The prevalence of proliferative kidney disease from the kidney and muscle of rainbow and brown trout in Aragón (Spain)

The prevalence and parasite density of PKX (the unknown myxosporean that causes proliferative kidney disease [PKD] of salmonids) were investigated in eight fishfarms in Aragon, Spain. Tissue sections stained with the biotynilated lectin GS-I revealed the presence of this protozoan in only one of the farms. In rainbow trout, the renal prevalence and parasite density peaked in July, but in brown trout the maximum renal prevalence and maximum renal parasite density were reached in May and in July, respectively. In rainbow trout, after the acute phase of the disease, the number of PKX decreases in the kidney but increases in the muscle. In this species of fish, the prevalence and parasite density are much higher in the muscles than in the kidney in October.     

The first report of Gyrodactylus salaris Malmberg, 1957 (Platyhelminthes, Monogenea) on Italian cultured stocks of rainbow trout (Oncorhynchus mykiss Walbaum)

The monogenean Gyrodactylus salaris Malmberg, 1957 is considered one of the most important parasites of wild salmonids in the European Community due to the heavy ecological and economical damage it has inflicted on Atlantic salmon (Salmo salar) parr populations. Rainbow trout (Oncorhynchus mykiss) is susceptible to G. salaris and can act as a suitable carrier host and, consequently, its trade in EU territory is restricted in relation to the status of “recognized free” zones. Despite the economic importance of rainbow trout farming in Italy, information on the Italian gyrodactylid fauna is lacking and prior to this study, G. salaris had not been officially reported. During a routine health examination of farmed rainbow trout stock throughout Central and Northern Italy in 2004–2005, five fish farms were found to be infected with G. salaris alongside three other gyrodactylids. Morphological and molecular characterisation confirmed the presence of G. salaris, Gyrodactylus teuchis Lautraite, Blanc, Thiery, Daniel et Vigneulle, 1999 and Gyrodactylus derjavinoides Malmberg, Collins, Cunningham et Jalali, 2007, while Gyrodactylus truttae Gläser, 1974 was identified by morphological analysis only. The findings from this study extend the distribution of G. salaris within Europe and highlight the importance of the rainbow trout trade in its dissemination.     

Field Evaluation of Sensitivity and Specificity of a Polymerase Chain Reaction (PCR) for Detection of Nucleospora salmonis in Rainbow Trout

Abstract

We determined the sensitivity and specificity of a nested polymerase chain reaction (PCR) for detection of the microsporidian parasite Nucleospora salmonis in kidney tissue of rainbow trout Oncorhynchus mykiss. Kidney tissues were sampled on three dates from 162 juvenile rainbow trout obtained from a California State fish hatchery where the organism was endemic. Kidney tissues were used to prepare imprints stained with May–Grünwald Giemsa and for extraction of genomic DNA for a nested PCR test for N. salmonis. Positive PCR results for N. salmonis were obtained from 1 of 100, 2 of 32, and 27 of 30 kidneys collected on the first, second, and third sample dates, respectively. Kidney tissues from 3 of 27 trout in the third sample that tested positively by PCR also had microscopic evidence of parasites in stained kidney imprints. No parasites were detected in the remaining 159 kidney samples examined microscopically. Sensitivity and specificity of the PCR assay were estimated by using maximum likelihood estimation based on cross-classified test results. This method yielded estimates of sensitivity of 99.99% and specificity of 99.87%. This field evaluation supports experimental evidence that the nested PCR test will be a valuable diagnostic tool for prevention and control of N. salmonis as well as for risk assessment associated with fish movements.     

Effects of Salinity on Yersinia ruckeri Infection of Rainbow Trout and Brown Trout

Abstract

Juvenile rainbow trout Oncorhynchus mykiss and brown trout Salmo trutta acclimated to freshwater or salinities of 9.0‰ or less were exposed to Yersinia ruckeri, the bacterial pathogen that causes enteric redmouth disease (ERM). Both species of fish were kept in the same recirculating systems after bacterial exposure. Rainbow trout mortality was significantly (P < 0.05) different in each salinity: 96.5% in freshwater, 89.5% in water of 1.1‰ salinity, 81.3% in 3.0‰ salinity, and 75.0% in 9.0‰ salinity (model SE = 1.0). All deaths occurred between 3 and 12 d after exposure to Y. ruckeri. Only 2.3% of brown trout in all salinities died, and differences among treatments were not significant. For both fish species, Y. ruckeri was isolated from liver, spleen, and trunk kidney of fish dying during this experiment, and lesions of rainbow trout were consistent with ERM. Yersinia ruckeri was not isolated from brown trout surviving for 21 d after bacterial exposure but was isolated from 3 of 24 surviving rainbow trout; a polymerase chain reaction assay detected the DNA of Y. ruckeri in 3 additional rainbow trout survivors. Neither the lesions of fish with ERM nor the percentage of surviving fish subclinically infected with Y. ruckeri was affected by salinity. Bacterial growth in vitro was not affected by low (≤9.0‰) salinity; however, bacterial adhesion to polystyrene was significantly reduced as salinity increased. Although mortality caused by Y. ruckeri was significantly lower for rainbow trout in water with slightly increased salinity, none of the salinities tested was effective in preventing serious losses caused by this pathogen in recirculating systems.     

