A high abundance of dactylogyrid monogeneans is commonly found on the spotted rose snapper Lutjanus guttatus under culture conditions. This study provides the first description of the haematology and gill histology of L. guttatus infected by dactylogyrids. The fish were infected by cohabitation with naturally infected fish. Infection levels were categorized as low (mean intensity of 22.1 parasites per fish), intermediate (100.6 parasites per fish) or high (199.8 parasites per fish). The attachment of dactylogyrids increased the erythrocyte sedimentation rate in fish with a low level of infection. Glucose was high for all three levels of infection. An increase in the infection load was positively correlated with the total leucocyte count, thrombocyte percentage ratio and the percentage of granulocytes, whereas increased infection level was negatively correlated with the leucocyte percentage ratio and the number of lymphocytes. A high infection level on the gills led to a severe proliferative epithelial response, a high number of mucous cells, a moderate increase in chloride cells, infiltrating inflammatory cells and lamellar fusion. These host responses reflect a sublethal condition and indicate a potential risk posed by dactylogyrid infection on cultured spotted rose snapper.
The present study investigated the effect of β-1,3/1,6-glucan on growth, haematology, innate immunity and resistance against dactylogyrids on the spotted rose snapper (Lutjanus guttatus). Fish were fed during 5 weeks with commercial diet (control group) and same diet supplemented with three levels of β-glucans (0·05%, 0·1% and 0·5%/kg feed). The results showed that at concentrations of 0·05% and 0·1%, fish growth was enhanced, and in weeks 2 and 4, an increase in cellular responses such as percentage of monocytes, neutrophils, respiratory burst activity and nitric oxide activity was observed. In diet with 0·5%β-glucans, changes were registered at the end of the experiment. At week 2, 0·05%β-glucans showed the best response to most of the analysed parameters. In a second trial, diet with 0·05%β-glucans was chosen to prove its effect on the resistance of infected fish with monogeneans. The results showed that fish reduce significantly the number of dactylogyrids, and parameters such as WBC, percentage of lymphocytes, neutrophils and eosinophils were increased. In addition, WBC and percentage of thrombocytes increased in the control (+). The β-glucans have the potential to be use in diet formulations of the spotted rose snapper and to limit the adverse effects of dactylogyrids.
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