Dr. Sascha Hallett, Associate Professor (Senior Research)

 Hallett photo    Office  535 Nash Hall
Phone  541-737-4721
FAX  541-737-0496
Email  halletts@oregonstate.edu

Research Interests: I find parasites fascinating organisms and have always been drawn to the aquatic environment. Thus, I am interested in parasites of marine and freshwater fish. Most of my research has focused on one phylum - the Myxozoa and I've never dissected a fish without encountering at least one of these microscopic, spore-forming, endoparasitic metazoans. Over 2000 are found in fish world-wide and most do not harm their host, but there are several that cause serious diseases (Ceratomyxa shasta, Parvicapsula minibicornis, Myxobolus cerebralis) in the Pacific Northwest of North America. My current research focuses on answering questions about these parasites so that we can make informed management decisions and reduce their impact on native fishes.

Courses Taught:  MB 499/599 General Parasitology Laboratory with Dr. M. Kent; MB 491/591, Fish Diseases, and the Salmonid Workshop, contributed lectures; MB 480 General Parasitology eCampus

BARTH chloromyxum Hallett page
New Species:  Chloromyxum auratum from feral goldfish in Oregon.

  Specific Interests

  • Discovery and description of novel species
  • Taxonomy (morphological & molecular characterization)
  • Nomenclature (accurate and unambiguous identification and naming of organisms)
  • Discovering life cycles
  • Molecular diagnoses
  • Ecosystem impacts



I undertook a PhD at the University of Queensland, Brisbane, Australia, with Professor Bob Lester and Peter O'Donoghue on "Myxozoan Parasites of Oligochaetes and Fish". I was then awarded an Alexander von Humboldt Research Fellowship to study with Professor Mansour El-Matbouli at the University of Munich, Germany for two years still focusing on myxozoan parasites. Then I procured a post-doc position with Dr Jerri Bartholomew at Oregon State University, USA, in 2003 where I have been studying myxozoan parasites of the Pacific Northwest.

Current Projects

Disease Risks Associated With Hatcheries in the Willamette River Basin;
Funded by: US Army Corp of Engineers; Began: 2011
Personnel: Jerri Bartholomew, Tony Amandi, Rich Holt, Michelle Jakaitis, Sean Roon, Gerri Buckles

The transmission of pathogens between wild and hatchery fishes is not well understood. Wild fish may provide a source of pathogens for hatchery fish and hatchery operations may increase disease risks to wild populations if they serve as point sources that amplify local pathogens. We are combining a range of approaches (sentinel fish exposures, water sampling, wild fish sampling) and lab experiments to gather information on the types and levels of pathogens entering and being released from hatcheries in the Willamette Basin. My major role is to develop water sampling methodologies to directly detect and quantify the following bacterial pathogens: Flavobacterium psychrophilum, F. columnare, Aeromonas salmonisida and Renibacterium salmoninarum; these were selected based on hatchery outbreaks.

 Hallett 2
Hallett 2


 Sentinal fish cages in inflow to Leaburg Hatchery.

 Collecting a water sample from the hatchery outflow.

 Hallett 5


Water filtration setup.

Filtered water sample: camptured material
on a 0.22µm disc


Development of a Database for Pathogens of Oregon Fishes; Funded by: Oregon Department of Fish and Wildlife, Sports Fish Restoration; Began: 2011
Personnel: Stephen Atkinson, Casey Dinsmore, Jerri Bartholomew, Tony Amandi, Craig Banner

We are working with The Oregon Department of Fish and Wildlife and OSU’s College of Science IT group (COSINe) to create a comprehensive, searchable, web-based source of information on pathogens of Oregon’s fishes. This database will provide the public (e.g. fishers, hobbyists, students), fish biologists, fish propagation and fish health services staff a reference that will have pertinent information and images of pathogens. The project will create a unique compilation of pathogens of both freshwater and marine fishes, from wild, naturally reared and cultured populations. The database uses a SQL database engine with Drupal webpage interfaces. We plan to go live July 2013.

