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Pathogenicity of Aeromonas Hydrophila and Some Other Bacteria Isolated from Japanese Eels Anguilla Japonica Reared Farm Water

Md. Mer Mosharraf Hossain
Dept of Aquaculture & Fisheries, Jessore Science & Technology University, Ambottola, Jessore Sadar, Jessore-7407, Bangladesh & Dept of Aquaculture, Faculty of Agriculture, Kochi University, Nankoku shi, Kochi 783-8502, Japan

Kenji KAWAI
Dept of Aquaculture, Faculty of Agriculture, Kochi University, Nankoku shi, Kochi 783-8502, Japan

Abstract/ Executive Summary:

Studies were conducted to determine the pathogenicity for Japanese eels Anguilla japonica and ayu Plecoglossus altivelis of several strains of the species Aeromonas hydrophila, A. jandaei, A. sobria, A. caviae. Plesiomonas shigelloides. Vibrio anguillarum, Pseudomonas fluorescens and Shewanella putrefaciens isolated from an eel farm. Virulence of 20 isolates obtained during epizootics from diseased eels, asymptomatic carriers and tank water and of 20 isolates obtained during the absence of disease from healthy eels and water was evaluated by 2 challenge methods: intraperitoneal injection (IP) and bath exposure. Of the isolates tested by IP, all strains of A. hydrophila and those of A. jandaei mainly from epizootics, were pathogenic for eels (LD₅₀ dose 10^5.4 to 10^7.5 cfu fish^-1). Strains of these species caused an ulcerous disease by bath exposure to 10⁷ to 10⁸ cfu ml^-1. In contrast, ayu were less susceptible to infection caused by A. hydrophila and A. jandaei isolates, perhaps because of the lower water temperatures at which they were held during challenge. The epizootics in the eel populations occurred in spring and summer when water temperatures were 17 to 22^oC. The data indicate that A. hydrophila and A. jandaei are Japanese eel pathogens and that infections may occur by waterborne transmission. Therefore, the presence of these bacteria in the farm environment probably constitutes a health hazard for eels.

Keywords: Aeromonas hydrophila, Japanese eels Anguilla japonica, pathogenicity, challenge test

Location: Dept of Aquaculture & Fisheries, Jessore Science & Technology University, Ambottola, Jessore Sadar, Jessore

Start Date:

End Date:

Program Area: Animal Health and Management

Commodity: Eel

Objectives:

1. Its objective was to determine the virulence of the bacterial species (Aero- monas hydrophila, A. jandaei, A. sobria, A. caviae, Plesiomonas shigelloides, Vibno anguillarum, Pseudo- monas fluorescens and Shewanella putrefaciens) isolated from the affected Japanese eel farm during epizootics as well as during routine surveys, so that their importance as Japanese eel pathogens could be assessed.

Methodology:

Test fish: The infectivity study was con-ducted using juvenile Japanese eels Anguilla japonica (elvers-young eel) (5 to 12 g) and ayu Plecoglossus altivelis (4 g). Groups of 6 fish (elvers or ayu) were transferred to 24 l plastic tanks supplied with dechlorinated municipal water. Water temperatures were optimal for these fish, ranging from 20 to 22°C for elvers and from 13 to 15 "C for ayu. Effluent water from the tanks was treated with sodium hypochlorite in sufficient concentration to kill pathogens present. Tanks were treated with 0.1N HCL and irradiated with ultra- violet light when the experiment was over. Bacterial strain: A number of strains of the species Aeromonas hydrophila (10), A. jandaei (B), A. sobria (4), A. caviae (3), Plesiornonas shigelloides (2), Vibrio anguillarum (4), Pseudomonas fluorescens (7) and Shewanella putrefaciens (2) were evaluated for their virulence . The number of isolates tested for each species approximately reflected the numerical relationship previously observed among these species. Oxidative bacilli (P. fluorescens and S. putrefaciens) were identified using the tests. Fermentative bacilli (Aeromonas spp., V anguillarum, and P. shigelloides) were classified by a numerical taxonomy study. Some biochemical characteristics of the strains tested. For challenging, previously lyophilized strains were used. Challenge test - IP challenge: Bacterial strains were grown overnight on TSA-1 at 25°C. For each strain, 6 fish (elvers or ayu) were injected intraperitoneally (IP) with 0.1 m1 of a suspension containing l09, 108, 107, 106, 105 and 104 cells ml-1 in phosphate-buffered saline (PBS, pH 7.0). Six fish (controls) injected with 0.1 m1 of sterile PBS were held under the same conditions as the challenged specimens. Mortalities were recorded daily for 7 d (Santos et al. 1988). Liver and kidney samples from freshly dead fish were cultured on TSA-1, and isolates were subsequently tested for their biochemical properties. Mortalities were only considered if the 50 % lethal dose (LDS,) was calculated. Strains that exhibited LD₅₀≥108 cfu fish^-1 were considered avirulent. Bath challenge: Only the most virulent strains in IP experiments were selected for bath challenges. Strains were grown for 24 h in tryptic soy broth (Oxoid) supplemented with 1 % NaCl (TSB-1). For each strain, duplicate groups of 6 elvers (average weight 12 g) were held for 60 min in an aerated bath (61) containing 2 and 20 ml broth culture l-1 and then returned to their tanks. Fish were exposed to 107 and to 108 cells ml-l during each treatment. Mortalities were recorded daily for 14 days (Baxa et al. 1990). Samples from ulcers, livers, and kidneys of freshly dead elvers were examined for the presence of the challenge bacterium. At 14 days post- exposure, samples of gills, liver and kidney from all survivors were also analyzed. Statistical analysis: The association between pathogenecity for eels and origin of the isolates (epizootics/non- epizootics) was assessed using the Chi-square (X2) test.

Source of Information: BANGLADESH RESEARCH PUBLICATIONS JOURNAL; ISSN: 1998-2003, Volume: 5, Issue: 1, Page: 22-30, January - February, 2011

Article Link (Online Journal):

Funding Source:

Total Budget (Tk.) :

Achievement/Findings:

The data indicate that A. hydrophila and A. jandaei are Japanese eel pathogens and that infections may occur by waterborne transmission. Therefore, the presence of these bacteria in the farm environment probably constitutes a health hazard for eels.

Knowledge Management: Journal