Expanded geographical distribution of Myxobolus cerebralis: first detections from Alaska

Myxobolus acanthogobii Hoshina, 1952 and Myxobolus selari n. sp. (Myxosporea: Myxobolidae) infecting brain of commercial fishes in Terengganu, Malaysia

Myxosporean infection in marine water fishes has drawn less attention than in freshwater fishes, which resulted in a higher taxonomic variety in freshwater in Malaysia. This study aimed to address the gap by conducting a myxosporean survey on two commercially significant marine fish species, Nemipterus furcosus (Valenciennes) (Eupercaria incertae sedis: Nemipteridae) and Selar crumenophthalmus (Bloch) (Carangiformes: Carangidae), collected from the northeastern part of peninsular Malaysia. During the examination of the organs, two distinct Myxobolus Bütschli, 1882 species were discovered in the brain tissue of these fishes, despite the absence of any observable pathological signs. The two Myxobolus species were characterized through morphometry, morphology, and analysis of partial small subunit ribosomal RNA (18S rDNA) gene. As a result, Myxobolus acanthogobii Hoshina, 1952, which infects 2.3% of N. furcosus, is synonymous with a myxobolid species commonly found in Japanese waters, based on its morphological traits, tissue tropism, and molecular diagnostics. Furthermore, a novel species, Myxobolus selari n. sp., was described, infecting the brain of one (11%) individual S. crumenophthalmus. This unique species displayed distinctive features, placing it within a well-supported subclade primarily comprising brain-infecting myxobolids. Maximum likelihood analysis further revealed the close relationships among these brain-infecting myxobolids, underscoring the significance of tissue tropism and host taxonomy for myxobolids. This study represents the initial documentation of Myxobolus species within the southern South China Sea, shedding light on the potential diversity of marine myxosporean in this region. This article was registered in the Official Register of Zoological Nomenclature (ZooBank) as urn:lsid:zoobank.org:pub:7C400E35-7CB8-4DEE-92B7-F75FF3926441.

Effect of substrate on the proliferation of Myxobolus cerebralis in the mitochondrial lineages of the Tubifex tubifex host

The study goal was to examine the effects of sand and mud on the propagation of Myxobolus cerebralis, the whirling disease agent, in four mitochondrial 16S ribosomal DNA lineages (I, III, V, VI) of its oligochaete host, Tubifex tubifex (Tt). In all the lineage groups held continuously in either substrate (non-shifted) or transferred from sand to mud (shifted), substrate influenced parasite proliferation only in lineage III. Sporogenesis and release of triactinomyxon spores (TAMs) were more prevalent in lineage III Tt in mud compared to sand. Low-infection prevalence and lack of parasite development in lineage I is associated with the greater number of resistant worms and were not affected by substrate type. Substrate did not impact Tt from lineages V and VI that failed to develop any parasite stages in either substrate even after shifting from sand to mud. The relationship between the microbial community in the substrate and parasite proliferation in lineage III was described but not analyzed due to small sample size. Substrate-associated bacteria were hypothesized as essential dietary source for the oligochaete host feeding selectively on fine (mud)-microflora. Progeny was produced by all lineage groups shifted to mud with disparate survival profiles in lineage V and VI and high mortalities in lineage III. Our study demonstrates that substrate type can alter parasite proliferation in lineage III. Conversely, parasite development and infectivity were not altered in lineage V and VI that are refractory to the parasite nor among the more resistant phenotypes (I), regardless of substrate type.

A Perspective: Molecular Detections of New Agents in Finfish-Interpreting Biological Significance for Fish Health Management

The increased sensitivity of advanced molecular techniques greatly exceeds the sensitivities of traditional detection methods for infectious agents. This sensitivity causes difficulty in interpreting the biological significance of such detections in fish (and shellfish), especially when the agent(s) cannot be cultured in the laboratory. In the Pacific Northwest, including Canada and Alaska, molecular detections of "new" (unknown or known but discovered in a different geographic location or fish host) potentially infectious agents in fish have received extensive media attention and misinterpretation that call for resource agencies to change current fish health surveillance practices or policies to include these agents. Fish health specialists from several of these agencies and organizations (see Acknowledgments) advise that any policy changes should be made only after further investigations to avoid wasting resources to conduct surveillance for organisms that are not significant to fish health or for noninfectious genetic material that does not represent a viable agent. Molecular detection is not proof of agent viability within or on host tissues and requires further investigation regarding the agent's ability to replicate and evidence that the agent causes substantial risk of disease to exposed fish populations. This document provides examples of molecularly detected agents causing public concern that were accompanied by little or no data to provide context and assessment of biological significance, highlights important questions to be answered regarding these detections, and provides a suggested pathway of investigative criteria to determine viability and pathogenicity of such agents that are necessary for consideration of any changes to aquatic animal health practices and policies.