Distribution of the Myxozoan Parasite Ceratomyxa shasta in the Klamath River; Funded by the U.S. Bureau of Reclamation; Began: 2006
Personnel: Jerri Bartholomew, Karuk and Yurok Tribes, Gerri Buckles, Adam Ray, Charlene Hurst, Gerri Buckles

Ceratomyxa shasta is an intestinal parasite of salmon and trout. It is only found in the Pacific northwest of North America and it is negatively impacting Chinook and coho salmon in the Klamath River (which straddles the Oregon/California border). The parasite has two waterborne stages in its lifecycle. We have collected river water samples at established index sites in the Klamath River since 2006. The triplicate 1L samples were filtered, the captured DNA extracted and any C. shasta-DNA present measured using quantitative PCR (TaqMan probe based; Hallett et al. 2006; 2012). There are multiple ITS1 genotypes of the parasite simultaneously present in the river and these differentially impact the various salmonid species. Thus it is important to know not only total parasite abundance but also which genotypes are present; a subset of water samples has been genotyped. We are currently analyzing the data for spatial and temporal trends and how these vary year to year.

 Hallett 8

Clarification of the Pacific North West ‘Manayunkia speciosa’;
Personnel: Stephen Atkinson, Julie Alexander, Jerri Bartholomew, Greg Rouse

There is one species of freshwater polychaete in the Pacific North West of North America. It is host to several parasites, including two myxozoans, Ceratomyxa shasta and Parvicapsula minibicornis. Investigations into the genetic diversity of the worm revealed that it differs to the type described from the Great Lakes, and elsewhere. This has prompted assignment of the PNW species to a new species and a comprehensive morphological characterization, including light and scanning electron microscopy.

Investigations into Myxozoans in Waterfowl; Funded by OSU General Research Fund, Dept. Microbiology Tartar Award
Personnel: Stephen Atkinson, Jerri Bartholomew

A nation-wide collaboration resulted in the description of the first myxozoan known from birds, Myxidium anatidum. This discovery questions whether we have witnessed the recent emergence of a novel parasite or a host-switching event (e.g. from fish to aquatic bird). Dr. Bartholomew and I have a grant from the OSU General Research Fund to further explore this intriguing relationship and to determine if all six documented US cases were caused by one or more species of myxozoans and to determine the infectious cycle. This will involve molecular methods and a field trip to Georgia, USA.

 BARTH Hallett page ducks



Bartholomew, J.L., Atkinson, S.D., Hallett, S.L., and Alexander, J.D.  2021.  Myxobolus cerebralis; in Fish Parasites:  A handbook of protocols for their isolation, culture, and transmission  Eds Sitjà-Bobadilla, A., Bron, J.E., Wiegertjes, G., Piazzon, M.C.; 5M Books, Sheffield, UK. 

Bartholomew, J.L., Atkinson, S.D., Hallett, S.L., and Alexander, J.D.  2021.  Ceratonova shasta; in Fish Parasites:  A handbook of protocols for their isolation, culture and transmission  Eds Sitjà-Bobadilla, A., Bron, J.E., Wiegertjes, G., Piazzon, M.C.; 5M Books, Sheffield, UK.

Hallett, S.L., Hartigan, A., and Atkinson, S.D.  2015.  Myxozoans on the move: dispersal modes, exotic species and emerging diseases. In: Myxozoan Evolution, Ecology and Development Okamura B, Gruhl A, Bartholomew JL (eds). Springer.

Hallett, S.L., Atkinson, S.D., Bartholomew, J.L. and Székely, C.S.  2015.  Myxozoans exploiting homeotherms. In: Myxozoan Evolution, Ecology and Development Okamura B, Gruhl A, Bartholomew JL (eds). Springer.

Fontes, I., Hallett, S.L., and Mo, T.A.  2015.  Comparative epidemiology of myxozoan diseases. In: Myxozoan Evolution, Ecology and Development Okamura B, Gruhl A, Bartholomew JL (eds). Springer.

Alexander, J.D., Kerans, B.L., El-Matbouli, M., Hallett, S.L., and Stevens, S.L.  2015.  Annelid-myxosporean interactions. In: Myxozoan evolution, Ecology and Development Okamura B, Gruhl A, Bartholomew JL (eds). Springer.

