a-Clinical signs and lesions

▪️ The clinical signs vary from acute hemorrhagic septicemia to large ulcerative lesions in chronic stages depending on numerous factors such as virulence of the microorganism, fish immunity, and environmental stressors.

▪️ Typical signs of septicemia include reddened fins and anal opening, generalized skin hemorrhages, exophthalmia, abdominal distension, fin erosion, and protruded scales (lepidorthosis) due to accumulated fluids in the scale pockets (edema) (Fig 2) (Youssuf et al., 2020).

▪️ Chronic infections with A. hydrophila were associated with dermal ulcerative lesions with focal hemorrhages and inflammation.

➡️  Internally, hyperaemia of the internal organs and wall of swim-bladder, petechial haemorrhages on the liver, bloody to yellowish exudate in abdominal cavity, diffuse hemorrhages and congestion in the intestines and visceral organs, congestion of gills, grayish lesions in internal organs in chronic cases (Aly et al., 2023).

b- Laboratory diagnosis

▪️  Samples: Gills, intestine, and visceral organs of infected fish

I- Presumptive diagnosis

➡️Isolation and identification

▪️ It grows rapidly on common nutrient media such as Tryptic Soya Agar (TSA) or Brain Heart Infusion agar (BHIA) at 20 to 37°C producing creamy, round, slightly raised colonies that are commonly ∞-hemolytic on blood agar. On selective media such as Rimler Shotts agar it gives yellowish to green colonies (Shotts and Rimler, 1973).

▪️ The virulent bacterial strains are known to use myo-inositol as their only carbon source. They are also known to be oxidase positive, catalase positive, indole positive, ferment and oxidize glucose producing gas.

▪️ API 20E and API 20NE are commercial biochemical kits used for bacterial identification.

 II-Histopathological examination

▪️ Fish infected with Aeromonas spp revealed degenerative and necrotic changes in tissue with inflammatory response (Dong et al., 2017). A. hydrophila bacterium produces protease, elastase, and hemolysin that cause massive necrosis in the tissue (Yardimci and Aydin, 2011).

III-Confirmatory diagnosis

➡️ Molecular assay

▪️  PCR identification using sequence analysis of the 16S rRNA gene or the gyrB genes are the most common techniques for confirming the identity of motile aeromonad (El-Gohary et al. 2020).

▪️ Restriction fragment length polymorphism (RFLP)-PCR assay is also an easy technique that differentiate between the most relevant and antigenically related Aeromonas spp (Algammal et al., 2020)