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1.
Abstract

Two sizes of fingerling Snake River cutthroat trout Oncorhynchus clarkii behnkei and Colorado River rainbow trout O. mykiss were raised at hatcheries testing negative for Myxobolus cerebralis and stocked into the Dolores and Cache la Poudre rivers from 1999 to 2001. Populations were resampled over a 2-year period to determine which species and size combination had the highest growth and survival rates. Fish were tested for M. cerebralis via polymerase chain reaction and pepsin?trypsin digest analyses. Growth and survival rates between the species and size groups were not significantly different in either river. In the Dolores River, annual survival for both species and sizes of fish combined ranged from 0.063 to 0.12. In the Cache la Poudre River, survival for both sizes of rainbow trout was 0.004; survival for cutthroat trout ranged from 0.182 to 0.53. Larger fish had higher growth rates than smaller fish, and cutthroat trout had higher rates than similar sizes of rainbow trout. In both rivers, a higher percentage of the rainbow trout sample was infected than in the cutthroat trout sample. Rainbow trout also had a higher mean number of spores per head than cutthroat trout, and small rainbow trout had higher spore counts than large rainbow trout. Survival rates for cutthroat trout in the Cache la Poudre River were the highest of any of the groups, suggesting a difference that is biologically significant. Raising fingerlings to sizes greater than 100 mm can improve poststocking survival. If rainbow trout are stocked into contaminated waters, raising fingerlings to a larger size does not appear to improve growth or survival rates. Stocking rainbow trout in the spring could maximize growth rates but will expose fish to greater triactinomyxon densities, resulting in higher intensities of infection.  相似文献   

2.
Abstract

Laboratory exposures to the infectious stages (triactinomyxons) of Myxobolus cerebralis demonstrated a range of susceptibility to whirling disease among four species of inland salmonids. Replicate groups of each species were exposed to two concentrations of triactinomyxons, a low dose (100–200 per fish) and a high dose (1,000–2,000 per fish). Exposed fish were evaluated for clinical signs, for severity of microscopic lesions at 35 d, 2 and 5 months, and for spore concentrations in the head cartilage at 5 months. A standard strain of rainbow trout Oncorhynchus mykiss matched for age served as a susceptible species control. Rainbow trout, westslope cutthroat trout O. clarki lewisi, Yellowstone cutthroat trout O. clarki bouvieri, and bull trout Salvelinus confluentus were susceptible to M. cerebralis infections. Clinical signs, including radical swimming (“whirling”) and black tails, were observed at 7 weeks postexposure among rainbow and cutthroat trout challenged at 3 weeks of age. Clinical signs were rare among bull trout exposed at an age of 4 weeks and absent among rainbow and cutthroat trout exposed at 3 months posthatch. Most rainbow, cutthroat, and bull trout were found to be infected when examined at 5 months postexposure. The most severe microscopic lesions among infected fish at 5 months postexposure were found among rainbow trout. Cutthroat trout had less severe lesions, bull trout had mild infections, and no evidence of infection was found among Arctic grayling Thymallus arcticus. Mean spore concentrations among infected fish correlated with the severity of microscopic lesion scores. Rainbow trout had mean concentrations of spores in head cartilage reaching 106, whereas more resistant species such as bull trout had 104 spores; no spores were found among Arctic grayling at 5 months postexposure.  相似文献   

3.
Abstract

The intensity and prevalence of whirling disease was tested by exposure of 2-monthold fry and 1-, 2-, and 3.5-year-old adults of rainbow trout Oncorhynchus mykiss to a known number of laboratory-produced Myxobolus cerebralis at the actinosporean triactinomyxon stage. Fry exposed to graded concentrations of infectivity (triactinomyxons) for 3 h were individually examined for spores of Myxobolus cerebralis 5 and 6 months later. Exposure of fish to the lowest doses, 1 and 10 triactinomyxons per fish, did not result in detectable myxosporean spores. Fish that became lightly infected by a dose of 100 triactinomyxons per fish experienced a decrease in the incidence of infection between 5 and 6 months after exposure. A linear relationship was found between the numbers of recovered myxosporean spores and doses of 100–10,000 triactinomyxons per fish, and the spore burden appeared to plateau at doses of 10,000–100,000 triactinomyxons per fish. Adult fish continuously exposed to the highest dose of triactinomyxons for 3.5 months were infected and asymptomatic, however, the severity of myxosporean infection decreased with increased age of fish. This information may help in controlling whirling disease in salmonids.  相似文献   

4.
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  相似文献   

5.
Abstract

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 × 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.

