Expression of infection-related immune response in European sea bass (Dicentrarchus labrax) during a natural outbreak from a unique dinoflagellate Amyloodinium ocellatum.

In the Mediterranean area, amyloodiniosis represents a major hindrance for marine aquaculture, causing high mortalities in lagoon-type based rearing sites during warm seasons. Amyloodinium ocellatum (AO) is the most common and important dinoflagellate parasitizing fish, and is one of the few fish parasites that can infest several fish species living within its ecological range. In the present study, A. ocellatum was recorded and collected from infected European sea bass (Dicentrarchus labrax) during a summer 2017 outbreak in north east Italy. Histological observation of infected ESB gill samples emphasized the presence of round or pear-shaped trophonts anchored to the oro-pharingeal cavity. Molecular analysis for small subunit (SSU) rDNA of A. ocellatum from gill genomic DNA amplified consistently and yielded 248 bp specific amplicon of A. ocellatum, that was also confirmed using sequencing and NCBI Blast analysis. Histological sections of ESB gill samples were addressed to immunohistochemical procedure for the labelling of ESB igm, inos, tlr2, tlr4, pcna and cytokeratin. Infected gills resulted positive for igm, inos, pcna and cytokeratin but negative to tlr-2 and tlr-4. Furthermore, ESB immune related gene response (innate immunity, adaptive immunity, and stress) in the course of A. ocellatum infection using quantitative polymerase chain reaction (qpcr) for infected gills and head kidney was analysed. Among the twenty three immune related gene molecules tested, cc1, il-8, il-10, hep, cox-2, cla, cat, casp9, and igt were significantly expressed in diseased fish. Altogether, these data on parasite identification and expression of host immune-related genes will allow for a better understanding of immune response in European sea bass against A. ocellatum and could promote the development of effective control measures.