A New Parasitic Archamoeba Causing Systemic Granulomatous Disease in Goldfish Extends the Diversity of Pathogenic Endolimax spp.

Simple Summary It is almost 50 years since amoeba-associated granulomatosis was first described in goldfish (Carassius auratus), but the aetiologic agent has never been identified. In this study, a new archamoeba species, Endolimax carassius, is characterised as the causative agent of systemic granulomatous disease in goldfish. Phylogenetic analyses determined the new goldfish parasite species as a genotype sister of Endolimax piscium, the causative agent of systemic granulomatous disease in Senegalese sole. The fish genotypes clustered with other available genotypes from mammals in a well-supported Endolimax clade within the Mastigamoebidae. Endolimax is a genus of intestinal amoebae which stands among the least known human protists. Previous studies on amoebic systemic granulomatosis of a marine fish (Solea senegalensis) resulted in the unexpected characterization of a new organism which was related to Endolimax and named E. piscium. The existence of multiple reports of systemic granulomatosis caused presumptively by unidentified amoebae in goldfish lead us to investigate the organism involved in goldfish disease. Analysed goldfish presented small whitish nodules in the kidney, which correspond to chronic granulomatous inflammatory reactions with a ring-layer of amoebae in the periphery. Amoebae were amitochondriate and were located in a parasitophorous vacuole within macrophages, as previous studies on this condition in goldfish and other freshwater fish pointed out. SSU rDNA characterization confirmed a new Endolimax lineage which appears closely related to E. piscium, but the molecular evidence, distinct pathological features and lack of ecological overlapping between the hosts support their assignment to a new species, E. carassius. The results support the existence of a considerable unexplored diversity of Endolimax spp. among fish, and their proper characterization can contribute to an understanding of Archamoebae evolution and pathogenic potential.

Automated summary

A new parasite species, Endolimax carassius, was identified as the causative agent of systemic granulomatous disease in goldfish. Phylogenetic analyses showed that this goldfish parasite species is closely related to Endolimax piscium, the causative agent of systemic granulomatous disease in Senegalese sole. The discovery of a considerable diversity of Endolimax spp. among fish highlights the importance of characterizing these parasites in order to understand their evolution and pathogenic potential.