Morphological and molecular characterization of a novel Myxobolus species from the gastrointestinal tract of brown trout (Salmo trutta) in Spain

The genus Myxobolus Bütschli, 1882 is the largest group within the class Myxosporea and includes 905 nominal species, 18 of which have been found to infect fish belonging to the family Salmonidae. In the present study, microscopic analysis enabled detection of myxospores in 43 of 613 (7.0%) gastrointestinal tracts from brown trout (Salmo trutta) captured in several rivers in the northwest of the Iberian Peninsula. Measurement of the whole myxospores, polar capsules and other morphological characteristics, together with identification of the site of infection, has led us to propose a novel salmonid-myxobolid species, Myxobolus compostellanus n. sp. Molecular analysis of the small subunit ribosomal RNA (SSU-rRNA) gene yielded the same consensus sequence of 2039 bp in 14 fish specimens. A BLAST search indicated 97.6% similarity to Myxobolus neurobius. Phylogenetic analysis revealed that M. compostellanus n. sp. is clustered with other salmonid-infecting myxobolids. The present findings contribute to the existing knowledge about the genus Myxobolus, providing both morphological and molecular data on a novel species of Myxobolus found to infect the gastrointestinal tract of salmonids, M. compostellanus n. sp. in the brown trout (S. trutta).

An updated geographic distribution of Myxobolus cerebralis (Hofer, 1903) (Bivalvulida: Myxobolidae) and the first diagnosed case of whirling disease in wild-caught trout in the south-eastern United States

Myxobolus cerebralis (Bivalvulida: Myxobolidae), the aetiological agent of salmonid whirling disease, was detected in 2 river basins of North Carolina during 2015, which initiated the largest spatial-temporal monitoring project for the disease ever conducted within the south-eastern United States (focused mainly in eastern Tennessee and western North Carolina). A total of 2072 rainbow trout Oncorhynchus mykiss, 1,004 brown trout Salmo trutta and 468 brook trout Salvelinus fontinalis were screened from 113 localities within 7 river basins during June 2017 through October 2019. Infections were detected by pepsin-trypsin digest, microscopy and the species-specific nested polymerase chain reaction (PCR) in 19 localities across 6 river basins. Myxospore morphology was indistinguishable from the published literature. In 2019, five rainbow trout that symptomatic for whirling disease (sloping neurocranium and lordosis) were captured and processed for histopathology. Myxospores were detected in the calvarial cartilage of two deformed trout with associated erosion of the cartilage consistent with reported whirling disease lesions. This is the first report of M. cerebralis in Tennessee and the first histologically confirmed cases of whirling disease in southern Appalachian (south-eastern United States) rivers and streams and expands the distribution of M. cerebralis throughout western North Carolina and eastern Tennessee.

Proliferative kidney disease in Alaskan salmonids with evidence that pathogenic myxozoans may be emerging north

Proliferative kidney disease (PKD) of salmonids, a chronic immunopathology caused by the myxozoan parasite Tetracapsuloides bryosalmonae, is exacerbated by increased water temperatures. PKD causes economic concerns to trout farmers and contributes to the decline of wild salmonid populations in North America and Europe. The parasite occurs as far north as Norway and Iceland in Europe and was confirmed from California to southern British Columbia in the American continent. In mid-September 2011 adult chum salmon (Oncorhynchus keta) were sampled from Kantishna River, a tributary to Yukon River in Alaska. Clinical PKD was diagnosed based on the macroscopic appearance of mottled kidneys that were uniformly swollen and by the detection of tumultuous histozoic extrasporogonic and coelozoic sporogonic stages of T. bryosalmonae in renal tissue by histopathology. Archived samples provided the molecular confirmation and local strain identification, representing the first confirmed case of PKD in wild adult chum salmon, also co-infected with Parvicapsula minibicornis that represents another novel myxozoan detection in Alaska. Our investigation was extended to another case from August/September 1997, with mortality following furunculosis and ectoparasite co-infections, in sockeye salmon (Oncorhynchus nerka) pre-smolts net-pen reared in English Bay Lakes, Alaska. Immunohistochemistry on archived histological preparations confirmed T. bryosalmonae sporogonic and extrasporogonic stages, indicating a severe to resolving PKD, with concomitant Chloromyxum spp. infection. Those cases provide the first documentation that this parasite is present in Alaska and causes PKD in wild and cultured salmonids in the region. The known geographic range of T. bryosalmonae can be extended to ~267 km south of the Arctic Circle, representing the northernmost detection in America. Given the vast size of Alaska and small resident population, it is likely that T. bryosalmonae remained undetected, but more recently became evident due to the clinical manifestation of PKD, possibly linked to increasing water temperatures reported at the sample locations.

