Spironucleus species: economically-important fish pathogens and enigmatic single-celled eukaryotes

Diplomonads are aerotolerant anaerobic, binucleate flagellates, which are commonly found in the intestinal tract of wild and farmed fish. Of the diplomonad genera, Spironucleus, composed of opportunistic pathogens, poses the greatest threat to aquaculture. Immunocompromised hosts or fish without acquired immunity are thought to be more susceptible to parasitism by these otherwise commensal agents. Accumulation of flagellates along the intestinal tract often leads to systemic Spironucleosis causing high mortality of both ornamental and food fish in aquaculture. The life cycle of these piscine diplomonads is direct, consisting of a motile, parasitic trophozoite and a resilient encysted stage, which facilitates water-borne transmission. Confusion in the nomenclature, as well as numerous reassignments of taxa, hampers our understanding of host range and geographical distribution of fish diplomonads. Accurate identification requires transmission electron microscopy to characterise intricate ultrastructural features. Additionally, sequencing of the small subunit ribosomal RNA gene allows identification of cryptic Spironucleus spp. In vitro culture provides a convenient source of flagellates for biochemical and physiological research, allowing the identification of novel parasite-specific molecular pathways such as H2 production within Spironucleus sp. This provides insight into the pathogenicity of these organisms and offers potential new targets for chemotherapy. Restrictions on the administration of the current drug of choice, metronidazole, in aquacultural settings, as well as reported cases of drug resistance, means that control of Spironucleosis is especially difficult. Allium sativum (garlic)-derived compounds have proven highly effective at inhibiting parasite growth in vitro, showing great potential as a novel alternative therapy in the treatment of Spironucleosis. Further characterisation of the biochemistry, pathogenicity and taxonomy of fish diplomonads is required in order to fully appreciate the true impact and economic consequences of Spironucleus spp. in aquaculture