LAUSR

Abstract A sudden outbreak of disease associated with high mortality rates (75–85%) was reported in market-sized striped mullet (Mugil cephalus), farmed in earthen ponds in Manzala, Egypt, during the summer (July) 2019. We identified poor water quality parameters, including a sharp decrease in dissolved oxygen (DO) (3.5 mg/l) accompanied by elevated levels of unionized ammonia (0.90 mg/l), nitrite (0.05 mg/l), hydrogen sulfide (0.07 mg/l), nickel (0.7 mg/l), copper (0.6 mg/l), and iron (1.80 mg/l). Diseased and recently dead fish exhibited general septicemic signs. A. hydrophila and V. parahaemolyticus were isolated from diseased fish by culture on selective media and further identified by biochemical tests. PCR targeting the 16S rRNA gene and some virulence gene fragments of each bacterial isolate were performed, and genome sequencing and phylogenetic analysis confirmed the molecular identity of both isolates. The high molecular identity and close phylogenetic relationship confirmed that the isolates were genetically homologous to A. hydrophila and V. parahaemolyticus. Outbreaks involving multiple co-infecting pathogens have been reported in several fish species. The current study highlights a case report of synergistic interactions between poor water quality and the presence of aquatic microbial pathogens such as A. hydrophila and V. parahaemolyticus. This combination of factors resulted in the incidence of heavy mortalities in farmed striped mullet M. cephalus, during the summer season in Manzala Egypt.