Haematological and gill responses to an experimental infection of dactylogyrid monogeneans on the spotted rose snapper Lutjanus guttatus (Steindachner, 1869)

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.