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USGS1993

Effects of diet and environmental salinity on the growth, mortality, and tissue structure of juvenile striped bass

Juvenile striped bass Morone saxatilis (approximately 60 d old) were fed either a commercial trout feed or an open-formula salmon feed for 16 weeks and reared in 24°C fresh water or saline water prepared from sodium chloride or synthetic sea salt. Fish that were fed salmon feed were larger, contained higher levels of body lipid, and had livers with uniform glycogen vacuolation. Fish that were fed trout feed were smaller, contained less body lipid, and had necrotic liver tissue. More than 95% of the fish fed the trout feed and reared in water containing sodium chloride at 10‰ salinity died within the first 2 weeks. Mortality of fish fed the salmon feed under the same rearing conditions was 3% in 2 weeks. An osmoregulatory dysfunction from an imbalance of environmental calcium and sodium is suggested as the cause of the mortality. Although no definitive cause was established for the diet-related histological changes in the livers of fish that were fed the trout feed, type and level of dietary essential fatty acids are suspected

Journal of Aquatic Animal Health↗

USGS1994

Effects of elevated water temperature and immunosuppressant treatment on prevalence and titer of infectious pancreatic necrosis virus in naturally infected brook trout

Using fingerlings of brook trout Salvelinus fontinalis naturally infected with infectious pancreatic necrosis virus (IPNV), we demonstrated that elevated water temperature and treatment with the immunosuppressant triamcinolone acetonide (generic Kenalog®) significantly increases the titer of IPNV and probably also the prevalence of the infection. Stress-promoting treatments could potentially enhance the capability to detect various fish viral agents.

Journal of Aquatic Animal Health↗

USGS1994

Plasma chloride and gill epithelial response of channel catfish to infection with Ichthyophthirius multifillis

Because infection with Ichthyophthirius multifiliis has been reported to result in osmotic stress, genesis of the plasma chloride and chloride cell response of channel catfish Ictalurus punctatus to the infection was studied from day 0 to day 7 postexposure (PE). Parasite population densities ranged from 0.08 to 0.22 trophonts per gill filament during days 1–5 PE and increased to 2.1 at day 7 PE. Population increases were the result of infection by a new generation of infective theronts and reproduction by the parasite on the host. Mean plasma chloride concentrations of fish were not significantly different among days 0–5 PE (range of means, 93.01–97.87 meq/L), but by day 7 PE the mean had declined significantly, to 83.73 meq/L. Maintenance of relatively stable plasma chloride concentrations for 5 d was associated with chloride cell proliferation. The number of interlamellar filament chloride cells had significantly increased by day 1 PE, and the number of chloride cells in the lamellae had significantly increased by day 5 PE. No further increase had occurred by day 7 PE. Vacuolization of chloride cells, examined by electron microscopy, became more common as infection progressed, and many chloride cells appeared to be degenerating by day 5 PE.

Journal of Aquatic Animal Health↗

USGS1994

Observations on the occurrence of bacterial gill disease and amoeba gill infestation in rainbow trout cultured in a water recirculation system

Spontaneous outbreaks of bacterial gill disease (BGD) occurred in 17-44-g rainbow trout Oncorhynchus mykiss after they were placed in a water recirculation system. In each of five groups stocked from September 1991 through July 1992, BGD occurred within 6–8 d after stocking. In each instance, BGD was followed by a secondary amoeba infestation. The spontaneous BGD outbreaks did not occur among previously stocked groups that had recovered from earlier BGD disease outbreaks. Examination of gill tissue by Gram stain and indirect fluorescent antibody technique showed increased numbers of filamentous bacteria associated with BGD after the rainbow trout were stocked into the system. Bacterial numbers decreased after a 1-h treatment with chloramine-T at concentrations of 9–15 mg/L but increased within 2 d after treatment. Although the chloramine-T treatments controlled mortality related to BGD, the amoeba infestation persisted. Histological examination of gills showed some focal hyperplasia before the rainbow trout were placed into the recirculation system, but hyperplasia became more extensive and lamellar fusion and mild telangiectasis developed within a week after placement in the system. The density at which the fingerling rainbow trout were stocked and the suspended solids present in tank water may have contributed to the BGD outbreaks. Exposure of juvenile rainbow trout to tank water from the recirculation system before they were placed into the system did not afford them protection against BGD after stocking.

