Unusual tumour-like pathologies caused by mysterious cells termed 'X-cells' have been reported from numerous fish groups worldwide. After nearly 100 years of research, the tumour-like growths have recently been shown to be caused by a protozoan parasite. In the present study, histopathology and small subunit ribosomal DNA (SSU rDNA) sequences are used to assess whether the X-cell parasite infecting Atlantic dab Limanda limanda L. is distinct from the X-cell parasite infecting Japanese flounder and goby, and to determine their systematic position within the protists. SSU rDNA from Scottish dab was 89.3% and 86.7% similar to Japanese X-cell sequences from flounder and goby respectively, indicating that the parasite infecting dab in the Atlantic is distinct from the Pacific species. Histological studies revealed significant gill pathology and demonstrated the precise location of the parasites within the gill tissues using specific in situ hybridization probes. Phylogenetic analyses showed that the X-cell parasites from Scotland and Japan form a monophyletic group within the Myzozoa, and are basal alveolates. However, ultrastructure of X-cells from dab fails to confirm this systematic placement.
Iridoviruses, especially megalocytiviruses, are related to severe disease resulting in high economic losses in the aquaculture industry worldwide. The ornamental fish industry has been affected severely due to Megalocytivirus infections. Megalocytivirus is a DNA virus that has three genera; including red sea bream iridovirus, infectious spleen and kidney necrosis virus, and turbot reddish body iridovirus. Megalocytivirus causes non-specific clinical signs in ornamental fish. Cell culture, histology, immunofluorescence test, polymerase chain reaction (PCR) assay, and loop-mediated isothermal amplification assay have been used to diagnose megalocytiviruses. Risk factors such as temperature, transportation (export and import), and life stages of ornamental fish have been reported for the previous cases due to Megalocytivirus infections. In addition, other prevention and control methods also have been practiced in farms to prevent Megalocytivirus outbreaks. This is the first review of megalocytiviruses in ornamental fish since its first detection in 1989. This review discusses the occurrences of Megalocytivirus in ornamental fish, including the history, clinical signs, detection method, risk factors, and prevention measures.