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 outstanding questions about these parasites so that we can make informed management decisions and reduce their impact on native fishes.
Nomenclature (accurate and unambiguous identification and naming of organisms)
Discovering life cycles
Molecular diagnoses
Ecosystem impacts
Background:
I undertook a PhD at the University of Queensland, Brisbane, Australia, with Prof 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 Prof 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 North West.
Determination of the susceptibility of local Tubifex tubifex oligochaete populations to the myxozoan parasite Myxobolus cerebralis and correlation of this phenotype with their genotype to understand the distribution of the parasite and its impact on fish populations in Oregon.
Tubifex tubifex are obligate invertebrate hosts in the life cycle of Myxobolus cerebralis, the myxozoan parasite that causes whirling disease in salmonid fishes. Introduced to the USA in the 1950's, the exotic parasite has now been recorded from 26 states and is established to varying degrees across Oregon's Columbia River System (Pacific Northwest). Characteristics of local T. tubifex populations are likely to play a role in the pattern of disease occurrence. To better understand these patterns, we collected T. tubifex from three Oregon river basins (Grande Ronde, Deschutes and Willamette), determined their genotype (mitochondrial 16S rDNA lineage and RAPD genotype) and exposed 10 different populations to M. cerebralis in the laboratory.
Clinical signs of whirling disease: include black tail, skeletal deformities
Life cycle of Myxobolus cerebralis: The waterborne actinospore stage released from the oligochaete host infects the fish host in which it develops into the myxospore stage.
Collecting oligochaetes in the Deschutes River basin.
RAPD patterns from a range of oligochaetes.
Correlation of parasite abundance in water samples with severity of disease in fish in the Klamath River.
We recently developed a water sampling protocol (collection, filtration, DNA extraction & QPCR) to detect and quantify the myxozoan parasite Ceratomyxa shasta in river water samples. Ultimately, it is hoped that the method can replace laborious and lengthy fish exposures to detect and monitor the spatial and temporal distribution of the parasite. One outstanding question is ‘What dose is required to cause disease in different species of fishes in the Klamath River?'. To address this, we are simultaneously exposing fish (coho, Chinook, steelhead) and collecting water samples. A better understanding of this relationship will allow us to make timely management decisions that will reduce the currently high impact of this parasite on Klamath River salmonids.
Polychaete experiments.
We have recently established laboratory
cultures of Ceratomyxa's invertebrate host, Manayunkia speciosa. this
setup will allow us to address several outstanding data gaps in our
understanding of the Ceratomyxa shasta life cycle, which are relevant to managing the disease in the Klamath River.
Two important research questions are: After release from their host
(myxospores from fish and actinospores from worms). for how long do C.
shasta spores remain viable and able to infect their next host? Is
this survival temperature dependent? This data will provide insight as
to how long spores may survive at different times of the year and,
combined with flow data, how far they may travel downstream once
becoming waterborne and remain infective.
Other
questions include: How long does it take the parasite to develop in its
worm host? Does developmental time change with water temperature? We
know that development in the fish host may take 21-90 days, depending
on the water temperature (development is faster at higher
temperatures). The worm information will inform us as to how long the
parasite's full life cycle takes to complete.
Life cycle of Ceratomyxa shasta, a myxozoan parasite of salmonids
Investigations into myxozoans in waterfowl
As the result of a nation-wide collaboration, we recently described 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.
Teaching:
General Parasitology Laboratory MB 499/599 Summer term (with Prof. Mike Kent).
Contributed lectures and labs to MB491/591 Fish Diseases and the Salmonid Disease Workshop.
OSU Publications (2005-):
Hallett S.L., Lorz H.V., Atkinson S.D.,
Rasmussen C., Xue L., & BartholomewJ.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.
Hallett, S.L. & 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
(in press).
Székely Cs., Hallett S.L., Atkinson
S.D. & MolnárK. (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 SL & AtkinsonSD (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. & 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., Brown J.D. (2008) Myxozoan parasitism in waterfowl. International Journal for Parasitology 38: 1199-1207. Online article
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. & Foott, J.S. (2007) Distribution and Abundance of the Salmonid Parasite Parvicapsula minibicornisDiseases of Aquatic Organisms 78: 137-146. (Myxozoa) in the Klamath River Basin (Oregon-California, USA).
Arsan E.L., Hallett S.L. & 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. & Bartholomew J.L. (2007) Expanded geographical distribution of Myxobolus cerebralis: first detections from Alaska. Journal of Fish Diseases 30: 483-491.
AtkinsonS.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, HallettS.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. & 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. & 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. & Bartholomew, J.L. (2005) Countering morphological ambiguities: development of a PCR assay to assist the identification of Tubifex tubifexHydrobiologia 543: 305-309. oligochaetes.
Dr. Sascha Hallett, Research Associate
