High mortalities caused by Aeromonas veronii: identification, pathogenicity, and histopathological studies in Oreochromis niloticus A. M. Abd El Latif 1 & Hiam Elabd 1 & Aziza Amin 2 & A. I. Noor Eldeen 3 & A. A. Shaheen 1 Received: 6 April 2019 /Accepted: 5 July 2019/ # Springer Nature Switzerland AG 2019 Abstract The current study planned to identify suspected causes of mass mortalities in Oreochromis niloticus at private fish farms in Port Said Governorate, Egypt, during the 2017 summer season. One hundred twenty diseased moribund O. niloticus fish were randomly collected from different locations. The examined fish showed lethargy, dark skin, bilateral exophthalmia with corneal opacity, and varying degrees of ulcers. Internal examination revealed generalized inflammation, congestion, hemorrhaging, and enlargement of most internal organs. Identification was accomplished through analytical profile index (API) 20E and polymerase chain reaction (PCR) and confirmed by sequencing and phylogenetic analysis. An antibiogram was constructed based on results from a disc diffusion test. Experimental infection was also performed and showed 80% mortality. Virulence genes were also assayed. Histopathological examina- tion of experimentally challenged fish revealed prominent pathological lesions (mainly necrosis), degenerative changes, and cellular vacuolation. In conclusion, Aeromonas veronii can be considered one of the main causes of summer mass mortalities in O. niloticus in the study area. Keywords Identification . Aeromonas veronii . Antibiogram . Histopathology . Virulence Introduction Aeromonas spp. infections are well known in aquaculture and usually cause severe losses among cultured fishes (Tukmechi et al. 2010; Cai et al. 2012; Eissa et al. 2015; Dong et al. 2017; Peatman et al. 2018). Mortality caused by Aeromonas in fish exposed to temperature stress under certain environmental conditions can reach up to 80% (Noga 1996). Also, in intensive aquaculture, mortality caused by A. hydrophila infections showed an increase in levels from the end of spring season to the beginning of summer (Faisal et al. 1989). Aquaculture International https://doi.org/10.1007/s10499-019-00429-8 * A. A. Shaheen adel.shaien@fvtm.bu.edu.eg; shaheen_aa@yahoo.com Extended author information available on the last page of the article