Bartholomew J.L. and  Hallett S.L.   2013.   Myxobolus cerebralis. In: Invasive Species Compendium. CABI, Oxfordshire, UK. http://www.cabi.org/isc/datasheet/73782

Hallett S.L. and Bartholomew J.L.   2012.  Chapter 8: Myxobolus cerebralis and Ceratomyxa shasta. In: Fish Parasites: Pathobiology and Protection. Editors: Woo P.T.K. & Buchmann K. CABI, Oxfordshire, UK.


Chinh, N.N., Van Ha, N., Doanh, P.N., Violetta, Y., Yoshinaga, T., Shirakashi, S., Hallett, S.L., and Whipps, C.M.  2022.  Morphological and molecular characterization of Ceratomyxa binhthuanensis n. sp. (Myxosporea: Ceratomyxidae) from the gall bladder of blacktip grouper Epinephelus fasciatus (Perciformes: Serranidae) in the East Sea of Vietnam.   Parasitol Res. 121(2):613-621.  doi:  10.1007/s00436-021-07419-x.

Borzák, R., Borkhanuddin, M.H., Cech, G., Molnár, K., Hallett, S.L., and Székely, C.  2021.  New data on Thelohanellus nikolskii Achmerov, 1955 (Myxosporea, Myxobolidae) a parasite of the common carp (Cyprinus carpio, L.):  the actinospore stage, intrapiscine tissue preference and molecular sequence.  Intl. J. for Parasitol.  Parasites and Wildlife 15:  112-119.

Lehman, B.M., Johnson, R.C., Adkison, M., Burgess, O.T., Connon, R.E., Fangue, N.A., Foott,  J.S., Hallett, S.L., Martinez–López, B., Miller, K.M., Purcell, M.K., Som, N.A., Valdes–Donoso, P., and Collins A.L.  2020.  Disease in Central Valley Salmon: Status and Lessons from Other Systems. San Francisco Estuary and Watershed Sci. 18 (3) Article 2: 1-31. https://doi.org/10.15447//sfews.2020v18iss3art2

Robinson, H.E., Alexander, J.D., Hallett, S.L., and Som, N.A.   2020.   Management Brief: Prevalence of infection in hatchery‐origin Chinook Salmon correlates with abundance of Ceratonova shasta spores: implications for management and disease risk. North American J. of Fish. Mgmt. 40: 959-972.

Richey, C.A., Kenelty, K.V., Van Stone Hopkins, K. Stevens, B.N., Martinez-Lopez, B., Barnum, S.M., Stevens, B.N., Hallett, S.L., Atkinson, S.D., Bartholomew, J.L., and Soto, E.  2020.  Validation of environmental DNA sampling for determination of Ceratonova shasta (Cnidaria:  Myxozoa) distribution in Plumas National Forest, CA.  Parasit. Res. https://doi.org/10.1007/s00436-019-06509-1.

Howell, C.K., Atkinson, S.D., Bartholomew, J.L., and Hallett, S.L.  2019.  Development and application of a qPCR assay targeting Ichthyophthirius multifiliis in environmental water samples.  Dis. of Aquatic Org. 134:43-55. 

Atkinson, S.D., Hallett, S.L., Diaz Morales, D., Bartholomew, J.L. and deBuron, I.  2019.  First myxozoan infection (Cnidaria:  Myxosporea) in a marine polychaete from North America and erection of actinospore collective group Saccimyxon.  J. of Parasit. 105:252-262.

Javaheri, A., Babbar-Sebens, M., Miller, R.N., Hallett, S.L., and Bartholomew, J.L.  2019.  An adaptive ensemble Kalman filter for assimilation of multi-sensor, multi-modal water temperature observations into hydrodynamic model of shallow rivers.  J. of Hydrology 572:682-691.

Javaheri, A., Babbar-Sebens, M., Alexander, J.A., Bartholomew, J.L., and Hallett, S.L.  2018.  Global sensitivity analysis of water age and temperature for informing salmonid disease management.  J. of Hydrology 561:89-97.

Atkinson, S.D., Hallett, S.L., and Bartholomew, J.L.  2018.  Genotyping of individual Ceratonova shasta (Cnidaria:  Myxosporea) myxospores reveals intra-parasite ITS-1 variation and invalidates the distinction of genotypes II and III.  Parasit. 145:1588-1593.