Journal of Aquatic Animal Health↗

USGS1994

Loma sp. salmonids from the Eastern United States: Associated lesions rainbow trout

A microsporidian of the genus Loma was noted in the gills of rainbow trout Oncorhynchus mykiss from a state hatchery (Buford Trout Hatchery) in Georgia. Mortalities of varying severity occur at this hatchery every fall, and the microsporidian was noted during an experiment from August 1991 to January 1992 to determine the effects of water source on disease. Infections first appeared to be systemic in the October sample; xenomas were observed in heart, spleen, and peripheral vessel walls. The presence of unidentified intracellular material preceded the appearance of xenomas in all tissues, but whether this material was associated with inflammation or represented immature stages of the parasite has yet to be determined. These structures were also noted in the intestine and liver, although xenomas were not noted in these organs. Mature xenomas did not elicit an inflammatory response but appeared to be short-lived. When the xenoma wall ruptured and released spores, an inflammatory response was again observed. The prevalence and severity of the infection were determined in fish maintained in troughs with well water, Chattahoochee River water, or hatchery (treated river) water. The infection tended to be more prevalent and more severe in fish maintained in the hatchery or river water than in those maintained in the well water. Stress induced by poor water quality may increase mortality from this parasite. This report extends the range of Loma sp. into the eastern United States.

Journal of Aquatic Animal Health↗

USGS1995

Pathological and behavioral manifestations of the “Cayuga syndrome,” a thiamine deficiency in larval landlocked Atlantic salmon

The “Cayuga syndrome” is a maternally transmitted, naturally occurring thiamine deficiency that causes 100% mortality of larval landlocked Atlantic salmon Salmo salar in several of New York's Finger Lakes, Results of multiyear studies to qualify and quantify the neurobehavioral and gross pathological signs of this condition are described, Affected sac fry became weak and ataxic and responded atypically to stimuli 1–2 weeks before death. Quantitative assays of stimulus-provoked swimming revealed a significant neuropathy whereby the sac fry exhibited abnormal thigmotactic and phototactic behaviors. Gross lesions observed in Cayuga sac fry included yolk-sac opacities, subcutaneous edema, vitelline hemorrhage or congestion, pericardial edema, retrobulbar edema, branchial congestion, foreshortened maxillae, hydrocephalus, and occasional caudal fin deformities, Lesion frequency in progeny differed significantly between dam source. Yolk conversion efficiency was decreased at least 1 week before death, suggesting that the bioenergetics of the fish was compromised and thereby supporting the thiamine residue and treatment data reported elsewhere, Comparisons with coagulated-yolk, blue-sac and swim-up syndromes are presented, The pathological signs of the Cayuga syndrome represent a unique departure from the lesions induced by toxicants or pathogens in other piscine models, and for the first time profile the profound effects of thiamine deficiency on cardiovascular and neurological systems of larval fish.

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USGS1996

Epizootiological study of bacterial cold-water disease in Pacific salmon and further characterization of the etiologic agent, Flexibacterin psychrophila

Isolates of Flexibacter psychrophila were obtained from chinook salmon Oncorhynchus tshawytscha and coho salmon O. kisutch that had previously sustained epizootics of coldwater disease. The pathogen was readily isolated from kidney and mucus of convalescent fish. The organisms were relatively inert in most standard microbiological media but were structurally and serologically homogenous by examination of whole cell protein lysates by sodium dodecyl sulfate polyacrylamide gel electrophoresis. In contrast to the homogeneity observed in phenotypic and serologic assays, the isolates studied elaborated varied ribotypes. All isolates produced a single rDNA spacer amplification product of about 240 base pairs.

