Review of piscine diplomonad flagellate Spironucleus spp.: updating and new insight into S. salmonis

Objective: Aquaculture is a crucial element of food supply worldwide, and aquatic production needs to be doubled in the next 50 years, to meet this demand. A big challenge facing aquaculture is diseases including those caused by parasites. Pathogenic intestinal protozoans, including diplomonads, are responsible for clinically important infections in fish worldwide. Diplomonads have been reported from wild, farmed and aquarium fishes in cold, temperate and warm waters. They have been found in various fish families mostly from cyprinids, cichlids, salmonids, and gadids in North America, Europe and Asia. However, characterization, life cycle/transmission, and pathogenecity remain poorly understood. Method & Materials: A key element of effective monitoring the infections is the accurate identification of parasites. Early studies used light microscopy is recognized as being inadequate to distinguish genera and species of diplomonads. A combination of electron microscopy (SEM & TEM) used for further characters of taxonomic value at the species level, such as surface adornments, cytoskeleton, and cytoplasmic organelles. These approaches are improved by sequence of the SSU rRNA gene. We established cultures where we observed novel life cycle stages. A quantitative pathology study was conducted by sectioning the pyloric region and liver, and staining with H&E and PAS/AB. Results & Conclusion: We recognized that piscine diplomonads belong to the genus Spironucleus and not Hexamita as reported earlier. SEM showed an unadorned cell surface, with caudal projection; TEM showed new functional morphology, and multi-lobed apices of the nuclei; the SSU rRNA gene showed S. salmonis is a sister taxon to S. vortens. The direct life cycle of diplomonads means they can become a problem in aquaculture, since high stocking densities allow rapid transmission of parasites directly from fish to fish. We observed trophozoites attached by the tip of posterior flagella to debris/posterior flagella of other trophozoites, and colonies formed, which would ensure simultaneous ingestion of numerous cysts, presumably exceeding the minimum infective dose for initiating infections. We suggest transmission can be targeted for treatment, by inhibition of attachment, and mechanical/chemical disruption of adhesive colonies. In pathology view, there was significant hypertrophy by goblet cells in pyloric region of intestine. This new data suggests that hyper-production of mucus in infected fish is not due to goblet cell hyperplasia, but is primarily due to hypertrophy/increased release of mucus by goblet cells. These alterations in enterocytes and goblet cells may decrease nutrient absorption and underlie impaired growth in infected fish.