Tetracapsuloides

Tetracapsuloides bryosalmonae is a myxozoan parasite of salmonid 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. The disease can result in losses of up to 90% in infected populations.[1]

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[2]. The absence of mature spores in salmonid hosts, the lack of fish-to-fish transmission, and seasonality of the disease suggesting that the life cycle of PKX was completed in another host and that infection of salmonids could be accidental. Aleksei Korotnev (Korotneff) observed a myxozoan in the bryozoan, Plumatella fungosa, in 1892, which he described as Myxosporidium bryozoides.[3] Myxozoan infection of bryozoans was 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.[4] In 2000, the PKX organism was scientifically described as Tetracapsuloides bryosalmonae[5], and assigned to a new class, Malacosporea, within the phylum Myxozoa.[6] In the same year, another group of researchers described the PKX organism from Arctic char, Salvelinus alpinus, as Tetracapsuloides renicola[7]. According to the rules of binomial nomenclature however, the first given name takes priority.