Richey, C.A., Kenelty, K.V., Van Stone Hopkins, K., Stevens, B.N., Martinez-Lopez, B., Barnum, S.M., Hallett, S.L., Atkinson, S.D., Bartholomew, J.L., and Soto, E.  2018.  Distribution and prevalence of Myxobolus cerebralis in postfire areas of Plumas National Forest:  Utility of environmental DNA sampling.  J. of Aquatic Animal Hlth. 30:130-143. 

de Buton, I., Hill-Spanik, K.M., Haselden, L., Atkinson, S.D., Hallett, S.L., and Arnott, S.A.  2017.  Infection dynamics of Kudoa inornata (Cnidaria:  Myxosporea) in spotted seatrout, Cynoscion nebulosus (Teleost:  Sciaenidae).  Dis. of Aquat. Org. 127:29-40.

Chiaramonte L.V., R. A. Ray, R. Alex Corum, Toz Soto, S.L. Hallett and Bartholomew, J.L.   2016.   Klamath River Thermal Refuge Provides Juvenile Salmon Reduced Exposure to the Parasite Ceratonova shasta. Transactions of the American Fisheries Society 145: 810-820.

Foott J.S., Stone R., Fogerty R., True K., Bolick A., Bartholomew J.L., Hallett S.L., Buckles G.R., and Alexander J.D.  2016.   Production of Ceratonova shasta myxospores from salmon carcasses: Carcass removal is not a viable management option. Journal of Aquatic Animal Health 28: 75-84.

Hurst C.N., Wong P., Hallett S.L., Ray R.A., and Bartholomew J.L.   2014.   Transmission and persistence of Ceratonova shasta genotypes in Chinook salmon. Journal of Parasitology 100: 773-777.

Alexander, J.A., Hallett, S.L., Stocking, R. W., Xue, L. and Bartholomew, J.L.   2014.   Host and parasite populations after a ten year flood: Manayunkia speciosa and Ceratonova (syn Ceratomyxa) shasta in the Klamath River. Northwest Science 88: 219-233.

Hallett, S.L., Ray, R.A., Hurst, C.N., Holt, R.A., Buckles, G.R., Atkinson, S.D. and Bartholomew, J.L.   2012.  Density of the Waterborne Parasite, Ceratomyxa shasta, and Its Biological Effects on Salmon. Applied and Environmental Microbiology 78: 3724-3731.

Zielinski C.M., Lorz H.V., Hallett S.L., Xue L.  and Bartholomew J.L.   2011.  Comparative susceptibility of Deschutes River (Oregon, USA) Tubifex tubifex populations to Myxobolus cerebralis. Journal of Aquatic Animal Health 23: 1-8.

Hallett, S.L. and Bartholomew, J.L.    2009.  Development and application of a duplex QPCR for river water samples to monitor the myxozoan parasite Parvicapsula minibicornis. Diseases of Aquatic Organisms 86: 39-50.

Hallett S.L., Lorz H.V., Atkinson S.D., Rasmussen C., Xue L. and Bartholomew J.L.   2009.   Propagation of the myxozoan parasite Myxobolus cerebralis by different geographic and genetic populations of Tubifex tubifex: An Oregon perspective. Journal of Invertebrate Pathology 102:57-68.

Székely Cs., Hallett S.L., Atkinson S.D. and Molnár K.   2009.   Complete life cycle of Myxobolus rotundus Nemeczek, 1911 (Myxosporea: Myxobolidae), a gill myxozoan of the common bream (Abramis brama). Diseases of Aquatic Organisms 85: 147-155.

Molnár K., Székely Cs., Hallett S.L. and  Atkinson S.D.   2009.  Some remarks on the occurrence, host specificity and validity of Myxobolus rotundus Nemeczek, 1911 (Myxozoa: Myxosporea). Systematic Parasitology 72: 71-79. DOI 10.1007/s11230-008-9161-7

Hallett, S.L. and  Bartholomew, J.L.  2008.   Effects of water flow on the infection dynamics of Myxobolus cerebralis. Parasitology 135: 371-384 doi: 10.1017/S0031182007003976

Bartholomew J.L., Atkinson S.D., Hallett S.L., Lowenstine L.J., Garner M.M., Gardiner C.H., Rideout B.A., Keel M.K., and Brown J.D.  2008.   Myxozoan parasitism in waterfowl. International Journal for Parasitology 38: 1199-1207.

