Amyloodinium ocellatum: milestones, doubts and future goals

A meta-analytical approach was followed to collect and review data on the biological and ecological characteristics of Amyloodinium ocellatum (AO) from the available literature (N=116 publications). Disaggregated data have implemented an Excel spreadsheet to be rapidly extracted and evaluated. Milestones. AO is the only protozoan belonging to the Amyloodinium genus (class Dinophyceae) and the aetiological agent of amyloodiniosis. This protozoan is distributed worldwide in tropical and temperate regions and found in various aquatic habitats (sea and brackish water). AO is an obligate parasite dependent on euryhaline animals to survive and complete its triphasic life cycle. The AO triphasic cycle consists of a stationary parasitic stage (trophozoite/trophont), a free-living reproductive phase (tomont) and a free-swimming infective stage (dinospore). Additionally, the protozoan is not species-specific and can infect elasmobranchs, teleosts, crustaceans, mollusca and flatworms. AO can be diagnosed by light microscopy and rapid molecular diagnostic tools. Amyloodiniosis is a major threat for semi-intensive aquaculture (lagoon and land-based farming) in the Mediterranean basin especially in warmer months with high mortality rates (up to 100%) and significant economic losses. Against this background, to mitigate amyloodiniosis effective and licensed treatments do not exist, but formalin and copper sulphate (toxic to fish and highly polluting) are commonly used, being efficacious on trophonts and dinospores. Conversely, other potentially effective treatments (e.g. freshwater baths) are not resolutive or exploitable in all rearing realities. Doubts. Tomont persistence and dinospore lifespan in critical environments or without hosts have not been elucidated yet. Further, there are no specific immunogenic antigens identified for the development of a vaccine. Additionally, the information about molecular and genetic aspects is lacking. No data about the efficacy of alternative therapies on the dinospores/trophonts are available. Besides, immunohistochemistry results should be newly investigated on host tissue response. Future goals. The literature revision was the core to generate innovative data in the perspective to limit AO impacts on fish rearing systems. The availability of AO genome sequence, host gene expression against parasite and its antigenic profile could open the door for targeted drugs (eco-sustainable and licensed) and vaccines discovery.