Elimination of Myxobolus cerebralis in Placer Creek, a Native Cutthroat Trout Stream in Colorado

Placer Creek, a tributary of Sangre de Cristo Creek in Colorado's San Luis Valley, supported an allopatric core conservation population of native Rio Grande Cutthroat Trout Oncorhynchus clarkii virginalis during much of the 20th century. After the failure of gabion barriers in the late 1990s, Brook Trout Salvelinus fontinalis infected with Myxobolus cerebralis invaded from Sangre de Cristo Creek. By 2005, whirling disease (WD) and competition from Brook Trout reduced Rio Grande Cutthroat Trout numbers to less than 10% of the total trout population. New barriers were constructed in 2006 and the stream was treated with rotenone in 2007 and 2009 to eliminate all fish prior to the reintroduction of Rio Grande Cutthroat Trout. Results of WD research studies in Montana, California, and Colorado indicated it might be possible to break the life cycle of the parasite in some situations. Our management interventions included (1) reducing the fish population in the stream to zero for approximately 14 months, (2) introducing lineage V and VI Tubifex tubifex worms, which are not susceptible to M. cerebralis, and (3) eliminating a small off-channel pond that provided optimal habitat that sustained a localized high-density population of lineage III T. tubifex, the oligochaete host susceptible to M. cerebralis. Electrofishing during the fall of 2009 and spring of 2010 indicated the drainage was devoid of fish. Fry, juvenile, and adult Rio Grande Cutthroat Trout were stocked in September and October of 2010 and 2011. Approximately 975,000 lineage V and VI T. tubifex were introduced into Placer Creek between 2010 and 2012 as possible oligochaete competitors for the lineage III worms. The off-channel pond was filled in, and the surface was reseeded in April 2012. No evidence of M. cerebralis infection was detected among more than 280 Rio Grande Cutthroat Trout tested between July 2012 and July 2016, indicating the parasite had been eradicated from the Placer Creek basin upstream of the barriers.

Accelerated deactivation of Myxobolus cerebralis myxospores by susceptible and non-susceptible Tubifex tubifex

In the 1990s, the Tubifex tubifex aquatic oligochaete species complex was parsed into 6 separate lineages differing in susceptibility to Myxobolus cerebralis, the myxozoan parasite that can cause whirling disease (WD). Lineage III T. tubifex oligochaetes are highly susceptible to M. cerebralis infection. Lineage I, IV, V and VI oligochaetes are highly resistant or refractory to infection and may function as biological filters by deactivating M. cerebralis myxospores. We designed a 2-phased laboratory experiment using triactinomyxon (TAM) production as the response variable to test that hypothesis. A separate study conducted concurrently demonstrated that M. cerebralis myxospores held in sand and water at temperatures ≤15°C degrade rapidly, becoming almost completely non-viable after 180 d. Those results provided the baseline to assess deactivation of M. cerebralis myxospores by replicates of mixed lineage (I, III, V and VI) and refractory lineage (V and VI) oligochaetes. TAM production was zero among 7 of 8 Lineage V and Lineage VI T. tubifex oligochaete groups exposed to 12500 M. cerebralis myxospores for 15, 45, 90 and 135 d. Among 4 mixed lineage exposure groups, TAM production averaged 14641 compared with 2202495 among 12 groups of Lineage III oligochaetes. Among the 6 unexposed Lineage III experimental groups seeded into original Phase 1 substrates for the 45, 90 and 135 d treatments during the Phase 2 portion of the study, TAM production was reduced by 98.9, 99.9 and 99.9%, respectively, compared with the average for the 15 d exposure groups. These results are congruent with the hypothesis that Lineage V and Lineage VI T. tubifex oligochaetes can deactivate and destroy M. cerebralis myxospores.