Journal of Aquatic Animal Health↗

USGS1996

Incidence of Renibacterium salmoninarum infections in juvenile hatchery spring chinook salmon in the Columbia and Snake Rivers

From 1988 through 1992, we assessed the prevalence (frequency of occurrence) and severity (degree of infection) of Renibacterium salmoninarum (RS) among fish in marked groups of Columbia River basin and Snake River basin hatchery spring chinook salmon Oncorhynchus tshawytscha before release and during their seaward migration. During the study, prevalence of RS infection decreased (from >90% to <65%) in six of the eight hatchery groups. We attributed this decrease to changes in hatchery practices that reduced vertical and horizontal transmission. Fish from Snake River hatcheries had a higher prevalence of infection when sampled at dams (mean >90%) than in the hatchery (mean <70%), but there were no differences in similar comparisons of Columbia River fish. Although prevalence and severity of RS infection were not correlated in the groups studied, it appears that fish from the Snake River were more severely infected than those from the Columbia River. Some groups of Snake River fish had higher severity of infection at dams than in the hatchery, but infection in fish from Columbia River hatcheries did not change. These differences between Snake River and Columbia River fish might have resulted from differences in river conditions and the distances from hatcheries to dams.

Oregon, Washington↗

USGS1996

Nature of Aeromonas salmonicida carriage on asymptomatic rainbow trout maintained in a culture system with recirculating water and fluidized sand biofilters

An asymptomatic carrier population of rainbow trout Oncorhynchus mykiss was examined for Aeromonas salmonicida by primary dilution counts on Coomassie Brilliant Blue agar and also by streaking on bacteriological media after a 24–48-h pre-enrichment in tryptic soy broth. The pathogen was detected by primary dilution plate counts in 1 spleen, 15 gill, and 19 mucus samples of the 100 trout examined. Aerornonas samonicida was detected only after 48-h preenrichment in the one spleen that had already tested positive via primary dilution counts. The pathogen was not detected in kidney, liver, and intestinal samples. The occurrence of the pathogen in mucus and gills suggests a predominantly external nature of asymptomatic carriage of A. salmonicida within this population of fish. Repeated examination of fluidized biofilters and tank water showed that A. salmonicida did not become established in the recirculation system.

Journal of Aquatic Animal Health↗

USGS1996

Control of external Aeromonas salmonicida: topical disinfection of salmonids with Chloramine-T

Brook trout Salvelinus fontinalis were treated with single 60-min static baths of 250 mg formalin/L, 3% NaCl, and 15 mg Chloramine-T/L to evaluate the efficacy of these compounds against external infections of Aeromonas salmonicida. Prevalence of A. salmonicida was significantly lower in brook trout treated with Chloramine-T than among those treated with formalin or salt. Further laboratory tests substantiated the therapeutic value of a single treatment of ChloramineT (15 mg/L) against A. salmonicida . In two experiments, viable counts of A. salmonicida in mucus did not vary among replicate groups of treated brook trout, but the counts for treated fish were significantly (P < 0.05) lower than those for untreated controls. In vitro tube dilution assays indicated that mean minimum inhibitory concentrations of Chloramine-T for 10 isolates of A. salmonicida were 9.0 mg/L for 1 h and 2.25 mg/L, for 24 h. In field trials at the White River National Fish Hatchery (Bethel, Vermont), the pathogen was detected principally as an external infection of juvenile Atlantic salmon Salmo solar maintained in two culture ponds. In one pond, the bacterium accounted for 100% of the total distribution of tnicroflora isolated from mucus. Seven days after treatment with Chloramine-T, A. sahnonicida accounted for 11% of the total bacterial counts identified from these fish. In the second pond, A. salmonicida composed 3% of the counts of bacteria isolated from the mucus of fish before treatment but was not isolated after treatment.

