Co-infection of Cryptocaryon irritans and Vibrio harveyi impairs osmoregulatory and respiratory functions in orange-spotted grouper Epinephelus coioides.

Cryptocaryon irritans and Vibrio harveyi are common parasitic and bacterial pathogens infecting mariculture fish such as the orange-spotted grouper (Epinephelus coioides), and they cause significant economic losses. Epidemiological evidence indicated that V. harveyi commonly causes secondary bacterial infection in teleost infected with C. irritans. However, the pathogenic mechanisms of C. irritans and V. harveyi co-infection in grouper remain unclear. In this study, a co-infection model was established in vivo that the co-infection increased the mortality of grouper and facilitated the replication of V. harveyi. Blood smear analysis showed that the percentage of white blood cells, including monocytes, lymphocytes and neutrophils, obviously increased in co-infected grouper. Histopathological and ultrastructural observations showed that co-infection induced the pathological changes in gills and trunk kidney. Excessive mucus was produced in gills and apoptosis was induced in trunk kidney. Enzymatic assays revealed a significant change in serum ion and protein, respiratory enzyme, and osmoregulatory enzyme in co-infected grouper. Moreover, detection of peripheral oxygen saturation and osmotic pressure demonstrated that the death of co-infected grouper was associated with the asphyxiation and osmotic pressure imbalance. Therefore, our study could facilitate further research on the pathogenic mechanisms of C. irritans and V. harveyi co-infection in orange-spotted grouper.