Whirling disease revisited: pathogenesis, parasite biology and disease intervention

Whirling disease (WD) is an ecologically and economically debilitating disease of rainbow trout Oncorhynchus mykiss caused by the actinosporean spores of the parasite Myxobolus cerebralis. M. cerebralis has a complex, 2-host life cycle alternating between salmonid fish and the oligochaete host Tubifex tubifex. The parasite alternates between 2 spore forms as transmission stages: an actinosporean triactinomyxon spore that is produced in the oligochaete host and a myxosporean spore that develops in the salmonid host. Waterborne triactinomyxon spores released from infected T. tubifex oligochaetes attach to the salmonid host by polar filament extrusion elicited by chemical (nucleoside) and mechanical (thigmotropy) stimuli-a process which is rapidly followed by active penetration of the sporoplasms into the fish epidermis. Upon penetration, sporoplasms multiply and migrate via peripheral nerves and the central nervous system to reach the cartilage where they form trophozoites which undergo further multiplication and subsequent sporogenesis. M. cerebralis myxospores are released into the aquatic environment when infected fish die and autolyse, or when they are consumed and excreted by predators. Myxospores released into the water are ingested by susceptible T. tubifex where they develop intercellularly in the intestine over a period of 3 mo through 4 developmental stages to give rise to mature actinospores. In this article, we review our current understanding of WD-the parasite and its alternate hosts, life cycle and development of the parasite in either host, disease distribution, susceptibility and resistance mechanisms in salmonid host and strategies involved in diagnosis, prevention and control of WD.

A review of molecular approaches for investigating patterns of coevolution in marine host-parasite relationships

Parasites and their relationships with hosts play a crucial role in the evolutionary pathways of every living organism. One method of investigating host-parasite systems is using a molecular approach. This is particularly important as analyses based solely on morphology or laboratory studies of parasites and their hosts do not take into account historical evolutionary interactions that can shape the distribution, abundance and population structure of parasites and their hosts. However, the predominant host-parasite coevolution literature has focused on terrestrial hosts and their parasites, and there still is a lack of studies in marine environments. Given that marine systems are generally more open than terrestrial ones, they provide fascinating opportunities for large-scale (as well as small-scale) geographic studies. Further, patterns and processes of genetic structuring and systematics are becoming more available across many different taxa (but especially fishes) in many marine systems, providing an excellent basis for examining whether parasites follow host population/species structure. In this chapter, we first highlight the factors and processes that challenge our ability to interpret evolutionary patterns of coevolution of hosts and their parasites in marine systems at different spatial, temporal and taxonomic scales. We then review the use of the most commonly utilized genetic markers in studying marine host-parasite systems. We give an overview and discuss which molecular methodologies resolve evolutionary relationships best and also discuss the applicability of new approaches, such as next-generation sequencing and studies utilizing functional markers to gain insights into more contemporary processes shaping host-parasite relationships.

Susceptibility to Myxobolus cerebralis among Tubifex tubifex populations from ten major drainage basins in Colorado where Cutthroat Trout are endemic

Establishment of Myxobolus cerebralis (Mc) resulted in declines of wild Rainbow Trout Oncorhynchus mykiss populations in streams across Colorado during the 1990s. However, the risk for establishment and spread of this parasite into high-elevation habitats occupied by native Cutthroat Trout O. clarkii was unknown. Beginning in 2003, tubificid worms were collected from all major drainages where Cutthroat Trout were endemic and were assayed by quantitative PCR to determine the occurrence and distribution of the various lineages of Tubifex tubifex (Tt) oligochaetes. Over a 5-year period, 40 groups of Tt oligochaetes collected from 27 streams, 3 natural lakes, 2 private ponds, and a reservoir were evaluated for their relative susceptibility to Mc. Exposure groups were drawn from populations of pure lineage III Tt, mixed-lineage populations where one or more of the highly resistant (lineage I) or nonsusceptible lineages (V or VI) were the dominant oligochaete and susceptible lineage III worms were the subdominant worm, or pure lineage VI Tt. Experimental replicates of 250 oligochaetes were exposed to 50 Mc myxospores per worm. The parasite amplification ratio (total triactinomyxons [TAMs] produced / total myxospore exposure) was very high among all pure lineage III Colorado exposure groups, averaging 363 compared with 8.24 among the mixed-lineage exposure groups. Lineage III oligochaetes from Mt. Whitney Hatchery in California, which served as the laboratory standard for comparative purposes, had an average parasite amplification ratio of 933 among 10 exposed replicates over a 5-year period. Lineage I oligochaetes were highly resistant to infection and did not produce any TAMs. Lineages V and VI Tt did not become infected and did not produce any TAMs. These results suggest that the risk of establishment of Mc is high for aquatic habitats in Colorado where Cutthroat Trout and lineage III Tt are sympatric.