Myxobolus cerebralis infection in rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) exposed under natural stream conditions.

From early April into mid-June 1977, sequential groups of juvenile rainbow trout (Oncorhynchus mykiss) and brown trout (Salmo trutta) were each exposed for 10 days to the parasite Myxobolus cerebralis by immersion in a stream inhabited by infected wild trout. Following incubation in a M. cerebralis-free facility, trout were subsequently killed, and heads and gill arches were examined by routine histologic methods. A grading scale to quantify lesion severity was developed and applied. Percentage infected, lesion severity scores, effects of water temperature and flow rates on percentage infected and lesion severity scores, and resulting pathology were determined for each species at each exposure period. The percentage of rainbow trout infected with M. cerebralis was significantly higher than the percentage of brown trout infected for each exposure period. The percentages of rainbow trout infected in exposure periods later in the calendar year were significantly higher than those in earlier periods. The percentages of brown trout infected were not significantly different among exposure periods. Overall average lesion severity scores were significantly higher in rainbow than in brown trout. Lesion severity scores in rainbow trout increased over time (a positive correlation with exposure period). Lesion severity scores were not significantly different for brown trout among exposure periods. A significant correlation existed between water temperature and percentage of rainbow trout infected; a significant correlation also existed between water temperature and lesion severity scores in rainbow trout. Similar correlations did not exist for percentage of brown trout infected or accompanying lesion severity scores. In rainbow trout, ventral calvarium was the most common site of M. cerebralis replication, followed by gill arches. In brown trout, lesions were virtually confined to gill arches. Early lesions consisted of foci of cartilage necrosis with small numbers of M. cerebralis developmental stages. More advanced lesions consisted of multifocal areas of cartilage necrosis with numerous M. cerebralis developmental stages and/or mature myxospores bordered and/or infiltrated by mono- and multinuclear leukocytes. Lesions in brown trout were smaller and had fewer associated leukocytes and M. cerebralis developmental stages and/or mature myxospores. Higher infection rates, lesion severity scores, and differences in lesion location in rainbow versus brown trout explain in part why numbers of rainbow but not brown trout have fallen in western rivers inhabited with M. cerebralis-infected trout.     

California Golden Trout Oncorhynchus mykiss aguabonita Are Susceptible to Proliferative Kidney Disease

Abstract

An outbreak of proliferative kidney disease affecting farmed California golden trout Oncorhynchus mykiss aguabonita in the United Kingdom is described. The fish displayed the clinical signs of the disease, such as characteristic swelling of the posterior kidney and spleen. Confirmation of infection with Tetracapsuloides bryosalmonae in the renal tissue was done by means of electron microscopy and immunohistochemistry using a monoclonal antibody specific to T. bryosalmonae.     

The Effects of Myxobolus cerebralis on the Physiological Performance of Whirling Disease Resistant and Susceptible Strains of Rainbow Trout

Abstract

The development of rainbow trout Oncorhynchus mykiss strains that are resistant to whirling disease has shown promise as a management tool for populations in areas where Myxobolus cerebralis is present. However, the physiological effects of the disease on characteristics necessary for fish survival in natural river conditions have not been tested in many of these strains. Five rainbow trout strains were evaluated for their swimming ability and growth characteristics in relation to M. cerebralis exposure: the resistant German rainbow trout (GR) strain (Hofer strain), the susceptible Colorado River rainbow trout (CRR) strain, and three intermediate (hybrid) strains (F₁ = GR × CRR; F₂ = F₁ × F₁; B₂ = backcross of F₁ × CRR). Three broad response patterns among strain and exposure were evident in our study. First, exposure metrics, growth performance, and swimming ability differed among strains. Second, exposure to the parasite did not necessarily produce differences in growth or swimming ability. Exposure to M. cerebralis did not affect batch weight for any strain, and critical swimming velocity did not differ between exposed and unexposed families. Third, although exposure did not necessarily affect growth or swimming ability, individuals that exhibited clinical deformities did show reduced growth and swimming performance; fish with clinical deformities were significantly smaller and had lower critical swimming velocities than exposed fish without clinical deformities. Research and management have focused on GR × CRR hybrid strains; however, given the performance of the GR strain in our study, it should not be discounted as a potential broodstock. Additional field trials comparing the GR and F₁ strains should be conducted before wholesale adoption of the GR strain to reestablish rainbow trout populations in Colorado.

