Abstract

Several pathogenic organisms infecting aquatic animals such as oysters, newts, and fish have been characterized as members of the genus Dermocystidium.² Since their initial classification; however, some of these organisms have been redescribed. For example, the demonstration of an apical complex in biflagellate zoospores of Dermocystidium marimum⁵ prompted Levine¹ to reassign this oyster pathogen to a new genus (Perkinsus) and class (Perkinsea) within the phylum Apicomplexa. Recent descriptions of uniflagellate zoospores lacking an apical complex in Dermocystidium salmonis from Salmon gills⁴ and presence of hyphal forms among Dermocystidium sp. from Atlantic salmon (Salmo salar)³ further illustrate the diversity present among this assemblage. Relationships among certain of these organisms have been asserted based upon similarities in morphologic and ultrastructural characteristics, in vitro growth requirements, and pathology associated with infection. Still exact taxonomic descriptions of the Dermocystidium-like parasite and a related organism, the rosette agent, remain problematic. Recent molecular comparisons of the fish pathogens Dermocystidium salmonis, the rosette agent and Ichthyophobia hoferi, and the crayfish parasite Psorospermium Haeckel, have demonstrated homology of their small subunit ribosomal RNA gene sequences.⁶ We report here on the first description of zoo sporulation in the Dermocystidium-like organism and the rosette agent, providing further evidence that these parasites are the same and are related to Dermocystidium salmonis.

Kidney tissue from a 3-year-old adult Sacramento River winter run chinook salmon (Oncorhynchus tshawytschae) infected with the rosette agent was incubated with the chinook salmon embryo cell fine (CHSE-214) at 15°C and the rosette agent has been continuously maintained in vitro with CHSE-214 cells since its initial isolation. We have also been maintaining cultures of the rosette agent originally isolated from chinook salmon from Washington, and the Dermocystidium-like organism isolated from Atlantic salmon from a private hatchery in California. All three parasites infect monolayers of CHSE-214 cells and replicate intra cytoplasmically. Continuous cultures are maintained by periodic trypsinization of the infected monolayers and supplementation with uninfected CHSE-214 cells. Recently, we were able to induce another, previously undescribed form of the two rosette agent isolates and the Dermocystidium-like organism. Cultures of these parasites were transferred to flasks containing distilled water, artificial sea water, phosphate buffered saline (PBS), or minimal essential media at 15°C. By 3 days, the organisms had begun to zoo sporulate in the distilled water, producing active zoospores. The body was approximately 1-2 µm in diameter, with a flagellum of approximately 10 µm. Zoo sporulation continued for up to 23 days in the cell-free cultures in distilled water. No zoospores were ever detected in preparations in artificial sea water, MEK or PBS. Zoospore development proceeded at a range of pH 6.0-8.0. In control cultures containing MEM plus CHSE-214 cells, the monolayers were infected with aggregates of 2-6 µm organisms, seen both intra cytoplasmically and attached to the cell surface.

Tissue from a winter run chinook salmon naturally infected with the rosette agent also yielded zoospores when incubated in distilled water at 15° C. This free-swimming form of the rosette agent and Dermocystidium-like organism may represent the infectious stage of these parasites. This is the first observation of zoo sporulation of the rosette agent and the Dermocystidium-like organism and further supports their association with Dermocystidium salmonis.

References

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3.  McVicar, A-H., and R. Wootten. 1980. Disease in farmed juvenile Atlantic salmon caused by Dermocystidium sp. In: Ahne, W. (ed.). Fish diseases. Third COPRAQ-Session, Springer-Verlag Berlin Pp. 165-173.

4.  Olson, R.E., C.F. Dungan, and R.A. Holt. 1991. Water-borne transmission of Dermocystidium salmonis in the laboratory. Diseases of Aquatic Organisms 12:41-48.

5.  Perkins, F.O. 1976. Zoospores of the oyster pathogen Dermocystidium marinum.

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7.  Ragan, M.A. and eight coauthors. 1996. A novel clade of protistan parasites near the animal-fungal divergence. Proceedings of the National Academy of Sciences 93:11907-11912.