Journal of Aquatic Animal Health↗

USGS1996

Hepatic neoplasms in wild common carp

Common carp Cyprinus carpio from West Point Lake, Georgia-Alabama, were sampled ( N = 81) during fall 1991. Gross examination revealed single nodules in the livers of four fish; representative tissues from all sampled fish were preserved for histological examination. One of the four nodules was a hepatocellular adenoma and the other three were hepatocellular carcinomas. Two additional neoplasms, a cholangioma and a cholangiocellular carcinoma, were found in one of the nodule-containing livers. Nonneoplastic liver changes that were found microscopically included nuclear pleomorphism, hepatic megalocytosis, and occurrence of clear cell foci. Possible predisposing factors, including exposure to polycyclic aromatic hydrocarbons, are discussed. These are the first reported cases of hepatic neoplasms in common carp, a species generally considered resistant to hepatocellular neoplasia.

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USGS1997

Effects of ultraviolet-B radiation on fish: Histologic comparison of a UVB-sensitive and a UVB-tolerant species

Lahontan cutthroat trout Oncorhynchus clarki henshatvi were sensitive to simulated solar ultraviolet-B radiation (UVB) and exhibited grossly visible signs of sunburn upon exposure. Razorback suckers Xyrauchen texanus , however, were tolerant to simulated solar UVB and showed no grossly visible signs of exposure. Cutthroat trout also had considerably less of an unidentified, possibly photoprotective, substance in the skin than did razorback suckers. In an attempt to characterize the cellular response to simulated solar UVB exposure in the skin of these two species, we examined sections from UVB-exposed fish by light and electron microscopy. Cutthroat trout showed grossly visible signs of sunburn by 48 h. Histologic observations included a sloughing of the mucous cells, necrosis and edema in the epidermis and dermis, and, in some cases, secondary fungal infections. Razorback suckers did not show any visible signs of sunburn during 72 h of experimental exposure. Histologic analyses revealed that cell necrosis had occurred, but the severe necrosis and sloughing noted in cutthroat trout was not observed. An increase in epidermal thickness, apparently due to hypertrophy and hyperplasia of large PAS-negative cells, occurred in the razorback suckers. These cells contained a large central region of low electron density and appeared to be club cells. In some, extensive interdigitation of the electron-lucent cytoplasm with adjacent epithelial cell margins occurred. Near the surface of the epidermis these cells were larger and the interface with epithelial cells lacked complex interdigitation. These cells may contain the substance that appears to protect razorback suckers against UV-B radiation.

Journal of Aquatic Animal Health↗

USGS1997

Virulence of Renibacterium salmoninarum to salmonids

Virulence of Renibacterium salmoninarum isolates representing five origins was evaluated in eight salmonid hosts; four origins were of Lake Michigan and the fifth was of the Pacific Northwest. The species type strain, ATCC (American Type Culture Collection) 33209, was also included. Each isolate was grown in a kidney disease medium (KDM2) supplemented with 1 % ATCC 33209 culture metabolite; serial 10-fold dilutions were prepared, and groups of fish were challenged by intraperitoneal injection with 0.1 mL of each dilution. A 70-d observation period followed, and bacterial kidney disease (BKD) was diagnosed by the fluorescent antibody technique. Virulence of isolates was quantified as a dose lethal to 50% of fish (LD50) for each host–isolate challenge. In the first set of experiments, 23 isolates were used to challenge groups of brook trout Salvelinus fontinalis. The mean LD50 was 1.087 x 10 6 colony-forming units per milliliter (cfu/mL; SD = 2.022 x 10 6 ), and the LD50 values ranged from 8.457 x 10 6 to 2.227 x 10 4 cfu/mL. Analysis of variance to evaluate the effect of isolate origin on virulence in brook trout revealed no significant difference ( F = 1.502; P = 0.243). Susceptibilities of the other salmonid hosts were evaluated by challenge with six isolates of R. salmoninarum representing each origin and the species type strain. For many of the host–isolate challenge combinations, time to death was highly dependent on the dilution (number of bacteria) injected. In general, the isolates MCO4M, B26, and A34 (all of Lake Michigan origin) tended to be more virulent. Also, LD50 values were dispersed throughout a wider range among the more susceptible hosts. Lake trout Salvelinus namaycush, rainbow trout Oncorhynchus mykiss, and brook trout were relatively resistant to challenge with the strains, whereas coho salmon O. kisutch, domestic Atlantic salmon Saltno salar, and chinook salmon O. tshawytscha were relatively susceptible. Another challenge evaluated the effect of washing R. salmoninarum MCO4M cells before injection into brook trout. The calculated LD50 value, 2.009 x 10 5 cfu/mL, was similar ( X 2 = 0.878; P = 0.645) to that of the unwashed cells (1.163 x 10 5 cfu/mL). Furthermore, times to death for successive dilutions were similar regardless of whether or not the cells were washed.