Garner M.M., Atkinson S.D., Hallett S.L., Bartholomew J.L., Nordhausen R.W., Reed H., Adams L. and Whitaker B.   2008.   Renal myxozoanosis in weedy sea dragons, Phyllopteryx taeniolatus (Lacepède), caused by Sinuolinea phyllopteryxa n. sp. Journal of Fish Diseases 31: 27-35.

Bartholomew J.L., Atkinson S.D., Hallett S.L., Zielinski, C.M. and Foott, J.S.   2007.   Distribution and Abundance of the Salmonid Parasite Parvicapsula minibicornis (Myxozoa) in the Klamath River Basin (Oregon-California, USA). Diseases of Aquatic Organisms 78: 137-146.

Arsan E.L., Hallett S.L. and Bartholomew J.L. (2007) Tubifex tubifex from Alaska and their susceptibility to Myxobolus cerebralis. Journal of Parasitology 93: 1332-1342.

Arsan E.L., Atkinson S.D., Hallett S.L., Meyers T. and Bartholomew J.L. (2007) Expanded geographical distribution of Myxobolus cerebralis: first detections from Alaska. Journal of Fish Diseases 30: 483-491.

Atkinson S.D., Hallett S.L. & Bartholomew J. L (2007) The life cycle of Chloromyxum auratum (Myxozoa) from goldfish (Carassius auratus) involves an antonactinomyxon actinospore. Journal of Fish Diseases 30: 149-156.

Székely Cs, Hallett S.L., Al-Samman A., Dayoub A. (2007) First description of actinospores (Myxozoa) from Syria: novel records of hexactinomyxon, triactinomyxon and endocapsa types. Diseases of Aquatic Organisms 74: 127-137.

Bartholomew J.L., Lorz H.V., Atkinson S.D., Hallett S.L., Stevens D.G., Holt R.A., Lujan K. & Amandi A. (2007) Evaluation of a Management Strategy to Control the Spread of Myxobolus cerebralis in a Lower Columbia River Tributary. North American Journal of Fisheries Management 27: 542-550.

Hallett, S.L. and Bartholomew, J.L. (2006) Application of a real-time PCR assay to detect and quantify the myxozoan parasite Ceratomyxa shasta in river water samples. Diseases of Aquatic Organisms 71: 109-118.

Bartholomew, J.L., Atkinson, S.D. and Hallett, S.L. (2006) Involvement of Manayunkia speciosa (Annelida: Polychaeta: Sabellidae) in the life cycle of Parvicapsula minibicornis, a myxozoan parasite of pacific salmon. Journal of Parasitology 92: 742-748.

Hallett, S.L., Atkinson, S.D., Holt, R.A., Banner, C.R. & Bartholomew, J.L. (2006) A new myxozoan from feral goldfish (Carassius auratus). Journal of Parasitology 92: 357-363.

Hallett, S.L., Atkinson, S.D., Erséus, C.  and El-Matbouli, M. (2006) Myxozoan parasites disseminated via oligochaete worms as live food for aquarium fishes: descriptions of aurantiactinomyxon and raabeia actinospore types. Diseases of Aquatic Organisms 69: 213-225.

Hallett, S.L., Atkinson, S.D., Erséus, C. and El-Matbouli, M. (2005) Dissemination of triactinomyxons (Myxozoa) via oligochaetes used as live food for aquarium fishes. Diseases of Aquatic Organisms 65:  137-152.

Hallett, S.L., Atkinson, S.D. and Bartholomew, J.L. (2005) Countering morphological ambiguities: development of a PCR assay to assist the identification of Tubifex tubifex oligochaetes. Hydrobiologia 543: 305-309.

Hallett, S.L., Atkinson, S.D., Erséus, C. and El-Matbouli, M. (2004) Molecular methods clarify morphometric variation in triactinomyxon spores (Myxozoa) released from different oligochaete hosts. Systematic Parasitology 57: 1-14.

Hallett, S.L., Atkinson, S.D., Schöl, H. and El-Matbouli, M. (2003) Characterisation of two novel types of hexactinomyxon spores (Myxozoa) with subsidiary protrusions on their caudal processes. Diseases of Aquatic Organisms 55: 45-57.

