Occurrence and distribution of bacterial pathogens of fish in the southern gulf of Lake Tana, Bahir Dar, Ethiopia

Anwar Nuru, Bayleyegn Molla* and Eshetu Yimer**

University of Gondar, Faculty of Veterinary Medicine, P. O. Box, 196, Gondar, Ethiopia
hamduanwar@yahoo.com
* Ohio State University, College of Veterinary Medicine, 1920 Coffe Rd, Columbus, OH 43210, Ohio, USA
** Ethiopian Health and Nutrition Research Institute, , P. O. Box, 181689, Addis Ababa, Ethiopia

Abstract

 Bacteriological study was conducted from September 2006 to March 2007 to estimate the occurrence and distribution of gram-negative bacteria from the kidney and intestine of apparently healthy fish and their aquatic environment in the southern gulf of Lake Tana.

Among the isolates Aeromonas (A.) hydrophila (7.1%), Aeromonas (A.) caviae (9.8%), Aeromonas (A.) sobria (15%), Edwardsiella (E.) tarda (2%), Vibrio spp (7.8%), Yersinia (Y.) ruckeri (3.1%), Edwardsiella (E.) ictaluri (1.2%) and atypical strains of Aeromonas (A.) salmonicida (13%) were isolated in both the kidney and intestinal samples. All the bacterial species, which were isolated from the water samples, were also recovered from fish. The isolation of bacterial pathogens in the kidney of the fish may be a risk for the occurrence of diseases outbreak whenever the fish are succumbed to stress factors. Moreover the recoveries of various organisms, which are potentially pathogenic to humans, in the fish suggest that if they are improperly handled, undercooked or consumed raw may cause diseases to susceptible individuals.

Keywords: Bahir Dar, fish, gram-negative bacteria, intestine, kidney, Lake Tana, water

Introduction

In Ethiopia, despite favorable physical conditions aquaculture is almost non-existing but fishing still depends on inland water bodies. The water body covers only 0.7% of the total area of the country. It comprises 10 lakes and the largest lake is Lake Tana. The southern gulf of Lake Tana which have relatively high fish density (Wudneh 1998), receives many contaminant inflows from the nearby farm lands with intensive fertilizers and untreated industrial, municipal, sewages, leaching from pit latrines and septic tankers, and they all remain the most important sources of pollution (Yimenu 2005).

In Ethiopian water bodies including lakes the only group of pathogens of fish that have been relatively well-studied are helminthes, however less attention have been given to bacterial pathogens of fish including those which have zoonotic importance. Therefore, the objective of the study was to estimate the occurrence and distribution of important gram-negative bacteria from the kidney and intestine of fish and their aquatic habitat.

Materials and methods

Study area

The study was conducted in Bahir Dar and Lake Tana (Figure 1). Bahir Dar is located in north western Ethiopia on the southern shore of Lake Tana, the source of the Blue Nile. The city has a latitude and longitude of 11°36′N 37°23′E and an elevation of 1840 m. a. s. l. The climate is characterized by a major rainy season with heavy rains during June to October and sometimes a minor rainy season during February to March. Maximum water temperature of the lake as a monthly average ranges between 21 and 26oC (Dejen 2003). The lake has low chlorophyll a concentration of 3.5 mg m-3, low salinity (143 mg1-1), low mean biomass (129 mg carbon m-2) (Wudneh 1998) and conductivity ranges from 136 to 234 μs cm-1 (Nagelkerke 1997). The largest fish family in the lake is the Cyprinidae and it is represented by three genera: Barbus, Garra and Varicorhinus. The lake has also contains one cichlid, Oreochromis niloticus (Nile tilapia) and the Catfish family (Clariidae), which is represented by one species, Clarias gariepinus (African catfish) (Negelkerke 1997).

Figure 1. Map of the study area

 Study design

The study was a cross-sectional conducted during September 2006 to March 2007. A total of 255 live fish and 62 water samples were sampled from the southern gulf of Lake Tana. Fish samples were obtained using multi-mesh monofilament survey gillnet that is composed of five different randomly distributed mesh size-panels, ranging from 60-120 mm with 50 m total length and 2 m width. One sampling day of the week was randomly selected and a simple random sampling technique was employed to select the samples after they are drawn from the water. Water samples were also collected from fish sampling sites about 15 cm down the water surface in 100 ml of sterile amber color bottles. Fish were identified for their species type using methodologies applied by Negelkerke (1997). Gonad maturity was assigned by viewing the gonads and ovaries were staged in I-V maturity scales according to Nicolsky (1963). Stage I and II was taken as immature stages, stage III to V was considered as mature.  

