Fish farming has been practiced in various parts of the world. It has been in practice since the ancient civilization of Egypt and China, thus fisheries sector persists as a significant source of food, nutrition, income, and livelihoods for hundreds of millions of people around the world (Deng, 2020). Fish and their products are an important source of easily digested and delicious animal protein with high biological value which compromise many essential amino acids, vitamins D and B, essential minerals (calcium, phosphorus, iodine, zinc, iron, and selenium) as well as amounts of trace elements which have beneficial effects for adult health and child cognitive development (Nagy et al., 2018; Deng, 2020). Also fish has the role in the improvement of human bodily functions and reduction in susceptibility to cardiovascular diseases and cancer, which arrived from polyunsaturated fatty acids particularly those from the omega-3 family (Usydus and Joanna, 2012).
Artisanal freshwater fishery is fishery sector and the most valuable economic activities in Ethiopia, which contribute in poverty alleviation in the country (Janko, 2014). It has been suggested that, the country has an estimated annual total exploitable fish potential of 51,481 tons, which can meet only 79% of the current actual demand at a time, 55% of the projected demand in 2010 and 44% of the projected demand in 2015, based solely on population size whereas the Lake Tana, Ashenge, Hayk, Koka, Ziway, Langano, Awassa, Abaya and Chamo are among the potential fish rich lakes found in Ethiopia (Tilahun et al., 2016).
Unlike in all animal production systems, in fish subjected to intensive culture practices disease outbreaks are one of the main and high significant limiting factors (Nagy et al., 2018). especially that result from bacterial infections are one of the major problems hampering production, development and expansion of the aquaculture industry and difficult to control (Kebede and Habtamu, 2016).
Among the most important bacterial diseases causing severe economic losses in fish farms of many countries is Edwardsiellosis, which are infectious diseases often associated with poor water quality and stress (Kebede and Habtamu, 2016). It is a serious systemic bacterial disease, which affects a variety of fish taxa include carp, tilapia, eel, catfish, mullet, salmon, trout and flounder, caused by Edwardsiella tarda, a Gram- negative bacilli that belongs to the Enterobacteriaceae family, which has many traits that are characteristic of many enterobacteria. These characteristics include it being a facultative anaerobe, motile, short, rod-shaped bacterium (1 µm in diameter and 2–3 µm long) (Mohanty and Sahoo, 2007). E. tarda is known for causing diseases in both humans and fish also may infect reptiles, amphibians, marine mammals and other warm-blooded animals (Castro et al., 2006).
E. tarda is commonly found in the normal gut flora of fish and humans, and can be an opportunistic pathogen in human (Verjan et al., 2005) with the risk factors of being vulnerable to aquatic environment, pre-existing liver diseases, iron over load and raw sea food ingestion (Wang et al., 2008). Many potential virulence factors have been identified and include siderophores, the ability to invade epithelial cells and internal fish tissues, resistance to serum and phagocyte-mediated killing and the production of toxins/exoenzymes, including dermatotoxins, catalase and haemolysins (Wang et al., 2010).
In fish E. tarda may affect internal organs such as liver, kidney, spleen and muscle and exhibit signs of distention of abdomen and swollen anus due to the accumulation of ascitic fluid, pigment loss, enlarged kidney, and abscesses on internal organs (Ishibe et al., 2008) and also a variety of clinical syndromes, such as bacteremia or septicemia, enteric fever, gastroenteritis, an asymptomatic carrier state, and localized infections with extensive skin lesions that can develop into necrotic abscesses (Nucci et al., 2002). The affected fish shows the loss of scales from some the body areas, congestion, exophthalmia and as well as protruded and congested vent, ulcer and hemorrhages all over the fish body especially at the base of the pectoral fin, anal opening, and tail rot while their postmortem examination revealed variety of lesions as well as distention of gall bladder and abdomen with ascetic fluids, hepato-pancreas and kidney enlargement (Nagy et al., 2018). It has been reported in humans as the cause of gastroenteritis, minimal diarrhea, nausea and generalized infections mainly among individuals with impaired immune systems (Lan et al., 2008).
E. tarda can be isolated on Edwardsiella Isolation Media (EIM), Brain Heart Infusion (BHI), Tryptic Soya Agar (TSA), Xylose Lysine Deoxycholate (XLD) and MacConkey Agar with characteristics of small, circular, raised, whitish with black center on XLD and pale on MacConkey agar, grow best at a range of 25°C-37°C temperature and 7–8 PH being positive on catalase, indole, lysine and glucose fermentation, while being negative on cytochrome oxidase, lactose fermentation, citrate, mannitol, dulcitol, sorbitol, inositol, xylose, rhamnose, alkaline slant and acid butt on Triple sugar iron Agar (Quinn et al., 2011). It does not produce urease and similar to Salmonella in that it is able to generate hydrogen sulfide on laboratory media (Kebede and Habtamu, 2016).
The practice of consuming uncooked fish meals, manual handling of fish and unhygienic practice during filleting in Ethiopia implies that the public is at higher risk of contracting the disease from infected fish meat. Therefore, the disease which cause with this bacteria or edwardsiellosis requires to give attention due to its potential threat to future aquaculture industry and public health. In Ethiopia, the bacterium has been isolated from apparently healthy fish of Lake Zeway and Langano (Kebede and Habtamu, 2016), Lake Tana (Nuru et al., 2012), crater lake around Bishoftu and Lake Hawasa (Nemo et al., 2017) and Lake Hayike (Tesfaye et al., 2018). Even though, the lake was re-emerged recently and using as source of income from fish meat for surrounding people, there is no work done in investigation of this bacterium on Lake Haramaya.
Therefore, this study was conducted with the objective of isolating and identification of E. tarda from the fishes (Catfish and Tilapia) of Lake Haramaya:
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To elucidate safety of fish products with respect to E. tarda contamination.
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To estimate the occurrence and distribution of E. tarda in the lake Haramaya from intestine, kidney, liver and spleen of Catfish and Tilapia.