A Sphaerospora-like myxosporidean from the kidneys of Atlantic salmon (Salmo salar)

Sphaerospora elwhaiensis sp.n. (Myxosporea: Sphaerosporidae) from landlocked sockeye salmon Oncorhynchus nerka (Salmoniformes: Salmonidae) in Washington State, USA

A new species of sphaerosporid myxosporean, Sphaerospora elwhaiensis sp. n., is described from kidney of non-anadromous sockeye salmon (kokanee) Oncorhynchus nerka (Walbaum) from Lake Sutherland in the northern Olympic Peninsula, Washington, USA. Infection with the parasite was detected in 45% of 177 kokanee examined over 5 years. While conforming to the morphological criteria by which members of the genus are defined, the parasite is distinguished from congeners in salmonids of western North America by a unique combination of valvular sculpting of the myxospore, the relatively large size of the myxospore and monosporous development within the pseudoplasmodium. In addition, nucleotide sequences of the parasite's small and large subunit ribosomal RNA gene are unique. Phylogenetic analyses of these sequences suggested that the parasite is most closely related to freshwater Myxidium spp. and Zschokkella spp. The molecular data have provided further evidence for a polyphyletic association previously recognized among members of the genus and emphasize the need for a taxonomic revision of Sphaerospora Thélohan, 1892 and related genera.

Sphaerospora ictaluri n. sp. (Myxosporea: Sphaerosphoridae) observed in the kidney of channel catfish, Ictalurus punctatus rafinesque

Sphaerospores were found in the kidneys of alevin channel catfish (Ictalurus punctatus) from a farm in Central California. Multicellular developmental stages, similar to C-blood protozoans described for Sphaerospora spp. from cyprinid fishes, were observed in circulating blood and numerous tissues. Upon a 2nd examination of the same population of fish 10 days later, sporogonic stages were seen developing into mature sphaerospores in the lumina of the kidney tubules. Sporogenesis was asynchronous with simple unicellular stages adjacent to more complex forms with developing polar capsules and valves. Only one elliptical spore (5.6 microns in width, 6.5 microns in thickness by 5.8 microns in length) developed within the surrounding pseudoplasmodium. Thin valves surrounded two sporoplasm cells and two subspherical polar capsules (1.7 x 1.9 microns) which contained a polar filament with four to five turns. The blood stages of the Sphaerospora sp. described here are similar to the trophozoites seen in channel catfish with proliferative gill disease (PGD). Early stages of PGD also observed in the same population of channel catfish containing developmental and sporogonic stages of this newly recognized Sphaerospora sp. may suggest a causal relationship between this new myxosporean and the gill disease.

Fish infected with Sphaerospora spp. Thélohan (Myxosporea) from waters enzootic for proliferative kidney disease of salmonids

Sphaerospores were found among three species of fish examined from waters known to be enzootic for proliferative kidney disease (PKD) of salmonids. They were detected in the renal tubules of both hatchery-reared rainbow trout (Salmo gairdneri) exposed to the infectious stage of PKD and in chubs (Gila bicolor) in the headwaters of a hatchery where PKD is enzootic. Sticklebacks (Gasterosteus aculeatus) collected near net pens where Pacific salmon had experienced a PKD epizootic were also found to harbor sphaerospores in the lumen of the kidney tubules. The latter two host species contained developmental stages of a myxosporidan in the blood and in the lumen of the kidney tubules which are similar to those of PKX, the causative agent of PKD in salmonid fish. The sphaerospores observed in the rainbow trout are the first to be observed in this species. The similarity to previously observed developmental stages, rarity, and presence of these sphaerospores in salmonid fish from a hatchery where PKD is enzootic suggest that they are the most mature stage of the PKX myxosporidan yet observed.