Harriet V. Lorz

Dynamics of Myxobolus cerebralis in the Lostine River, Oregon: Implications for Resident and Anadromous Salmonids

Abstract Myxobolus cerebralis, the myxozoan parasite causing whirling disease in salmonids, was first detected in northeastern Oregon in 1986. To better understand the potential impact of M. cerebralis on resident and anadromous salmonids in this region, a series of studies was conducted in the Lostine River, where infected fish were first reported. Rainbow trout Oncorhynchus mykiss fry were exposed in sentinel cages for 14 d at five sites from near the rivers mouth to the headwaters during the period from July 1998 to November 1999. A subsample of exposed fry was assayed for the presence of DNA of M. cerebralis by polymerase chain reaction. Following four exposures, fish were held in the laboratory for 5 months of observation. The severity of infection in these fish was determined by histological examination, enumeration of spores isolated from the cranial cartilage, and observation of clinical signs. Sentinel fry became infected throughout the river from March to November, although the prevalence of in...

Age-dependent susceptibility of Chinook salmon to Myxobolus cerebralis and effects of sustained parasite challenges.

Abstract Laboratory challenges with triactinomyxons of Myxobolus cerebralis demonstrate that Chinook salmon Oncorhynchus tshawytscha have a lower susceptibility to infection and the development of whirling disease than rainbow trout O. mykiss. In age-dependent challenges, triplicate groups of each species were exposed at 1, 3, 5, 7, and 9 weeks posthatch to 1,000 triactinomyxons per fish and juvenile Chinook salmon were exposed to a high dose of 10,000 triactinomyxons per fish. Assessment of infection at 5 months postchallenge showed that Chinook salmon acquire resistance to clinical disease after 3 weeks posthatch, as compared with 7 weeks posthatch for rainbow trout. Furthermore, all measures of infection severity, including myxospore burden and lesion score, were lower in Chinook salmon. To model natural exposure and evaluate the ability of the fish to retain resistance following repeated challenge, replicate groups of Chinook salmon and rainbow trout were exposed daily to low parasite doses (5–200 tri...

Propagation of the myxozoan parasite Myxobolus cerebralis by different geographic and genetic populations of Tubifex tubifex: an Oregon perspective.

Tubifex tubifex are obligate invertebrate hosts in the life cycle of Myxobolus cerebralis, the myxozoan parasite that causes whirling disease in salmonid fishes. This exotic parasite is established to varying degrees across Oregons Columbia River system (Pacific Northwest, USA) and characteristics of local T. tubifex populations likely play a role in the pattern of disease occurrence. To better understand these patterns, we collected T. tubifex from three Oregon river basins (Willamette, Deschutes, and Grande Ronde), determined their genotype (mitochondrial 16S rDNA lineage and RAPD genotype) and exposed 10 different populations to M. cerebralis in the laboratory. Four mt lineages were identified: I, III, V and VI. Lineage III was found in all river basins but dominated both central and eastern sites. The RAPD assay further divided these lineages into geographic sub-populations; no RAPD genotype was common to all basins. There was a significant difference in prevalence of infection and level of parasite production among the populations we exposed to M. cerebralis that was attributed to genotypic composition. Only lineage III worms released actinospores and only populations dominated by this lineage amplified the parasite. These populations had the lowest survival, however, the lineage dominant before exposure remained dominant despite the high prevalence of infection. The distribution and infection dynamics of susceptible T. tubifex throughout Oregon may contribute to the differences in M. cerebralis occurrence; our studies further support the influence of oligochaete genotypes on the manifestation of whirling disease in salmonid populations.

Detection of Myxobolus (Myxosoma) cerebralis in Salmonid Fishes in Oregon

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.

Comparative susceptibility of Deschutes River, Oregon, Tubifex tubifex populations to Myxobolus cerebralis.

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.