Oumouna, M., Hallett, S.L., Hoffmann, R.W. and El-Matbouli, M. (2003) Seasonal occurrence of actinosporeans (Myxozoa) and oligochaetes (Annelida) at a trout hatchery in Bavaria, Germany. Parasitology Research 89: 170-184.

Hallett, S.L., Atkinson, S.D. and El-Matbouli, M. (2002) Molecular characterisation of two aurantiactinomyxon (Myxozoa) phenotypes reveals one genotype. Journal of Fish Diseases 25: 627-631.

Oumouna, M., Hallett, S.L., Hoffmann, R.W. and El-Matbouli, M. (2002) Early developmental stages of two actinosporeans, Raabeia and Aurantiactinomyxon (Myxozoa), as detected by light and electron microscopy. Journal of Invertebrate Pathology 79:17-26.

Hallett, S.L., and Diamant, A. (2001) Ultrastructure and small-subunit ribosomal DNA sequence of Henneguya lesteri n.sp. (Myxosporea), a parasite of sand whiting Sillago analis (Sillaginidae) from the coast of Queensland, Australia. Diseases of Aquatic Organisms 46: 197-212.

Kent M.L., Andree K.B., Bartholomew J.L., El-Matbouli M., Desser S.S., Devlin R.H., Feist S.W., Hedrick R.P., Hoffmann R.W., Khattra J., Hallett S.L., Lester R.J.G., Longshaw M., Palenzeula O., Siddall M.E., and Xiao C. (2001). Recent advances in our knowledge of the Myxozoa. The Journal of Eukaryotic Microbiology 48: 395-413.

Hallett, S.L., Erséus, C., O’Donoghue, P.J. and   Lester, R.J.G. (2001) Parasite fauna of Australian marine oligochaetes. Memoirs of the Queensland Museum 46: 555-576.

Lester, R.J.G., Hallett, S.L., El-Matbouli, M. & Canning, E.U. (1999) Can a new species of Myxozoa be described based solely on their actinosporean stage? Reply. Parasitology Today 15: 508.

Hallett, S.L. & Lester, R.J.G. (1999) Actinosporeans (Myxozoa) with four developing spores within a pansporocyst: Tetraspora discoidea n.g., n.sp. and Tetraspora rotundum n.sp. International Journal for Parasitology 29: 419-427.

Hallett, S.L., Erséus, C. & Lester, R.J.G. (1999) Actinosporeans (Myxozoa) from marine oligochaetes of the Great Barrier Reef. Systematic Parasitology 44:49-57.

Lester, R.J.G., Hallett, S.L., El-Matbouli, M. & Canning, E.U. (1998) The case for naming actinosporeans using the Zoological Code. Parasitology Today 14: 476-477.

Hallett, S.L., O’Donoghue, P.J. & Lester, R.J.G. (1998) Structure and development of a marine actinosporean, Sphaeractinomyxon ersei n. sp. (Myxozoa). Journal of Eukaryotic Microbiology 45: 142- 150.

Hallett, S.L., Erséus, C. & Lester, R.J.G. (1997) Actinosporea from Hong Kong marine Oligochaeta. In: Morton, B. (ed) Proceedings of the Eighth International Marine Biological Workshop: the Marine Flora and Fauna of Hong Kong and Southern China, Hong Kong, 2-20 April 1995. Pp. 1-7. Hong Kong University Press: Hong Kong.

Hallett, S.L., O’Donoghue, P.J. & Lester, R.J.G. (1997) Infections by Kudoa ciliatae (Myxozoa: Myxosporea) in Indo-Pacific whiting Sillago spp. Diseases of Aquatic Organisms 30: 11-16.

Roubal, F.R., Hallett, S.L. & Lester, R.J.G. (1997) First record of triactinomyxon actinosporean in marine oligochaete. Bulletin of the European Association of Fish Pathologists 17: 83-85.

Hallett, S.L., Erséus, C. & Lester, R.J.G. (1995) An actinosporean from an Australian marine oligochaete. Bulletin of the European Association of Fish Pathologists 15: 168-171.

Hallett, S.L. & Roubal, F.R. (1995) Experiments on the infection dynamics of Caligus epidemicus (Copepoda: Caligidae) on the small marine fish, Ambassis marianus (Günther). Journal of Fish Diseases 18: 59-66.