Observations on the life stages of Sphaerothecum destruens n. g., n. sp., a mesomycetozoean fish pathogen formally referred to as the rosette agent

The rosette agent is an obligate intracellular parasite that causes morbidity and mortality in salmonid fish. In laboratory cultures, the spore stage (2-6 mum diam.) replicates in a salmonid cell line by sequential asexual division, giving rise to daughter cells. If infected cell cultures are transferred to distilled water, the spore stage undergoes internal division to give rise to at least 5 cell's each of which develops into a uniflagellated zoospore with a body of approximately 2 mum and a flagellum approximately 10 mum long. Zoosporulation does not occur in cell culture medium alone, artificial seawater, or phosphate-buffered saline. This parasite is currently classified as a member of the Class Mesomycetozoea (formerly Ichthyosporea) based on phylogenetic analyses of the small subunit ribosomal DNA of three different isolates from fish. Given these new morphological observations combined with the available molecular phylogenetic data on other mesomycetozoeans, we propose to classify the rosette agent as Sphaerothecum destruens, n. g., n. sp. This new genus has unique features including (1) intracellular development of spore stages in various organs eliciting a host granulomatous response; and (2) the differentiation of mature spores into multiple, flagellated zoospores. Taken together, these characteristics clearly distinguish it from the closely related genera Dermocystidium and Rhinosporidium.