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 examination
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.