Received December 28, 2011; accepted February 2, 2012  相似文献   

6.
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.  相似文献   

7.
Abstract

Susceptibility to infection by the myxosporean parasite Myxobolus cerebralis was compared among strains of cutthroat trout Oncorhynchus clarki in two separate exposure tests in the laboratory. In both tests, each strain was exposed to 1,000 triactinomyxons/fish for 2 h in 8.0 L of water. In the first test, three strains of 10-week-old cutthroat trout were compared: two strains of Bonneville cutthroat trout O. c. utah (Bear Lake and southern Bonneville strains) and Yellowstone cutthroat trout O. c. bouvieri. In the second test, these strains plus Snake River fine-spotted cutthroat trout O. c. subsp. and Colorado River cutthroat trout O. c. pleuriticus were exposed at either 5 or 10 weeks of age. The prevalence of the M. cerebralis infection was determined by single-round polymerase chain reaction (PCR) assay 5 weeks after exposure. In the first test, the prevalence was significantly lower in the Bear Lake strain of Bonneville cutthroat trout (78.5%) than in the Yellowstone (97.8%) or southern Bonneville (100%) strains when exposed at 10 weeks of age. In the second test, the Bear Lake strain also had significantly lower infection rates after exposure at 5 (54%) or 10 weeks (82%) of age than the other four strains, which did not differ from each other (94–100%). The severity of the infection was also significantly reduced in Bear Lake Bonneville cutthroat trout, as suggested by the strength of the product of the single-round PCR assay. These results suggest that intraspecific differences in susceptibility to M. cerebralis infection exist, further supporting the need to maintain the genetic diversity among subspecies and geographic variants of cutthroat trout.  相似文献   

8.
Abstract

Dams along the Deschutes River (DR) in central Oregon have blocked fish migration for over 40 years. Reestablishment of anadromous fish runs above the dams as part of a fish passage plan may introduce fish pathogens, such as Myxobolus cerebralis, the myxozoan parasite that causes salmonid whirling disease. This parasite is carried by adult salmon that stray into the DR system during their return to enzootic areas of the upper Columbia River basin, and it is now known to be established in at least one lower DR tributary. The life cycle of M. cerebralis involves two obligate hosts: a salmonid and the oligochaete worm Tubifex tubifex. To determine the likelihood of parasite establishment above the DR dams, we conducted benthic sediment surveys between 1999 and 2007 and found that T. tubifex had a patchy distribution and low relative abundance. Mitochondrial 16S ribosomal DNA gene analysis indicated that two lineages of T. tubifex (III and VI) were present both above and below the dams. Laboratory susceptibility studies to characterize differences in infection prevalence and parasite production between nine T. tubifex populations revealed that production varied considerably among exposed groups and was proportional to the number of lineage III worms present. Our results suggest that M. cerebralis could become established above the dams if infected fish are allowed passage into the upper DR system, but not all areas of the DR basin can be classified as having the same likelihood for parasite establishment, and the potential impact will be location dependent.

Received July 2, 2010; accepted October 24, 2010  相似文献   

9.
Abstract

Effects of temperature and aging on viability and infectivity of laboratory-produced actinosporean triactinomyxon spores (infective stage of the organism causing whirling disease) were studied. In vitro staining of triactinomyxon spores with vital fluorescein diacetate correlated with the ability of the spores to infect fry of rainbow trout Oncorhynchus mykiss. The experimentally produced actinosporean stage of Myxobolus cerebralis was short-lived, persisting for only 3–4 d at 12.5°C and for less time at warmer temperatures. The vital staining method has potential for screening therapeutants intended to control myxosporean infection of fish.  相似文献   

10.
Abstract

We used a simple technique for filtering the actinospores of Myxobolus cerebralis from natural waters to observe seasonal periodicity at eight sites in the upper Colorado River drainage. We used a tub lined with 20-μm-mesh Pecap screen to concentrate actinospores from 1,900-L samples and estimate density by microscope count. Identity of the observed actinospores as those of M. cerebralis was confirmed in 86 samples by the use of a polymerase chain reaction test. The 42-ha Windy Gap Reservoir appeared to be a point source of actinospores; the highest densities observed were consistently from samples taken at sites just below the reservoir. Both densities and the frequency of detection were much lower 26 km below the reservoir. The actinospores first appeared in abundance after the runoff in both years of the study. Actinospore densities tended to be greatest during summer and early fall and lowest during spring. In August 1997, a series of significant flow fluctuations and attendant water temperature swings appeared to alternately inhibit and stimulate the release of actinospores. Populations of rainbow trout Oncorhynchus mykiss continue to suffer recruitment failures throughout the study reach, apparently because of the effects of whirling disease in age-0 fish. This suggests that the detection of low numbers of actinospores by this technique at some sampling locations may indicate a level of infectivity that is destructive for the susceptible rainbow trout.  相似文献   

