''Tetracapsuloides bryosalmonae'' is a

myxozoa Myxozoa (etymology: Greek: μύξα ''myxa'' "slime" or "mucus" + thematic vowel o + ζῷον ''zoon'' "animal") is a subphylum of aquatic cnidarian animals – all obligate parasites. It contains the smallest animals ever known to have lived. O ...

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 ...

salmon Salmon () is the common name for several list of commercially important fish species, commercially important species of euryhaline ray-finned fish from the family (biology), family Salmonidae, which are native to tributary, tributaries of the ...

id fish. It is the only species currently recognized in the monotypic genus ''Tetracapsuloides''. It is the cause of proliferative kidney disease (PKD), one of the most serious parasitic diseases of salmonid populations in Europe and North America that can result in losses of up to 90% in infected populations.

Taxonomy

Until the late 1990s, the organism which caused PKD was enigmatic, thus called PKX organism. The causative agent of PKD was recognized as a form of Malacosporean, but the absence of mature spores in salmonid hosts, the lack of fish to fish transmission, and seasonality of the disease suggested that the life cycle of PKX was completed in another host and that infection of salmonids could be accidental. Korotneff observed a myxozoan in the

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 l ...

n, '' Plumatella fungosa'', in 1892, which he described as ''Myxosporidium bryozoides.'' Myxozoan infection of bryozoans was not reported again until 1996. Ecological investigations of freshwater bryozoans in

North America North America is a continent in the Northern Hemisphere and almost entirely within the Western Hemisphere. It is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South America and the Car ...

discovered parasitic sacs of a myxozoan species, freely floating in the body cavities of several bryozoans. Molecular analyses indicated that the

18S rDNA 18S ribosomal RNA (abbreviated 18S rRNA) is a part of the ribosomal RNA. The S in 18S represents Svedberg units. 18S rRNA is an SSU rRNA, a component of the eukaryotic ribosomal small subunit ( 40S). 18S rRNA is the structural RNA for the small ...

sequences of these sacs were indistinguishable from those of PKX. The PKX organism was scientifically described as ''Tetracapsuloides bryosalmonae'' Canning, Curry, Feist, Longshaw & Okamura 1999, which was assigned to a new

class Class or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used differentl ...

, the Malacosporea within the

phylum In biology, a phylum (; plural: phyla) is a level of classification or taxonomic rank below kingdom and above class. Traditionally, in botany the term division has been used instead of phylum, although the International Code of Nomenclature f ...

Myxozoa Myxozoa (etymology: Greek: μύξα ''myxa'' "slime" or "mucus" + thematic vowel o + ζῷον ''zoon'' "animal") is a subphylum of aquatic cnidarian animals – all obligate parasites. It contains the smallest animals ever known to have lived. O ...

. Around the same time, another group described the PKX organism from

Arctic char The Arctic char or Arctic charr (''Salvelinus alpinus'') is a cold-water fish in the family Salmonidae, native to alpine lakes and arctic and subarctic coastal waters. Its distribution is Circumpolar North. It spawns in freshwater and populatio ...

, ''Salvelinus alpinus'', as ''Tetracapsuloides renicola'' Kent, Khattra, Hedrick & Devlin 2000, but the first given name has priority according to the rules of the

binomial nomenclature In taxonomy, binomial nomenclature ("two-term naming system"), also called nomenclature ("two-name naming system") or binary nomenclature, is a formal system of naming species of living things by giving each a name composed of two parts, bot ...

Life cycle

''T. bryosalmonae'' has a two-host life cycle, as other myxosporeans, cycling between freshwater bryozoa and salmonid fish species, rather than 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 (biology), class of generally marine invertebrate, marine annelid worms, common name, commonly called bristle worms or polychaetes (). Each body segment has a pair of fleshy protrusions called parapodia that ...

worm Worms are many different distantly related bilateral animals that typically have a long cylindrical tube-like body, no limbs, and no eyes (though not always). Worms vary in size from microscopic to over in length for marine polychaete wor ...

as for ''

Myxobolus cerebralis ''Myxobolus cerebralis'' is a myxosporean parasite of salmonids (salmon and trout species) that causes whirling disease in farmed salmon and trout and also in wild fish populations. It was first described in rainbow trout in Germany in 1893, ...

