''Kudoa thyrsites'' is a

myxosporea Myxosporea is a class of microscopic parasites, belonging to the Myxozoa clade within Cnidaria. They have a complex life cycle which comprises vegetative forms in two hosts, an aquatic invertebrate (generally an annelid but sometimes a bryozoa ...

parasite Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has ...

of marine

fish Fish are aquatic, craniate, gill-bearing animals that lack limbs with digits. Included in this definition are the living hagfish, lampreys, and cartilaginous and bony fish as well as various extinct related groups. Approximately 95% of ...

es. It has a worldwide distribution, and infects a wide range of host species. This parasite is responsible for causing economic losses to the fisheries sector, by causing

post-mortem An autopsy (post-mortem examination, obduction, necropsy, or autopsia cadaverum) is a surgical procedure that consists of a thorough examination of a corpse by dissection to determine the cause, mode, and manner of death or to evaluate any di ...

"myoliquefaction", a softening of the flesh to such an extent that the fish becomes unmarketable. It is not infective to humans.

Taxonomy

The spores of ''K. thyrsites'' are stellate in shape, with 4 valves and 4 polar capsules. Upon infection by the actinosporean stage the

sporoplasm Sporoplasm is an infectious material present in the cytoplasm of various fungi-like organisms, such as members of class ''Microsporidia''. Sporoplasm is defined as a mass of protoplasm that gives rise to or forms a spore. The protoplasmic body t ...

migrates to a

muscle fibre A muscle cell is also known as a myocyte when referring to either a cardiac muscle cell (cardiomyocyte), or a smooth muscle cell as these are both small cells. A skeletal muscle cell is long and threadlike with many nuclei and is called a muscle ...

where it forms a

pseudocyst Pseudocysts are like cysts, but lack epithelial or endothelial cells. Initial management consists of general supportive care. Symptoms and complications caused by pseudocysts require surgery. Computed tomography (CT) scans are used for initial ima ...

. Within these pseudocysts are the developing spore stages. Comparison of 18S rDNA sequences of ''Kudoa'' species and other myxozoan species to determine their relationships. They show that ''Kudoa'' species are distinct from other myxozoans analyzed ('' Myxidium'' sp., '' Myxobolus'' sp., and ''

Henneguya zschokkei ''Henneguya zschokkei'' or ''Henneguya salminicola'' is a species of a myxosporean endoparasite. It afflicts several salmon in the genus ''Oncorhynchus''. It causes milky flesh or tapioca disease. ''H. zschokkei'' is notable for its absence of m ...

''). ''Kudoa thyrsites'' is an interesting member of this group in that apparently has very broad host specificity, infecting many fish species around the world (Table 1).

Pathology

Members of the

genus Genus ( plural genera ) is a taxonomic rank used in the biological classification of living and fossil organisms as well as viruses. In the hierarchy of biological classification, genus comes above species and below family. In binomial nom ...

Kudoa primarily infect muscle tissue of marine fishes, where they form nodules or pseudocysts containing a great number of individual spores. In lighter infections these pseudocysts are isolated from the fish's

immune system The immune system is a network of biological processes that protects an organism from diseases. It detects and responds to a wide variety of pathogens, from viruses to parasitic worms, as well as cancer cells and objects such as wood splint ...

within the muscle fibre. More intense infections can result in severe

inflammation Inflammation (from la, inflammatio) is part of the complex biological response of body tissues to harmful stimuli, such as pathogens, damaged cells, or irritants, and is a protective response involving immune cells, blood vessels, and molec ...

surrounding infected muscle fibres. Although apparently asymptomatic in all but heavy infections, they are associated with post-mortem degeneration of the tissue. This softening of flesh is most likely a result the release of

proteolytic enzyme A protease (also called a peptidase, proteinase, or proteolytic enzyme) is an enzyme that catalyzes (increases reaction rate or "speeds up") proteolysis, breaking down proteins into smaller polypeptides or single amino acids, and spurring the for ...

s by the parasite. This causes losses to both aquaculture operations, for instance, where

salmon Salmon () is the common name for several commercially important species of euryhaline ray-finned fish from the family Salmonidae, which are native to tributaries of the North Atlantic (genus ''Salmo'') and North Pacific (genus '' Oncorhy ...

are being reared in "sea-pens", and to capture fisheries. Losses are both direct, through the degradation of fish products, and indirectly, through the perception of the

consumer A consumer is a person or a group who intends to order, or uses purchased goods, products, or services primarily for personal, social, family, household and similar needs, who is not directly related to entrepreneurial or business activities. ...

that fish from a particular area are of a lower quality. The intensity of ''K. thyrsites'' infection is positively correlated with the severity of flesh softening in Atlantic salmon fillets. Softening of flesh always occurred with heavily infected fillets, while lightly infected fillets showed no softening. Prevention and/or control of ''K. thyrsites'' infections is problematic especially in open water netpens. Currently there are no available treatments. One approach to control may be to disrupt the life cycle in some way thereby minimizing the likelihood of infection.

Life cycle

The myxosporeans have been shown to have complex life cycles using more than one host. Usually a fish and an

oligochaete Oligochaeta () is a subclass of animals in the phylum Annelida, which is made up of many types of aquatic and terrestrial worms, including all of the various earthworms. Specifically, oligochaetes comprise the terrestrial megadrile earthworm ...

polychaete Polychaeta () is a paraphyletic class of generally marine annelid worms, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that bear many bristles, called chaetae, which are made ...

worm, and in one case a

bryozoa Bryozoa (also known as the Polyzoa, Ectoprocta or commonly as moss animals) are a phylum of simple, aquatic invertebrate animals, nearly all living in sedentary colonies. Typically about long, they have a special feeding structure called a ...

n. The life cycle of ''K. thyrsites'' is poorly understood. It has been hypothesized that ''K. thyrsites'' has an indirect life cycle involving some marine invertebrate. Experiments have shown direct transmission of the marine myxosporean ''Myxidium leei'' in sea bream. However, direct transmission of K. thyrsites failed when native fish were fed fresh myxospores. If K. thyrsites does have an indirect life cycle, the intermediate host has yet to be identified.

References

# # # # # # {{Taxonbar, from=Q3151525 Kudoidae Animal parasites of fish Veterinary parasitology Animals described in 1924