Received September 9, 2010; accepted May 27, 2011     

Developmental competence of eggs produced by rainbow trout Doubled Haploids (DHs) and generation of the clonal lines

Poor quality eggs produced by the fully homozygous doubled haploids (DHs) may impair generation of clonal lines in fish species. In the present research, gynogenetic development of rainbow trout (Oncorhynchus mykiss) was induced in eggs originated from the DH females. Eggs were activated with the UV‐irradiated grayling (Thymallus thymallus) spermatozoa and subjected to the high hydrostatic pressure (HHP) shock to provide diploid clonal individuals. Only two of four DH females produced eggs that were successfully activated by the irradiated spermatozoa and subsequently developed into the gynogenetic embryos. Survival rates of rainbow trout from the clonal lines equalled 21.5% and 19.8% during embryogenesis and decreased after hatching to 18.6% and 14.9%, respectively. Some of the dead rainbow trout clones collected between hatching and swim‐up stage were emaciated and exhibited spinal deformities including scoliosis. Provided results confirmed limited developmental competences of eggs produced by rainbow trout DH females. Clonal rainbow trout developing in such eggs exhibited reduced survival and increased frequency of the body abnormalities.     

Pathological effects of Arcobacter cryaerophilus infection in rainbow trout (Oncorhynchus mykiss Walbaum)

Arcobacter cryaerophilus was isolated from naturally infected rainbow trout (Oncorhynchus mykiss Walbaum), and its pathogenicity was tested by intramuscular injection into 40 healthy 1-year-old rainbow trout at 16 degrees C. The lethal dosage of 50% end point (LD50) for A. cryaerophilus was calculated 2.25 x 10(4) viable cells. Experimental infection caused deaths with gross clinical abnormalities such as degenerated opercula and gills, liver damage, haemorrhagic kidney and serous fluid in swollen intestines. The counts of A. cryaerophilus in kidney, liver and gills of experimentally infected fish ranged from 1.59 x 10(10) colony forming units (cfu)/g to 7.41 x 10(12) cfu/g. The means of erythrocyte (RBC) count, haematocrit level, serum cholesterol, triglyceride, albumin and total protein concentrations in the blood of the experimentally infected rainbow trout group were significantly lower than in the healthy fish. Leukocyte (WBC) counts of the experimentally infected rainbow trout were significantly higher than those of healthy fish. The present work shows that the selected blood characteristics may be good indicators of response to infections in rainbow trout.     

Alterations in the rainbow trout (Oncorhynchus mykiss) eggs exposed to ionizing radiation during induced androgenesis

Ionizing radiation (IR) is applied to inactivate nuclear genome in the salmonid eggs to induce androgenetic development. However, it has been considered that doses of IR used to damage maternal chromosomes may also affect morphology of the eggs and decrease their developmental potential. Thus, the main goal of the present research was to assess alterations in the rainbow trout (Oncorhynchus mykiss) eggs caused by the high dose of IR administered during androgenesis. In the present research, rainbow trout eggs were irradiated with 350 Gy of X‐rays, inseminated and exposed to the high hydrostatic pressure (HHP) shock to develop as androgenetic doubled haploids (DHs). The distribution of lipid droplets in the irradiated and non‐irradiated rainbow trout eggs, survival rates and morphology of larvae from androgenetic and control groups were compared. It has been observed that non‐irradiated and irradiated eggs exhibited altered distribution of lipid droplets. Most of the eggs before IR treatment displayed rather equal distribution of the oil droplets. In turn, majority of eggs studied after irradiation had coalesced lipid droplets, a pattern found in eggs with reduced quality. Incidences of abnormally developed larvae were more frequently observed among fish that hatched from the irradiated eggs. Observed changes suggest X‐rays applied for the genetic inactivation of rainbow trout eggs may lead to decrease of their developmental competence.     