Journal of Aquatic Animal Health↗

USGS1997

Recurrent amoebic gill infestation in rainbow trout cultured in a semiclosed water recirculation system

Five lots of commercially purchased juvenile rainbow trout Oncorhynchus mykiss (17–44 g) stocked in a continuous-production water recirculation system became infested with gill amoebae. The amoebae were introduced into the recirculation system, as evidenced by their presence on gills of fish held in quarantine tanks. Based on their morphology, as seen in histological sections and by electron microscopy, the amoebae appeared to be more closely related to the family Cochliopodiidae than to other taxa of free-living amoebae. Attempts to culture the amoebae in different media, at different temperatures of incubation, and in fish cell culture were not successful. Initial treatment of the recirculation system with formalin at 167 parts per million (ppm) for 1 h eliminated amoebae from the gills. Subsequent treatments of the entire system with formalin at 50–167 ppm reduced the intensity of further infestations.

Journal of Aquatic Animal Health↗

USGS1997

Evaluation by enzyme-linked immunosorbent assay (ELISA) of Renibacterium salmoninarum bacterins affected by persistence of bacterial antigens

Rainbow trout Oncorhynchus mykiss were injected intraperitoneally with a bacterin containing killed Renibacterium salmoninarum cells delivered alone or in an oil-based adjuvant. We evaluated the relative abilities of the batterins to prevent the initiation or progression of infection in fish challenged by waterborne exposure to R. salmoninarum. Sixty-one days after vaccination, fish were held for 24 h in water containing either no bacteria or approximately 1.7 x 103, 1.7 x 105, or 5.3 x 106 live R. salmoninarum cells/mL. An enzyme-linked immunosorbent assay (ELISA) was used to monitor changes in the levels of R. salmoninarum antigen in live fish before and after the immersion challenges. High levels of R. salmoninarum antigens were detected by ELISA in kidney-spleen tissue homogenates from vaccinated fish immediately before the challenges. Levels of those antigens remained high in the tissues of unchallenged fish throughout the study. We found that the ELISA used in this study may be unsuitable for evaluating the efficacy of batterins because it did not distinguish antigens produced by the challenge bacteria during an infection from those of the bacterins. Groups of control and vaccinated fish also were injected with either 1.7 x 104 or 1.7 x 106 R. salmoninarum cells and served as R. salmoninarum virulence controls. Relative survival among the various subgroups in the injection challenge suggests that adverse effects might have been associated with the adjuvant used in this study. The lowest survival at both injection challenge levels was among fish vaccinated with bacteria in adjuvant.

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USGS1997

Bactericidal activity of juvenile chinook salmon macrophages against Aeromonas salmonicida after exposure to live or heat-killed Renibacterium salmoninarum or to soluble proteins produced by R. salmoninarum

Macrophages isolated from the anterior kidney of juvenile chinook salmon Oncorhynchus tshawytscha in 96-well microtiter plates were exposed for 72 h to 0, 10 5 , or 10 6 live or heat-killed Renibacterium salmoninarum cells per well or to 0, 0.1, 1.0, or 10 μg/mL of R. salmoninarum soluble proteins. After treatment, the bactericidal activity of the macrophages against Aeromonas salmonicida was determined by a colorimetric assay based on the reduction of the tetrazolium dye MTT to formazan by viable bacteria. The MTT assay was modified to allow estimation of the percentage of bacteria killed by reference to a standard curve relating the number of bacteria added to microtiter wells to absorbance by formazan at 600 nm. The live and heatkilled R. salmoninarum treatments significantly ( P < 0.001) increased killing of A. salmonicida by chinook salmon macrophages. In each of the five trials, significantly ( P < 0.05) greater increases in killing occurred after exposure to 10 5 R. salmoninarum cells than to 10 6 R. salmoninarum cells per well. In contrast, treatment of macrophages with 10 ug/mL R. salmoninarum soluble proteins significantly ( P < 0.001) decreased killing of A. salmonicida , but treatment with lower doses did not. These results show that the bactericidal activity of chinook salmon macrophages is stimulated by exposure to R. salmoninarum cells at lower dose levels but inhibited by exposure to R. salmoninarum cells or soluble proteins at higher dose levels.

