From the early reports of unique species of diplomonads in each species of fish, via the assumption that all were Hexamita salmonis, to the more sophisticated identifications of today, the study of piscine diplomonads has come a long way! We now recognize that in wild fish, diplomonads appear to be commensals, while in aquaculture, several species are significant pathogens.
A key element of effective monitoring of the infections, and predicting likely host ranges, is the accurate identification of the parasites. Early studies used only light microscopy, which we now recognize as being inadequate to distinguish genera and species of these small flagellates, most of which are only about 10 µm long. The advent of electron microscopy brought much improved diagnosis, with identification of features that reliably distinguish genera, and we now recognize that piscine diplomonads belong to the genus Spironucleus and not Hexamita as reported earlier. Electron microscopy can also be used to assist with identification of species, by distinguishing surface and internal features (Saghari Fard et al. 2007). However, this approach alone is insufficient, since some diplomonads that appear identical, can only be distinguished and assigned to distinct species using a molecular approach by sequencing of the small subunit rRNA gene (Jørgensen and Sterud 2006). Currently there are five well characterized species of piscine diplomonads: S. barkhanus, S. salmonicida, S. salmonis, S. torosa, and S. vortens.
We cannot generalize the host-parasite relationships of piscine diplomonads. Much depends on the exact species of host and of parasite, and the environmental conditions. While there are no reports of diplomonads being pathogenic in wild fish, there are numerous reports of their association with disease in aquaculture and aquaria. Among pathogenic species are S. salmonicida causing systemic disease in salmon, S. salmonis associated with chronic weight loss in fry and fingerling trout, and S. vortens associated with disease in aquarium cichlids. While the clinical signs of these different diseases are well documented, pathogenesis is poorly understood. We also need to increase our knowledge of natural reservoir hosts to understand how and why diseases are only observed in fish kept in captivity.
The direct life cycle of diplomonads means that they can become a problem in aquaculture, since high stocking densities allow rapid transmission of the parasites directly from fish to fish. Most studies focus on the easily observed swimming trophozoite stage in the fish. Very little attention is given to the cysts, which are assumed to transmit the infection through the water. The conditions prompting encystment and excystment are not known, even though improved understanding of these environmental triggers may help to identify new treatment targets.
Treatment options remain limited, and usually employ metronidazole against the swimming trophozoites in the fish. However, anti-parasitic drugs administered orally do not easily combat systemic infections.
We gratefully acknowledge the Nachwuchsförderung des Landes Berlin (Nafög) for the award of a Ph.D. scholarship to Mr. Reza Fard, and the Deutsche Forschungsgemeinschaft for the award of a Mercator Visiting Professorship to Dr. Sarah Poynton.
1. Fard MRS, Jørgensen A, Sterud E, Bleiss W, Poynton SL (2007) Ultrastructure and molecular diagnosis of Spironucleus salmonis (Diplomonadida) from rainbow trout Oncorhynchus mykiss in Germany. Diseases of Aquatic Organisms (in press)
2. Jørgensen A, Sterud (2006) The marine pathogenic genotype of Spironucleus barkhanus from farmed salmonids redescribed as Spironucleus salmonicida n. sp. Journal of Eukaryotic Microbiology 53: 531-541.