Bench-top validation testing of selected immunological and molecular Renibacterium salmoninarum diagnostic assays by comparison with quantitative bacteriological culture

No gold standard assay exhibiting error-free classification of results has been identified for detection of Renibacterium salmoninarum, the causative agent of salmonid bacterial kidney disease. Validation of diagnostic assays for R. salmoninarum has been hindered by its unique characteristics and biology, and difficulties in locating suitable populations of reference test animals. Infection status of fish in test populations is often unknown, and it is commonly assumed that the assay yielding the most positive results has the highest diagnostic accuracy, without consideration of misclassification of results. In this research, quantification of R. salmoninarum in samples by bacteriological culture provided a standardized measure of viable bacteria to evaluate analytical performance characteristics (sensitivity, specificity and repeatability) of non-culture assays in three matrices (phosphate-buffered saline, ovarian fluid and kidney tissue). Non-culture assays included polyclonal enzyme-linked immunosorbent assay (ELISA), direct smear fluorescent antibody technique (FAT), membrane-filtration FAT, nested polymerase chain reaction (nested PCR) and three real-time quantitative PCR assays. Injection challenge of specific pathogen-free Chinook salmon, Oncorhynchus tshawytscha (Walbaum), with R. salmoninarum was used to estimate diagnostic sensitivity and specificity. Results did not identify a single assay demonstrating the highest analytical and diagnostic performance characteristics, but revealed strengths and weaknesses of each test.

Spatial and temporal variation of whirling disease risk in Montana spring creeks and rivers

Spring creeks are important spawning and rearing areas for wild trout, but the stable flows, cool temperatures, and high nutrient levels that characterize these unique habitats may also make them highly susceptible to establishment and proliferation of the whirling disease pathogen Myxobolus cerebralis. We evaluated the spatial and temporal dynamics in whirling disease risk by using sentinel rainbow trout Oncorhynchus mykiss fry in nine different spring creeks and their conjoining rivers or reservoirs in Montana over a 20-month period. Whirling disease risk was high in five of the seven pathogen-positive spring creek study sites; at these sites, prevalence levels exceeded 90% and over 50% of sentinel fry had moderate to high infection severity scores. Spring creeks generally had higher disease prevalence and severity than paired river or reservoir sites. Fine sediment levels varied widely among springs creeks with high and low whirling disease risk, and we found no significant association between fine sediment level and infection severity. The low risk measured for some spring creeks was likely attributable to the pathogen invasion being in its early stages rather than to environmental characteristics limiting the severity of infection. High whirling disease risk occurred over a wide range of temperatures at spring creek sites (4.5-13°C) and river sites (1.7-12.5°C). There was an unusual seasonal cycle of infection in spring creeks, with peak infection levels occurring from late fall to early spring and declining to near zero in late spring to early fall. The low infection risk during spring suggests that spring-spawning trout would be at a low risk of infection, even in spring creeks with otherwise high disease severity. In contrast, fry of fall-spawning trout may be much more susceptible to infection in spring creek environments.

Myxobolus neurotropus infecting rainbow trout in Alaska, a new geographic record

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.

