First description of Vibrio harveyi as the causative agent of morbidity and mortality in farmed flathead grey mullet Mugil cephalus

Flathead grey mullet Mugil cephalus is an important species in the aquaculture industry in the Mediterranean basin and throughout the world. During the last 10 yr, M. cephalus breeding stocks, larvae, and juveniles cultured in Eilat (Israel) have shown neurological signs such as uncoordinated circular swimming, while also presenting oral hemorrhages. Death follows days after the onset of the clinical signs, and mortality rates may reach 80% in some cases, causing high economical losses. Bacteriology isolations from different organs, including the brain, and a Koch’s postulate experiment, confirmed Vibrio harveyi as the causative agent. Histological analyses showed the presence of the bacterium in different organs. However, in the brain, the bacterium was observed only within blood vessels and meninges. In some samples, mild to severe brain tissue damage was seen. In order to understand the virulence and lethality of V. harveyi, a median lethal dose was calculated, and the result was 10⁶ colony-forming units fish⁻¹. To the best of our knowledge, this is the first report that describes V. harveyi isolated from the brain of M. cephalus and validates it as an etiological agent causing neurological signs in this fish species.