11.
Abstract

A series of experiments was carried out with infectious hematopoietic necrosis virus (IHNV; 193-110 isolate) in rainbow trout Oncorhynchus mykiss (weight, ～1.2 g) to determine the duration of the patent period and the timing of onset of the infectious periods. We first attempted to transmit IHNV to recipient fish from infected rainbow trout 2–3 d after they had been exposed. No infection transfer occurred despite high titers (104.79 to 104.91 plaque-forming units 5–8 d postexposure (dpe). To determine the number of secondary cases produced by one infectious individual, we exposed approximately 50 rainbow trout (weight, ～1.5 g) in each of seven replicate tanks to a donor fish that had been infected with virus by bath exposure 3 d earlier. The prevalence of infection in recipient fish rose from 0.84% at 2 dpe to 7.9% at 6 dpe. Maximum incidence (22 cases) occurred between 2 and 4 dpe. No disease-specific mortalities occurred in recipient fish during the experiment. The titer of virus in both recipient and donor fish increased from 2 to 4 dpe. There was a positive correlation between the level of infection among donors and prevalence values among recipient fish (r 2 = 0.60). The level of challenge by one infectious fish under the conditions provided was enough for infection transfer from sick cohabitant to susceptible fish but was not enough for initiation of a full-scale epizootic among recipients.  相似文献   

12.
Abstract

The Alaska Department of Fish and Game fish pathology laboratory received a rainbow trout Oncorhynchus mykiss from the Alaska Peninsula that was suspected of having whirling disease based on the display of aberrant swimming behavior and a deformed spine. We tested for Myxobolus cerebralis using standard pepsin–trypsin digest and molecular procedures, which yielded negative results. However, many oval shaped myxospores were observed in brain smears and were confirmed to be those of the morphologically similar M. neurotropus based on a diagnostic assay using PCR. The known geographic distribution of this parasite includes Idaho, Washington, Utah, Oregon, California, and now, Alaska. Whether this species is an emerging parasite is not known because it was only described a few years ago. Given the severe infection found in this rainbow trout, perhaps the considerable displacement of neurological tissues and subsequent pressure on peripheral nerves could have contributed to the spinal curvature and accompanied abnormal swimming. Conversely, the M. neurotropus infection may have been incidental and the spinal deformity may have actually been due to one of several nonspecific developmental or congenital causes. Further studies on geographic distribution and impact on host fitness will probably determine the importance of this species to fish health.

Received March 1, 2012; accepted June 15, 2012  相似文献   

13.
Abstract

Myxobolus (Myxosoma) cerebralis, the etiological agent of whirling disease, was detected in salmonid fish populations in northeastern Oregon. This is the first record of M. cerebralis in the Pacific Northwest of the USA. During an epizootiological survey for the parasite, two methods for spore detection were compared, and an efficient procedure for determining M. cerebralis infection in adult fish was developed. The enzyme digest method was more efficient than the plankton centrifuge procedure for examination of numerous individual lots of fish processed during the survey. Sampling only the area around the otoliths was at least as effective as sampling entire heads for detection of spores in infected fish.  相似文献   

14.
Abstract

Three myxosporeans were encountered in the cranial tissues of a California population of rainbow trout Oncorhynchus mykiss examined for the presence of Myxobolus cerebralis, the causative agent of whirling disease. Typical spores of M. cerebralis and a previously undescribed species of Myxobolus were found in the cranial tissues prepared by the pepsin HCl-trypsin digestion method. Henneguya zschokkei was also detected in digest preparations of cranial tissues, but was more numerous when branchial cartilage was included in the preparations. Microscopic examinations of tissues of individual rainbow trout showed occasional infections with both myxobolid species. Myxobolus cerebralis trophozoites and spores were found in the cranial and gill cartilage, and Myxobolus sp. was found in the brain and spinal cord. Henneguya zschokkei was also found within granulomas in the connective tissues below the gill arch. Both M. cerebralis and H. zschokkei were associated with a chronic inflammatory response in their respective tissues. In contrast, the Myxobolus sp. spores were found in pockets within the nervous tissues with no detectable host response. The spore measurements, calculated from fresh digests of infected tissues for the three myxosporeans (N = 20), for length × width × thickness in micrometers (SD) were 11.7 (0.6) without tails and 42.6 (5.2) with tails × 7.7 (0.8) × 7.0 (0.1) for H. zschokkei, 9.9 (0.4) × 8.4 (0.1) × 6.5 (0.3) for M. cerebralis, and 12.7 (0.7) × 10.5 (1.0) × 9.5 (0.8) for Myxobolus sp. Examined under scanning electron microscopy, the latter two species were morphologically similar although distinctive in size.  相似文献   

