Tetracapsuloides bryosalmonae is a myxozoan parasite of salmonid fishes, which causes Proliferative kidney disease (PKD), one of the most serious parasitic diseases of salmonid populations in Europe and North America, which causes losses of up to 90% in infected populations.

Taxonomy

---

Until the late 1990's, the organism which caused PKD was enigmatic. The "PKX organism", the causative agent of the disease, had been recognized as some form of myxozoan, but the absence of mature spores in salmonid hosts, the lack of fish to fish transmission, and seasonality of the disease suggest 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 bryozoan, Plumatella fungosa, in 1892, which he described as Myxosporidium bryozoides. Myxozoan infection of bryozoans were not reported again until 1996. Ecological investigations of freshwater bryozoans in North America discovered parasitic sacs of a myxozoan species, freely floating in the body cavities of several bryozoans. Molecular analyses indicated that the 18S rDNA sequences of these sacs were indistinguishable from those of PKX, and the PKX organism was scientifically described as Tetracapsuloides bryosalmonae Canning, Curry, Feist, Longshaw & Okamura 1999, which has been assigned to a new class, the Malacosporea within the phylum Myxozoa. Around the same time, another group described the PKX organism from Arctic char, 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.

Life cycle

---

T. bryosalmonae is highly unusual amongst the myxosporea, in that it uses a bryozoan as an alternate host, rather than an oligochaete or polychaete worm. To date, T. bryosalmonae has been found to parasitize five bryozoan species belonging to the genera Fredericella and Plumatella, all considered to be primitive genera. Problems lie in determining the fish host for this species. Although it infects salmonids, it does not mature in these fish, and bryozoans exposed to spores taken from salmon kidneys or urine remain uninfected. This implies that there may be another fish host which remains unidentified.

Pathology

---

Proliferative Kidney Disease is characterised by a swollen kidney and spleen, bloody "asictes" (fluid in the visceral cavity), and pale gills, which indicate the fish is anaemic. Note that these symptoms are common amongst many diseases of fish and do not specifically indicate an infection with Tetracapsuloides bryosalmonae.

Distribution

---

T. bryosalmonae has been recorded in Europe and North America. Phylogenetic analyses of internal transcribed spacer 1 sequences revealed a clade composed of all North American sequences plus a subset of Italian and French sequences. High genetic diversity in North America and the absence of genotypes 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 Myxobolus cerebralis and Ceratomyxa shasta.

Cited literature

---

Myxozoa | Fish diseases