Quantitative polymerase chain reaction (PCR) for detection of aquatic animal pathogens in a diagnostic laboratory setting

Real-time, or quantitative, polymerase chain reaction (qPCR) is quickly supplanting other molecular methods for detecting the nucleic acids of human and other animal pathogens owing to the speed and robustness of the technology. As the aquatic animal health community moves toward implementing national diagnostic testing schemes, it will need to evaluate how qPCR technology should be employed. This review outlines the basic principles of qPCR technology, considerations for assay development, standards and controls, assay performance, diagnostic validation, implementation in the diagnostic laboratory, and quality assurance and control measures. These factors are fundamental for ensuring the validity of qPCR assay results obtained in the diagnostic laboratory setting.

Geographical and host distribution patterns of Parvicapsula minibicornis (Myxozoa) small subunit ribosomal RNA genetic types

Parvicapsula minibicornis is a myxozoan parasite implicated in mortalities of both juvenile and pre-spawning adult salmon in the Pacific Northwest of North America. Disease severity and presentation varies between salmon species and geographical localities. To better characterize population structure of the parasite, we sought genetic markers in the P. minibicornis ribosomal RNA gene. We compared samples from California with the type specimen from British Columbia, identified sequence variations, and then sequenced 197 samples from fish, river water and the parasite's polychaete worm host. Although DNA sequences of the parasite were >98·9% similar, there was enough variation to define 15 genotypes. All genotypes were detected in fish samples, although not in all species. A single genotype only was found in sockeye and pink salmon in the Fraser River Basin, but was not detected in sockeye from the adjacent Columbia River Basin. All coho salmon, irrespective of river basin, were infected with a unique mix of 2 genotypes. These data indicated that the P. minibicornis population exhibited strong signals of structuring by both geography and salmonid host species. Particular genotypes may correlate with disease differences seen in salmon populations in the Pacific Northwest.

Molecular and morphological analysis of Myxobolus spp. of salmonid fishes with the description of a new Myxobolus species

While investigating the parasite fauna of wild coho salmon. Oncorhynchus kisutch (Walbaum, 1792), histological examination provided evidence of a new species of Myxobolus (Myxozoa: Myxosporea) infecting nerves of skeletal muscle. Spores were morphologically similar to those of the intramuscular Myxobolus insidiosus Wyatt and Pratt, 1963, both having pyriform spores with clavate polar capsules. However, the former developed exclusively in the nerves of skeletal muscle rather than in myocytes. We examined both species of Myxobolus derived from coho salmon; Chinook salmon, Oncorhynchus tshawytscha (Walbaum, 1792); cutthroat trout, Oncorhynchus clarkii (Richardson, 1836); and rainbow trout Oncorhynchus mykiss (Walbaum, 1792) from freshwater in Oregon. Spore morphology, small subunit ribosomal RNA gene (rDNA) sequences, and site of infection were compared. Myxobolus arcticus Pugachev and Khokhlov, 1979 has pyriform spores, infects the central nervous system of many salmonids, and is found in the Pacific Northwest. It was therefore included in the analyses to rule out conspecificity with the new species. Together, these data show that the Myxobolus sp. from peripheral nerves in the skeletal musculature of coho salmon, rainbow trout, and cutthroat trout is a new species, described herein as Myxobolus fryeri n. sp.

Potential for dissemination of the nonnative salmonid parasite Myxobolus cerebralis in Alaska

Myxobolus cerebralis, the myxozoan parasite responsible for whirling disease in salmonids, was first introduced into the United States in 1958 and has since spread across the country, causing severe declines in wild trout populations in the intermountain western United States. The recent detection of the parasite in Alaska is further evidence of the species' capability to invade and colonize new habitat. This study qualitatively assesses the risk of further spread and establishment of M. cerebralis in Alaska. We examine four potential routes of dissemination: human movement of fish, natural dispersal by salmonid predators and straying salmon, recreational activities, and commercial seafood processing. Potential for establishment was evaluated by examining water temperatures, spatial and temporal overlap of hosts, and the distribution and genetic composition of the oligochaete host, Tubifex tubifex. The most likely pathway of M. cerebralis transport in Alaska is human movement of fish by stocking. The extent of M. cerebralis infection in Alaskan salmonid populations is unknown, but if the parasite becomes dispersed, conditions are appropriate for establishment and propagation of the parasite life cycle in areas of south-central Alaska. The probability of further establishment is greatest in Ship Creek, where the abundance of susceptible T. tubifex, the presence of susceptible rainbow trout Oncorhynchus mykiss, and the proximity of this system to the known area of infection make conditions particularly suitable for spread of the parasite.