Swabs from the kidney and intestine were aseptically taken for isolation and identification of the bacteria just after searing the surface of the organs to avoid surface contaminants. The water samples were filtered using microfiltration membrane pore size of 0.1 µm and high vacuum based sucking machine. The filter paper along with the bacterial remnants was inserted in nutrient broth media for bacterial enrichment. The swabs and a loop full from enriched nutrient broth were streaked directly on Tryptic Soy Agar (TSA) plate. After incubation only colonies of gram-negative bacteria were re-streaked on new TSA plates to find pure colonies, and subjected to standard morphological and biochemical tests for identification. All the tests were done according to the flow chart outlined by Fisheries and Oceans Canada (2004). The Indole, Methyl-red, Voges-Proskaur and Citrate (IMViC) and different sugar tests were also applied for further identification. The species and subspecies of the genus Aeromonas and Plesiomonas were also further characterized according to Khardori and Fainstein (1988). 

Data management and analysis

Microsoft excel was employed for raw data entry. SPSS software was used to calculate the frequency and percentage of each bacterial isolate with respect to different variables and presented in tabulated form. 

Results

Among the fish samples caught for bacteriological examination, the majority of the fish were Barbs species (40.4%) followed by Oreocromis (O.) niloticus (19.6%), Clarias (C.) gariepinus (15.3%), Varicorhinus (V.) beso (12.9%) and Garra species (11.8%).  

Significant differences were observed in the number of different bacteria species isolated in the kidney and intestine. Large numbers of different bacteria species were isolated in the intestine than the kidney, and those, which were recovered from the kidney also isolated from the intestine (Table 1).  

 Even though individual differences occurred in the frequency of isolation of bacteria species some bacteria species like A. hydrophila, A. sobria, Vibrio spp and atypical species of A. salmonicida isolated in all fish species (Table 2). 

Despite the disparity observed in the frequency of isolation, no significant differences were observed in the type of bacteria species recovered between the maturation stages (Table 3). 

 Nine of the eighteen different bacteria species, which were isolated from the fish also recovered from water samples. Indicators of water pollution such as A. caviae, A. sobria and E. coli were isolated more frequently in Littoral than Sub-Litoral and Pelagic zones (Table 4). 

Discussion

In this study few bacteria species were isolated from the kidney, although the internal organs of healthy fish should be sterile (Sutine et al 2007). The isolation of bacteria species from the kidney, liver and spleen of apparently healthy fish have frequently been reported (Cahill 1990; Sousa and Silva-souza 2001; Eshetu 2000; Gebeyehu 2003; Apun et al 1999). Although Cahill (1990) and Magnadottir (2006) described the presence of microorganisms in internal fish organs could indicate the breakdown of immunological defense mechanisms, McVicar (1997) have pointed out that the occurrence of an infection in a fish could not be necessarily an abnormal event or it will lead to a disease situation. Moreover McVicar (1997) emphasized that under natural conditions, most infectious agents coexist with their host without causing significant disease. However, stress factors are frequently blamed for the incidence of many disease outbreaks and dissemination of infections.  

The frequency of some bacteria isolates were also found varying significantly between the different fish species but less between the two maturation stages as well as sex groups. The differences were more pronounced due to the variation of the frequency of bacteria isolates in the intestine of fish than the kidney. Generally it is accepted that fish posses a specific intestinal microbiota, but the bacteria composition may change with the type of fish species   (attribute to complexity of the fish digestive system), maturation status and the environmental conditions (salinity and bacterial load in the water) (Cahill 1990; Ringoa et al 2003). Characteristics of the microenvironment at various locations through the alimentary tract of each fish species also influence the taxonomic composition and the numerical abundance of bacteria present (Horsley 1997). 

Differences were also observed in the bacterial composition and frequency isolation across the sampling sites of the fish and water samples. This may be generally attributed to the relative distance and degree of exposure to the nearby point source pollution around the study area. This was in agreement with Fogarty et al (2003), who reported that pollution level decreases with an increase in distances from the source of pollutions.

Conclusions

The findings have shown that the presence of opportunistic fish pathogens both in the kidney and the intestine indicates the risk of the occurrence of disease outbreak any time when the fish are succumbed under stress. The recovery of organisms, which are potentially pathogenic to humans, in the fish suggest that if they are improperly handled, undercooked or consumed raw may cause diseases to susceptible individuals. The detection of similar bacterial species both in the fish and the water has some implications on the relationship between the fish and their aquatic environment (even though it requires some molecular characterizations of both isolates).  

Acknowledgements

The authors acknowledge Addis Ababa University, Faculty of Veterinary Medicine, and Amhara Regional Agricultural Bureau for financial support. We are also grateful to Bahir Dar Regional Veterinary Laboratory, Bahir Dar Fisheries and Other Aquatic Life Research Center and Amhara Regional Health Research Laboratory for their technical support and laboratory facilities. 

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Received 5 June 2012; Accepted 19 July 2012; Published 1 August 2012

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