Kudoa Mitigation Initiative Project

Kudoa thyrsites is a microscopic, multicellular parasite belonging to the Myxozoa. Myxozoans are mainly known as parasites of cold blooded invertebrates such as fishes and amphibians. They also have obligate, and lesser known, developmental stages within invertebrate hosts.

Waterborne spores act as infective stages between invertebrate and vertebrate hosts. Most myxozoan life stages are only visible via microscopy and they were considered to be protists until the 1990’s.

Phylogenetic analyses suggest that Myxozoans share a common ancestor with Cnidarians (e.g. jellyfish and corals) and are actually metazoans (animals) with reduced body plans.


The genus Kudoa is composed of over 90 species parasitic on marine and estuarine fishes. Species of Kudoa that infect skeletal muscle are recognized for their impacts on the marketability of commercially important fish hosts, as opposed to causing mortality. These effects usually present as visible cysts or flesh deterioration. No invertebrate hosts have been determined for any member of the genus, although related genera are found in polychaete worms.

K. thyrsites has a broad host range and is known to infect the muscle of over 30 different marine fish from several families around the world. Species familiar to people in BC include sea trout, herring, hake, wild and farmed salmon— both Atlantic and Pacific species – North Pacific arrow tooth flounder, and mahi mahi. Other fish species found to be susceptible are Dover sole and Pacific halibut.

K. thyrsites has no visible pathology but parasite derived enzymes degrade the flesh post mortem. Low levels of infection can go unnoticed in commercially sold fish, but at higher levels of infection the proteolytic activity of the parasite produces extensive myoliquefaction (dissolution or liquefaction of muscular tissue) and the fish meat is no longer commercially viable. K. thyrsites infections can result in substantial economic losses to the finfish aquaculture industry as infected fish have no signs of disease.

Husbandry methods that prevent or minimize exposure to K. thyrsites are unavailable due to limited life-history knowledge of the parasite. K. thyrsites requires an unknown alternate host, and until this host is discovered, empirical trials to understand infection dynamics and to evaluate husbandry methods using controlled exposures are precluded.

The BC Centre for Aquatic Health Sciences Society (BC CAHS) has been retained by Mowi Canada West to develop strategies to understand and mitigate the effects related to Kudoa thyrsites infection in Atlantic salmon (Salmo salar).


Kent, M.L., L. Margolis, D.J.Whitaker, G.E. Hoskins and T.E. McDonald. 1994. Review of Myxosporea of importance to salmonid fisheries and aquaculture in British Columbia. Folia Parasitologica 41: 27–37.

Marshall, W.L., MacWilliam, T. Brown, H., Lamson, H., Morrison, D., Afonso, LOB. 2016. Epidemiology of K. thyrsites (Myxozoa) infections in Atlantic Salmon (Salmo salar) farmed in heavily and lightly impacted regions of British Columbia. Journal of Fish Diseases 39:929-946.

Moran, J.D.W., D.J. Whitaker, M.L. Kent. 1999. A review of myxosporean genus Kudoa Meglistch, 1947, and its impact on the international aquaculture industry commercial fisheries. Aquaculture 172:163-196.



871A Island Hwy
Campbell River, BC
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