Co-infection with

A high mortality event occurred among Malabar groupers (Epinephelus malabaricus) reared for research. The affected fish had scattered reddish patches and ulcers on the skin, accompanied by parasite infestations on the skin surface. Histologic findings included ulcerative dermatitis and keratoconjunctivitis with gram-negative bacilli, and the parasites were often observed on the skin surface. Bacterial examinations and in situ hybridization revealed the presence of Vibrio harveyi in the affected tissues. The parasites were morphologically identified as the subfamily Benedeniinae (Monogenea: Capsalidae), likely Neobenedenia girellae. The monogeneans may have come into contact with the fish at a farm in Okinawa, Japan, and potentially facilitated the opportunistic infection with V. harveyi. This is the first report of vibriosis caused by V. harveyi in Malabar groupers.