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Abstract
The protozoan parasite Cryptocaryon irritans causes marine white spot disease in a wide range of fish hosts, including gilthead seabream, a very sensitive species with great economic importance in the Mediterranean area. Thus, we aimed to evaluate the immunity of gilthead seabream after a severe natural outbreak of C. irritans. Morphological alterations and immune cell appearance in the gills were studied by light microscopy and immunohistochemical staining. The expression of several immune-related genes in the gills and head kidney were studied by qPCR, including inflammatory and immune cell markers, antimicrobial peptides (AMP), and cell-mediated cytotoxicity (CMC) molecules. Serum humoral innate immune activities were also assayed. Fish mortality reached 100% 8 days after the appearance of the C. irritans episode. Gill filaments were engrossed and packed without any space between filaments and included parasites and large numbers of undifferentiated and immune cells, namely acidophilic granulocytes. Our data suggest leukocyte mobilization from the head kidney, while the gills show the up-regulated transcription of inflammatory, AMPs, and CMC-related molecules. Meanwhile, only serum bactericidal activity was increased upon infection. A potent local innate immune response in the gills, probably orchestrated by AMPs and CMC, is triggered by a severe natural outbreak of C. irritans.
Highlights
Nowadays, aquaculture provides nearly 52 percent of the fish for human consumption and represents the most productive food sector [1]
The control group was formed by fish from the same ba4tcohf o18f hatched eggs but that were located in other tanks with a controlled temperature at 20 ◦C
acidophilic granulocytes (AGs) are the most abundant and important phagocytic and antigen presenting cells in gilthead seabream and are functionally equivalent to mammalian neutrophils. Taking this into account and based on the data obtained regarding the mobilization of AGs to the gills, we further evaluated the transcription of pro-inflammatory cytokines (Figure 3)
Summary
Aquaculture provides nearly 52 percent of the fish for human consumption and represents the most productive food sector [1]. Gilthead seabream (Sparus aurata L.) is one of the most produced species in the Mediterranean area and is highly traded with more prosperous markets [2]. C. irritans is a holotrich ciliate protozoan that causes marine white spot disease or “marine ich” [4] and is considered to be the most devastating parasitic disease in both mariculture and in ornamental fish, commonly occurring when temperatures are above 19 ◦C, mainly between 20–30 ◦C, and generally between June and October [3,5,6,7,8,9,10]. C. irritans exhibits very low host specificity and is able to infect multiple fish species, with gilthead seabream being the most affected amongst the cultured species in the Mediterranean area [10]. C. irritans invades fish epithelium of the skin, gills, and eyes, altering their physiological functions [3,5,11]. The infective stage, the theront, infects the fish epithelial layer and develops to a trophont stage, which feeds on tissue debris and body fluids, matures, and leaves the host, becoming tomont cells, the external stage [3,9,11]
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