a-Clinical signs and lesions

▪️ The mortality rate of diseased fish may be acute or chronic depending on the degree of stressful environmental conditions that allow E. tarda infections to emerge.

▪️ Gross external lesions vary with species. In Nile tilapia the fish showed septicemic signs including haemorrhages on body surface and base of fins and congestion (Fig 1) (El-seedy et al., 2015), exophthalmia and cataracts (Fig 2 (Noor El Deen et al., 2017). Meanwhile, in striped bass, abscesses in internal organs were recorded, and necrotic abscesses in the muscle that emit a putrid odor when incised in channel catfish.  

▪️ The disease in eels is known by two forms: the suppurative interstitial nephritis and the suppurative hepatitis forms (Miyazaki and Egusa 1976a and b).

▪️ Generally, petechiae and dermal ulceration, iridial haemorrhage, reddening of the mouth and vent, exophthalmia, ascites, splenomegaly, renomegaly, and intestinal haemorrhage have been recorded among diseased fishes (López‐Porras et al., 2021).

Note: The fish clinical signs are comparable to those of other bacterial diseases, such as motile aeromonas septicemia, vibriosis, Pseudomonas septicemia; streptococcosis and consequently, biochemical, and molecular approaches are endorsed for diagnosis of disease.

b-Laboratory diagnosis

▪️ Samples:   liver, spleen, kidneys, and skin lesions of infected tissue from moribund or freshly dead fish are appropriate for bacteriological examination. Also, tissue specimens for histopathological analysis should be done.

I-Presumptive diagnostic assay

➡️  Isolation and identification

▪️ The bacterium is Gram-negative commonly motile but isolates from red seabream and yellowtail are non-motile (Matsuyama et al., 2005). The microorganism gives typical green colonies with black centers colonies on Edwardsiella isolation media (EIM) (Acharya et al., 2007) and producing indole from tryptone and an alkaline slant over an acid butt in triple sugar iron agar with production of H₂S and gas.

▪️ The most important biochemical tests that classified E. tarda into two groups: wild type and biogroup 1, included mannitol and arabinose fermentation, ornithine decarboxylase, citrate utilization, and hydrogen sulfide production. The fermentation of arabinose, mannitol and sucrose is negative in wild-type E. tarda, but it is positive for H₂S; meanwhile biogroup 1 are positive for the previous biochemical test. (Walton et al., 1993)

 II-Histopathological examination

▪️ Naturally diseased Nile tilapia revealed hydropic degeneration, coagulative necrosis, multi-focal areas of hemorrhages between the hepatic cells (Noor El Deen et al., 2017).

▪️ Mononuclear meningoencephalitis, granulomatous hepatitis and interstitial nephritis, splenitis, hemorrhagic enteritis, hemorrhagic branchitis, and ulcerative dermatitis were also recorded (Griffin et al., 2019).

III-Confirmatory diagnosis

➡️ Serological techniques

▪️ Agglutination tests, enzyme linked immunosorbent assays (ELISA), and fluorescent antibody techniques are valuable diagnostic procedures for disease (Swain and Nayak, 2003). Park et al. (1983) based on O-antigen extracts classified E. tarda into serotypes A, B, C and D.

➡️  Molecular assay

▪️ There are many investigations documented that PCR-based techniques can provide accurate, sensitive, and differential diagnosis.

▪️ Fluorescence in situ hybridization (FISH) technique by Ootsubo et al. (2002), and high-performance capillary electrophoresis (HPCE) by Yu et al. (2004) has been used to detect bacteria in infected tissues and pond water.

▪️ Multiplex nested PCR has been carried out to identify the fish pathogens; A. hydrophila, Edwardsiella tarda, Photobacterium damselae and S. iniae (Chang et al., 2009).

▪️ Recombinase polymerase amplification combined with lateral flow strip (RPA-LF) was carried out to monitor E. piscicida infection (Jiang et al., 2022).