[4] The PKX organism was scientifically described as Tetracapsuloides bryosalmonae Canning, Curry, Feist, Longshaw & Okamura 1999,[5] which was assigned to a new class, the Malacosporea within the phylum Myxozoa.

[6] Around the same time, another group described the PKX organism from Arctic char, Salvelinus alpinus, as Tetracapsuloides renicola Kent, Khattra, Hedrick & Devlin 2000,[7] but the first given name has priority according to the rules of the binomial nomenclature.

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[14].

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