The effects of Myxobolus cerebralis on the physiological performance of whirling disease resistant and susceptible strains of rainbow trout

The development of rainbow trout Oncorhynchus mykiss strains that are resistant to whirling disease has shown promise as a management tool for populations in areas where Myxobolus cerebralis is present. However, the physiological effects of the disease on characteristics necessary for fish survival in natural river conditions have not been tested in many of these strains. Five rainbow trout strains were evaluated for their swimming ability and growth characteristics in relation to M. cerebralis exposure: the resistant German rainbow trout (GR) strain (Hofer strain), the susceptible Colorado River rainbow trout (CRR) strain, and three intermediate (hybrid) strains (F1 = GR x CRR; F2 = F1 x F1; B2 = backcross of F1 x CRR). Three broad response patterns among strain and exposure were evident in our study. First, exposure metrics, growth performance, and swimming ability differed among strains. Second, exposure to the parasite did not necessarily produce differences in growth or swimming ability. Exposure to M. cerebralis did not affect batch weight for any strain, and critical swimming velocity did not differ between exposed and unexposed families. Third, although exposure did not necessarily affect growth or swimming ability, individuals that exhibited clinical deformities did show reduced growth and swimming performance; fish with clinical deformities were significantly smaller and had lower critical swimming velocities than exposed fish without clinical deformities. Research and management have focused on GR x CRR hybrid strains; however, given the performance of the GR strain in our study, it should not be discounted as a potential broodstock. Additional field trials comparing the GR and F1 strains should be conducted before wholesale adoption of the GR strain to reestablish rainbow trout populations in Colorado.     

Detection of Myxobolus cerebralis in Rainbow Trout and Oligochaete Tissues by Using a Nonradioactive in Situ Hybridization (ISH) Protocol

Abstract

A nonradioactive in situ hybridization (ISH) protocol was developed to detect Myxobolus cerebralis, the causative organism of whirling disease, in its primary host, rainbow trout Oncorhynchus mykiss, and in its alternate oligochaete host, Tubifex tubifex. A cocktail of three oligonucleotide primers (derived from the small subunit ribosomal DNA sequence) directed at target sequences of the parasite DNA was tailed at the 3′ end with digoxigenin-labeled deoxyuridine triphosphate (DIG-dUTP). Labeled probes were hybridized to parasite DNA present in deparaffinized tissue sections from infected trout and oligochaetes. The bound probes were visualized after modifications of existing ISH protocols. By using the new ISH procedure, the parasite was found in target tissues of subclinically and clinically infected fish and tubificid oligochaetes after exposures of these hosts to triactinomyxons and mature spores, respectively. The probe did not bind with salmonid tissues infected with two other myxosporean parasites, Ceratomyxa shasta or the PKX organism, or to a Myxobolus sp. infecting the cartilage of plain sculpin Myoxocephalus jaok. These initial results indicate that ISH is an effective and specific test for detecting Myxobolus cerebralis in its fish and oligochaete hosts.     

Host factors associated with the detection of Aeromonas salmonicida and Yersinia ruckeri in Ontario, Canada government fish hatcheries

This paper presents an epidemiological investigation of Ontario Ministry of Natural Resources Fish Health Laboratory data from 1981 to 1997, to determine whether fish species and age were associated with lot-level detection of Aeromonas salmonicida and Yersinia ruckeri in hatchery fish. In stepwise logistic regression, the species brook trout and back-cross (lake trout crossed with the hybrid “splake”) were more likely to test A. salmonicida-positive compared to all other species reared in the hatcheries. Similarly, the species brook trout was significantly more likely to test Y. ruckeri-positive compared to all other species. For both pathogens, the 1–5-month age group was associated significantly with detection. These findings suggest that purposive sampling of higher-risk fish lots could increase the likelihood of detecting both study pathogens.     

The dose–response relation for the antinociceptive effect of morphine in a fish,rainbow trout

Jones, S. G., Kamunde, C., Lemke, K., Stevens, E. D. The dose–response relation for the antinociceptive effect of morphine in a fish, rainbow trout. J. vet. Pharmacol. Therap.  35 , 563–570. There have been suggestions that analgesics be used by fish researchers. But in the absence of dose–response data for morphine, this suggestion seems imprudent. The purpose of the present study was to develop a dose–response relationship in fish using six doses of morphine. The response (movement of the fins or tail) to a noxious stimulus (electrical shock to the face region) was monitored before and after a dose of morphine intraperitoneally (i.p.). The i.p. dose of morphine ED₅₀ in rainbow trout was 6.7 ± 0.8 mg/kg (n = 12 at each dose). The plasma morphine concentration EC₅₀ was 4.1 ± 1.5 mg/L. In a second experiment, rainbow trout tested with equal amounts of morphine and naloxone (30 mg/kg) showed that the antinociceptive effect of morphine was blocked by naloxone. It has been suggested that stress‐induced analgesia has been a confounding factor in some fish studies. However, plasma cortisol levels in our study indicated that stress was not a confounding factor in the present experiments. The ED₅₀ for morphine in fish was higher than that reported for humans or other mammals. Our observation showing a dose–response relation for morphine using a noxious stimulus supports arguments for its effectiveness as an antinociceptive drug in fish.     