Evaluation of a management strategy to control the spread of myxobolus cerebralis in a lower Columbia River tributary

Abstract In October 2001, Myxobolus cerebralis, the myxozoan parasite that causes salmonid whirling disease, was detected in juvenile rainbow trout Oncorhynchus mykiss from a private hatchery that received water from Clear Creek, a tributary of the Clackamas River, Oregon. The Oregon Department of Fish and Wildlife closed the surface water portion of the hatchery in March 2003 and initiated a monitoring program to evaluate the success of the closure in containing further parasite spread. From 2002 to 2005, rainbow trout sentinels were held in live cages for 2 weeks at locations upstream, downstream, and within the area of the facility and then were tested for M. cerebralis infection. Infection prevalence in groups held in the hatchery pond, the outflow, and downstream of the facility was initially high; however, by May 2004 infection was no longer detected in Clear Creek, although the parasite continued to be detected in the hatchery pond. A single rainbow trout collected approximately 18 river kilometers...

Susceptibility of Juvenile and Yearling Bull Trout to Myxobolus cerebralis and Effects of Sustained Parasite Challenges

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.

Experimental Infection of Brook Trout with Infectious Hematopoietic Necrosis Virus Types 1 and 2

Abstract Fry of brook trout Salvelinus fontinalis became infected and diseased after immersion exposure to infectious hematopoietic necrosis virus (IHNV), but a long-lasting IHNV carrier state was not induced. Duplicate groups of 100 fish were immersed for 6 h in baths containing a type 1 (Round Butte, RB) or a type 2 (Rangen, RA) IHNV isolate at a high or low dose. Brook trout mortalities induced by immersion in a bath of the RB or RA IHNV isolate at 102 plaque-forming units (pfu) per milliliter were equivalent (1 and 0%), but fish were more susceptible to infection with RA IHNV. Only the single dead fish in the RB group was infected, but 24% of the RAexposed fish were infected 1 week after exposure. At a dose of 106 pfu/mL, exposure to RB IHNV resulted in a higher mortality (35%) and prevalence of infection (89% of live fish sampled at 1 week postexposure), but no infectious virus was detectable by 5 weeks after exposure. In contrast, RA IHNV exposure at a dose of 104 pfu/mL resulted in only 5% mortalit...

Prevalence of Myxobolus cerebralis at Juvenile Salmonid Acclimation Sites in Northeastern Oregon

Abstract Fry of rainbow trout Oncorhynchus mykiss held as sentinel fish became infected with Myxobolus cerebralis when held at all juvenile acclimation sites for spring Chinook salmon O. tshawytscha and steelhead O. mykiss (anadromous rainbow trout) and thus expand the known range of M. cerebralis in northeastern Oregon. In spring 2001, replicate cages of rainbow trout fry were placed at the intake site of each facility and sampled at approximately 14, 28, and 50 d. Infection prevalence, as determined by the presence of M. cerebralis DNA following polymerase chain reaction (PCR) amplification, was relatively low (<17.5%) at most acclimation sites. However, a significantly higher prevalence of infection in sentinel fry held at the Wallowa facility (65%) identified this site as presenting a higher risk for juvenile steelhead to M. cerebralis exposure than other sites. Further, PCR analysis of ossified cranial elements from steelhead juveniles held at the Wallowa facility for 6 weeks demonstrated a comparabl...

Detection of Myxobolus cerebralis in the Lower Deschutes River Basin, Oregon

Abstract The Deschutes River, located in central Oregon, is a tributary of the Columbia River. The migration of anadromous fish into the upper Deschutes River basin was blocked in the late 1960s by a series of dams constituting the Pelton Round Butte Hydroelectric Project (PRB). Plans to reintroduce anadromous fish runs above the PRB are complicated by concerns over the introduction of Myxobolus cerebralis (the agent of whirling disease), which is carried into the lower Deschutes River by adult salmon that stray on their return to enzootic regions of the upper Columbia River basin. To develop a fish passage strategy that minimizes the risk of parasite introduction into the upper Deschutes River basin, it is important to determine whether establishment of the M. cerebralis life cycle in waters below the PRB has occurred. Fry of susceptible rainbow trout Oncorhynchus mykiss and steelhead (anadromous rainbow trout) were exposed as sentinel fish at various locations in the Deschutes River and its tributaries ...