''. To date, ''T. bryosalmonae'' has been found to parasitize at last five freshwater bryozoans ''

Phylactolaemata Phylactolaemata is a class of the phylum Bryozoa whose members live only in freshwater environments. Like all bryozoans, they filter feed by means of an extensible "crown" of ciliated tentacles called a lophophore, and like nearly all bryoz ...

'' species belonging to the genera '' Fredericella'' and '' Plumatella'', all considered to be primitive genera. Infected bryozoans release mature T. bryosalmonae malacospores during overt infections when large spore sacs are freely floating within their coelomic cavity. Bryozoan dispersal strategies, including colony fragmentation, statoblast dispersal and the formation of migrating zooids allow their colonization of new habitats and the spreading of infective T. bryosalmonae stages.

Pathology

Proliferative kidney disease (PKD) is characterized by a swollen

kidney The kidneys are two reddish-brown bean-shaped organs found in vertebrates. They are located on the left and right in the retroperitoneal space, and in adult humans are about in length. They receive blood from the paired renal arteries; blood ...

and

spleen The spleen is an organ found in almost all vertebrates. Similar in structure to a large lymph node, it acts primarily as a blood filter. The word spleen comes .

, bloody

ascites Ascites is the abnormal build-up of fluid in the abdomen. Technically, it is more than 25 ml of fluid in the peritoneal cavity, although volumes greater than one liter may occur. Symptoms may include increased abdominal size, increased weight, ab ...

, and pale

gill A gill () is a respiratory organ that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s, indicating the fish becomes

anemic Anemia or anaemia (British English) is a blood disorder in which the blood has a reduced ability to carry oxygen due to a lower than normal number of red blood cells, or a reduction in the amount of hemoglobin. When anemia comes on slowly, th ...

at the late stage of the disease. Note that those symptoms are common amongst many other fish diseases and do not specifically indicate an infection with ''Tetracapsuloides bryosalmonae''. It is important to clarify the pathologic condition only happens in species particularly susceptible, or naïve, to T. bryosalmonae. In those cases, the parasite is allowed to cross the renal tubules wall to proliferate within the interstitial tissue of kidney (histozoic proliferation). This proliferation stage is a dead end for the parasite (extrasporogonic proliferation) but instead causes a tumultuous tumour-like tissue reaction in the kidney, inducing a chronic lymphoid hyperplasia marked by a strong parasite-driven immunosuppressant pathogenesis and a dysregulation of T-helper subsets.

Distribution

''T. bryosalmonae'' has been recorded in Europe and North America. Phylogenetic analyses of

internal transcribed spacer 1 Internal transcribed spacer (ITS) is the spacer DNA situated between the small-subunit ribosomal RNA (rRNA) and large-subunit rRNA genes in the chromosome or the corresponding transcribed region in the polycistronic rRNA precursor transcript. I ...

sequences revealed a

clade A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English term, ...

composed of all North American sequences plus a subset of Italian and French sequences. High genetic diversity in North America and the absence of

genotype The genotype of an organism is its complete set of genetic material. Genotype can also be used to refer to the alleles or variants an individual carries in a particular gene or genetic location. The number of alleles an individual can have in a ...

s which are characteristic of the North American clade in the rest of Europe implies that southern Europe was colonized by immigration from North America; however, sequence divergence suggests that this colonization substantially pre-dated human movements of fish. Furthermore, the lack of southern European lineages in the rest of Europe, despite widespread rainbow trout farming, indicates that ''T. bryosalmonae'' is not transported through fisheries activities. This result contrasts with the commonness of fisheries-related introductions of other pathogens and parasites such as ''

'' and ''

Ceratomyxa shasta ''Ceratonova shasta'' (syn. ''Ceratomyxa shasta'') is a myxosporean parasite that infects salmonid fish on the Pacific coast of North America. It was first observed at the Crystal Lake Hatchery, Shasta County, California, and has now been report ...

''. PKD is a serious immunopathology causing a high mortality rate, thus with a relevant economic impact for trout aquaculture in Europe and North America.

Cited literature

# # # # # # # # # # # # # # {{Taxonbar, from=Q7706223 Malacosporea Animal parasites of fish Kidney diseases Veterinary parasitology Monotypic cnidarian genera