2025 Volume 60 Issue 2 Pages 50-60
In October 2022, a disease associated with a sudden increase in morbidity and mortality occurred among one-year-old red-spotted grouper Epinephelus akaara cultured in land-based tanks in Tottori Prefecture, Japan. Daily monitoring of the disease showed that fish losing equilibrium at the bottom of the tanks appeared one after another, and chronic mortality continued over a period of three months. All moribund fish examined had a bloated swimbladder and were heavily infected with monogeneans of the genus Pseudorhabdosynochus on the gills (P. epinepheli and P. lantauensis), with infection intensities up to over one thousand worms per fish. In vitro and in vivo experiments showed that hydrogen peroxide was effective, removing over 98% of the worms through bathing in 350 ppm for 60 min or 700 ppm for 30 min at around 20°C. Histopathological examination revealed that the monogenean infection did not induce significant host reactions in the gills. In contrast, lesions were found in the brain and retina, and morbid fish tested positive for nervous necrosis virus (NNV) by RT-PCR. These results suggested that the morbidity and mortality associated with loss of equilibrium were caused by infection with NNV rather than that with the monogeneans.