Journal of Aquatic Animal Health↗

USGS1997

The North American strain of viral hemorrhagic septicemia virus is highly pathogenic for laboratory-reared Pacific herring ( Clupea pallasi )

Specific-pathogen-free Pacific herring Clupea pallasi were reared in the laboratory from eggs and then challenged at 5, 9, and 13 months of age by waterborne exposure to low (10 1.5–2.5 plaque-forming units [PFU] per milliliter), medium (10 3.5–4.5 PFU/mL), or high (10 5.5–6.5 PFU/mL) levels of a North American isolate of viral hemorrhagic septicemia virus (VHSV). The fish were extremely susceptible to the virus, showing clinical disease, mortality approaching 100%, and only a limited increase in resistance with age. Mortality began 4–6 d after exposure and peaked at approximately day 7 in fish exposed to high levels of virus. Whereas the mean time to death showed a significant dose response ( P < 0.001), the percent mortality and virus titers in dead fish were generally high in all groups regardless of initial challenge dose. External signs of disease were usually limited to 1–2-mm hemorrhagic areas on the lower jaw and isthmus and around the eye, but 2 of 130 infected fish exhibited extensive cutaneous hemorrhaging. Histopathologic examination of tissues from moribund fish sampled at 2–8 d after exposure revealed multifocal coagulative necrosis of hepatocytes, diffuse necrosis of interstitial hematopoietic tissues in the kidney, diffuse necrosis of the spleen, epidermis, and subcutis, and occasional necrosis of pancreatic acinar cells. Virus titers in tissues of experimentally infected herring were first detected 48 h after exposure and peaked 6-8 d after exposure at 10 7.7 PFU/g. Fish began shedding virus at 48 h after exposure with titers in the flow-through aquaria reaching 10 2.5 PFU/mL at 4–5 d after exposure, just before peak mortality. When the water flow was turned off for 3 h, titers in the water rose to 10 3.5 PFU/mL, and the amount of virus shed by infected fish (on average, greater than 10 6.5 PFU/h per fish) appeared sufficient to sustain a natural epizootic among schooling herring. Taken together, these data suggest that VHSV could be a significant limiting factor for populations of Pacific herring.

Journal of Aquatic Animal Health↗

USGS1998

Method for inducing saprolegniasis in channel catfish

A method was developed to uniformly and systematically induce saprolegniasis in channel catfish Ictalurus punctatus. Three different methods for inducing saprolegniasis were evaluated in waters containing known zoospore concentrations of Saprolegnia parasitica : (1) low-temperature shock to induce immunosuppression; (2) physical abrasion stress; and (3) a combination of both low temperature shock and abrasion stress. Low-temperature shock or abrasion stress alone were not effective for inducing saprolegniasis. Only 10% of fish stressed by low-temperature shock alone became infected. No fish receiving abrasion stress treatments alone became infected even though these fish were subject to significant abrasion and dewatering stress. A combination of low-temperature and abrasion stress, however, was sufficient to induce saprolegniasis in 100% of fish tested and resulted in 90% mortality. No fish became infected in the positive control group (exposed to zoospores of S. parasitica without stress) or in the negative control group. The combined-stress method should enable researchers to induce saprolegniasis in channel catfish at will to study its pathogenesis or to test the efficacy of candidate antifungal treatments. In conducting efficacy studies, therapeutic treatments must begin immediately when the first signs of saprolegniasis are observed because the disease progresses quickly and is deadly.

Journal of Aquatic Animal Health↗