in magnesium‐based treatments. Mortality rates in magnesium hardness treatments were consistent with those in the second experiment. These data suggest a calcium‐specific mechanism with respect to acute copper toxicity both in rohu and catfish.  相似文献   

16.
Amoebic gill disease (AGD), caused by the protozoan Neoparamoeba pemaquidensis (Page, 1987) is the most important disease affecting salmon farms in Tasmania. Reservoirs for this protozoan parasite are largely unknown. This study investigated wild fish as a potential reservoir of N. pemaquidensis . A total of 325 wild fish, comprising 12 different fish species, were caught from and around salmon farms and examined for the presence of AGD. None of the wild fish were infected with AGD. In a laboratory trial, seahorse, Hippocampus abdominalis , greenback flounder, Rhombosolea tapirina, and Atlantic salmon, Salmo salar, were challenged with N. pemaquidensis . Neoparamoeba pemaquidensis was detected on the gills on 10 of 15 (66.7%) flounder, nine of 24 (37.5%) seahorses, and six of six (100%) Atlantic salmon. However, paramoebae positive flounder and seahorse lacked the characteristic AGD gill pathology. It is concluded that AGD does not appear in wild fish and wild fish do not seem to be a reservoir of the pathogen.  相似文献   

17.
Amoebic gill disease (AGD) affects the marine culture phase of Atlantic salmon, Salmo salar L., in Tasmania. Here, we describe histopathological observations of AGD from smolts, sampled weekly, following transfer to estuarine/marine sites. AGD was initially detected histologically at week 13 post-transfer while gross signs were not observed for a further week post-transfer. Significant increases (P < 0.001) in the proportion of affected gill filaments occurred at weeks 18 and 19 post-transfer coinciding with the cessation of a halocline and increased water temperature at the cage sites. The progression of AGD histopathology, during the sampling period, was characterized by three phases. (1) Primary attachment/interaction associated with extremely localized host cellular alterations, juxtaposed to amoebae, including epithelial desquamation and oedema. (2) Innate immune response activation and initial focal hyperplasia of undifferentiated epithelial cells. (3) Finally, lesion expansion, squamation-stratification of epithelia at lesion surfaces and variable recruitment of mucous cells to these regions. A pattern of preferential colonization of amoebae at lesion margins was apparent during stage 3 of disease development. Together, these data suggest that AGD progression was linked to retraction of the estuarine halocline and increases in water temperature. The host response to gill infection with Neoparamoeba sp. is characterized by a focal fortification strategy concurrent with a migration of immunoregulatory cells to lesion-affected regions.  相似文献   

18.
This study examined the effects of water hardness and salinity on yolk sac larvae and swim‐up fry survival of Nile tilapia, Oreochromis niloticus (Chitralada strain), eggs during artificial incubation. Four experiments were conducted to evaluate the effects of hardness, salinity and the sources of saline incubation water. High water hardness treatments (500–4200 mg L?1 as CaCO3) resulted in higher yolk sac larvae and swim‐up fry survival than low water hardness treatments (50.0 and 132 mg L?1 as CaCO3); although yolk sac larvae and swim‐up fry survival did not differ among the high or low hardness treatments. Salinity of 4.0 g L?1 using seawater, and 4.0 and 8.0 g L?1 using unprocessed common salt resulted in the higher survival rate of yolk sac larvae and swim‐up fry than other salinity treatments. Yolk sac larvae and swim‐up fry survival was found to decrease with the increase in salinity and increase with the increase in water hardness. The present study demonstrated the positive effects of increased water hardness level (>132 mg L?1) on yolk sac larvae and swim‐up fry survival. The study also showed that seawater salinity of 4 g L?1 was the most appropriate salinity level for incubating Nile tilapia eggs.  相似文献   

19.
16S ribosomal RNA gene analysis was used to assess the bacterial community associated with Atlantic salmon, Salmo salar L., gills which were either affected by amoebic gill disease (AGD) or were AGD-negative, in order to determine the role that bacteria may play in the development of AGD. AGD-positive specimens were either infected in the laboratory with Neoparamoeba pemaquidensis, the causative agent of AGD, or were obtained from commercial salmon cages. Samples from laboratory fish maintained in sea water possessed a marine-type community while field samples which had been treated by a series of freshwater baths possessed a more diverse community which included variable proportions of different bacterial ecotypes, including groups typically associated with soil, skin surfaces and faeces. Samples from fish infected with AGD in the laboratory and a sample from one of two salmon cage fish specimens were dominated by a phylotype belonging to the strictly marine bacterial genus Psychroserpens (family Flavobacteriaceae, phylum Bacteroidetes). The phylotype was not detected in any of the AGD-negative samples or in one of two AGD-positive samples obtained from fish subjected to temporary freshwater immersion. The possibility of certain Psychroserpens species as potential opportunistic pathogens associated with salmonid AGD is proposed.  相似文献   

20.
The present study investigated the effects of water pH (5.0, 7.0 and 9.0), hardness (0, 20 and 120 mg CaCO3 L?1) and temperature (15, 23 and 30 °C) on the induction of sedation and anaesthesia, and subsequent recovery, of silver catfish exposed to eugenol. Moreover, the blood gas tensions (PvO2 and PvCO2) and blood pH in silver catfish acclimated to these temperatures were investigated after exposure to eugenol. Water pH, hardness, temperature and fish size affect the efficacy of eugenol in silver catfish, particularly at the lower concentrations tested (20 and 30 mg L?1). Sedation of this species can be induced at concentrations as low as 20 mg L?1, but for anaesthesia, a concentration of at least 40 mg L?1 of eugenol must be used to compensate for the influence of fish size and water quality. Blood gas tension and pH were affected by eugenol anaesthesia, but only in fish acclimated to 30 °C.  相似文献   

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