15.
Abstract

Sequential spread of infectious hematopoietic necrosis virus (IHNV) to tissues of rainbow trout Oncorhynchus mykiss was examined following immersion infection with two different isolates of IHNV, a pathogenic strain and a nonpathogenic strain from rainbow trout. Virus strain 193–110 was highly pathogenic to 1-month-old rainbow trout and caused 100% mortality within 13 d, whereas strain RB-76 was much less virulent, causing 50% mortality by the 19th day. Virus titers of 1-month-old fingerling fish dying soon after infection were significantly higher than titers of those dying later. Assays of dissected tissues showed that gills of infected 2-month-old fingerlings contained virus as early as 16 and 20 h postinfection, with definite replication occurring at 48 h. The early presence of the virus in the gills followed shortly by appearance of the virus in the kidneys and spleen indicated that the virus spreads rapidly to the target organs. Virus was detected in many other organs at lower levels on the third day and increased to higher levels during the following days. Heart tissue had high titers later in the infection. When 4-month-old rainbow trout were infected with strain 193–110, the mortality was reduced and delayed, whereas those infected with strain RB-76 produced no mortality. Assays on the day of death of these older fingerlings infected with strain 193–110 revealed that fish dying soon after infection also had higher titers than those dying later. Electron microscopic examination offish organs showed the presence of typical IHNV particles budding off from various tissue cells of affected organs, including gill tissue. The destructive effect of the virus was particularly noticeable in the disarrangement of heart muscle organelles.  相似文献   

16.
Abstract

The ability of two rhabdoviruses, infectious hematopoietic necrosis virus (IHNV) and viral hemorrhagic septicemia virus (VHSV), to infect fish skin was investigated by in vitro infection of excised tissues. Virus replication was determined by plaque assay of homogenized tissue extracts, and the virus antigen was detected by immunohistology of tissue sections. Gill, fin, and ventral abdominal skin tissues of rainbow trout Oncorhynchus mykiss that had been infected in vitro with a virulent strain of IHNV (193–110) produced substantial increases in virus titer within 24 h. Titers continued to increase up until day 3 of incubation; by this time, virus had increased 1,000-fold or more. This increase in IHNV titer occurred in epidermal tissues of fingerlings and of older fish. In another experiment, IHNV replicated in excised rainbow trout tissues whether the fish had been subject to prior infection with a virulent strain of IHNV (Western Regional Aquaculture Consortium isolate) or whether the fish had been infected previously with an attenuated strain of the virus (Nan Scott Lake, with 100 passes in culture). A virulent strain of VHSV (23/75) replicated effectively in excised gill tissues and epidermal tissues of rainbow trout and chinook salmon O. tshawytscha; however, the avirulent North American strain of VHSV (Makah) replicated poorly or not at all.  相似文献   

17.
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.  相似文献   

18.
Abstract

Arcobacter cryaerophilus was isolated from naturally infected rainbow trout Oncorhynchus mykiss, and its pathogenicity was tested by intramuscular injection into healthy 1-year-old high-body-weight (HBW) and low-body-weight (LBW) normally pigmented rainbow trout and albino crosses. Experimental infections caused deaths with gross clinical abnormalities such as exophthalmia, liver damage, bloody hemorrhagic kidney and heart, and swollen intestines. No significant differences in deaths were observed among the three infected fish groups. Hematocrit levels in blood of the experimentally infected HBW rainbow trout were significantly less than in healthy fish. No significant decreases were observed in the serum total protein of both the experimentally infected albino crosses and the high weight groups. Albumin and creatinine concentrations in serum were not significantly different among the three treatments.  相似文献   

19.
Abstract

Evaluation of the susceptibility of bull trout Salvelinus confluentus fry to Myxobolus cerebralis infection using two different laboratory challenge models demonstrated that the method of administering the exposure dose affected infection prevalence. Administration of a low parasite dose (500 per fish) in a single exposure did not establish infection, but when the same cumulative dose was administered over 21 d the prevalence of infection was 45%. The results of challenges at a high exposure dose (5,000 per fish) were similar, infections being detected in 24% of the fish receiving a single dose and 40% of those administered the same dose in multiple exposures. Clinical disease was not detected in fish exposed via either challenge method. The susceptibility of yearling bull trout was tested by means of a single high dose of 10,000 per fish, and infection was detected in only 5% of those fish.  相似文献   

20.
Abstract

Fry of rainbow trout Oncorhynchus mykiss were exposed to serotype VR-299 of infectious pancreatic necrosis virus (IPNV) by using a standardized immersion challenge. In concurrent experiments, fish were monitored for 11 d for excretion of IPNV or monitored for 9 d for excretion and transmission of IPNV to susceptible rainbow trout fry. Immersion-challenged fish began excreting virus within 2 d after challenge. The rate of IPNV excretion per fish increased steadily from about day 4 to day 8 and then decreased. Virus concentrations in tissues of immersion-challenged fish increased exponentially. Susceptible fish became infected with IPNV within 4 d after being introduced to immersion-challenged fish (e.g., 2 d after the challenged fish began excreting virus). By 9 d, 84% of the susceptible fish were infected with IPNV.  相似文献   

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