Blood Parameters and Immune Status of Rainbow Trout with Proliferative Kidney Disease

Abstract

Blood parameters, disease resistance, and the immune response were sequentially evaluated in rainbow trout Oncorhynchus mykiss with proliferative kidney disease (PKD). The fish were maintained under laboratory conditions, and the study group went through a full cycle of the disease. Hematological and serological changes occurred primarily in those fish with severe kidney lesions. Fish infected with the parasite that causes PKD demonstrated a greater resistance to bacterial challenge, and their immune responses were heightened when compared with those of uninfected fish. These data suggest that PKD alone is not a predisposing factor for secondary infections if the fish does not incur severe renal lesions.     

Comparison of hatchery and field performance between a whirling-disease-resistant strain and the Ten Sleep strain of rainbow trout

A whirling-disease-resistant strain of rainbow trout Oncorhynchus mykiss (GRHL strain) derived from a backcross of an F1 hybrid of two strains (German strain x Harrison Lake strain) with German strain females, was compared with the Ten Sleep (TS) strain of rainbow trout. The GRHL strain had consistently superior growth and feed conversion in two consecutive hatchery trials. Hatching and mortality rates were similar between strains. Both strains were stocked into two Utah reservoirs (Hyrum, Porcupine), and a third, Causey Reservoir, was monitored as a control for seasonal variation in prevalence of Myxobolus cerebralis. A total of 1,323 salmonids captured by gill net in spring and fall sampling between 2006 and 2008 were tested for M. cerebralis via pepsin-trypsin digest methods. Only eight of these (< 1% per species) had clinical signs consistent with whirling disease. In both reservoirs, GRHL survived better than the TS and had higher growth rates. The prevalence of M. cerebralis was significantly lower for GRHL (18.1%) than TS (50.0%) in Porcupine Reservoir. In Hyrum Reservoir the trend was similar, but prevalence was lower and did not significantly differ between GRHL (9.6%) and TS (23.1%). For infected fish, no significant differences were observed between strains in myxospore counts in either Hyrum (GRHL = 911-28,244 spores/fish [spf], TS = 1,822-155,800 spf) or Porcupine (GRHL = 333-426,667spf, TS = 333-230,511 spf) reservoirs. Unmarked rainbow trout in both reservoirs had significantly higher myxospore counts than stocked fish of either strain. There were significant differences in M. cerebralis prevalence and myxospore loads among other naturally reproducing salmonids in the reservoirs. The trend in susceptibility was cutthroat trout Oncorhynchus clarkii > kokanee Oncorhynchus nerka > brown trout Salmo trutta. The GRHL performed well in both hatchery and field settings and is recommended for stocking programs.     

Differing Resistance of Atlantic Salmon Strains and Rainbow Trout to Gyrodactylus salaris Infection

Abstract

Laboratory studies were conducted on the susceptibility of different strains of Atlantic salmon Salmo salar and rainbow trout Oncorhynchus mykiss to infection with the monogenean Gyrodactylus salaris. This parasite, probably originating from the Baltic Sea region, is known to minimally affect Neva River (Baltic Sea) Atlantic salmon. However, following its introduction into Norway, G. salaris has caused severe mortality and morbidity among Norwegian Atlantic salmon, which are considered a highly susceptible strain. The cohabitation experiment included one stock of rainbow trout and four different strains of Atlantic salmon from the Baltic Sea region (Mörrum River, Sweden), Europe (Skjern River, Denmark; Conon River, Scotland), and North America (Bristol Cove River, Canada). Fish were exposed to a Norwegian strain of G. salaris, and parasite population development and distribution were monitored for 7 weeks. Rainbow trout exhibited low susceptibility to G. salaris infection, whereas Conon River and Skjern River Atlantic salmon were highly susceptible and exhibited high mortality rates. Mörrum River Atlantic salmon exhibited intermediate susceptibility and low mortality. Bristol Cove River Atlantic salmon harbored relatively low parasite numbers, but fish mortality was high. Our experiment showed that the Danish Skjern River strain of Atlantic salmon is highly susceptible to G. salaris infection, further supporting the hypothesis that Atlantic Ocean strains are more susceptible to G. salaris infection than are Baltic strains.