Bacterial Septicaemia in Silver Carp Hypopthalmichthys molitrix (Valenciennes)

Unit for Ichthyopathology and Fish Health Protection
Freshwater Aquaculture Research and Training Centre of
Central Inland Fisheries Research Institute
Kausalyagang, VIa Bhubaneswar (Orissa), 751002 (INDIA)

NETWORK OF AQUACULTURE CENTRES IN ASIA
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BACTERIAL SEPTICAEMIA IN SILVER CARP, HYPOPHTHALMICHTHYS MOLITRIX (VALENCIENNES)

DILIP KUMAR and R.K. DEY
Unit for Ichthyopathology and Fish Health Protection,
Freshwater Aquaculture Research and Training Centre of
Central Inland Fisheries Research Institute,
Kausalyagang, Via Bhubaneswar (Orissa), 751002 (INDIA)

ABSTRACT

Bacterial septicaemia causing anaemia, emaciation, haemorrhagic lesions on the body, dydrops and mortality in silver carp is described for the first time from India. The causative organism isolated in pure culture from heart, kidney and coelomic fluid of the diseased specimens was characterized as a strain of Aeromonas hydrophila ssp. hydrophila on the basis of its morphology. Gram-staining property and other biochemical reactions. Significantly high levels of haemosiderin deposits were found in liver and ovary. Other marked histopathological changes in the kidney and liver are also discussed. Average haemotocrit value in diseased specimens was 27.63 as compared to 34.5 in healthy specimens. Drug sensitivity tests are also presented.

INTRODUCTION

The technology of composite fish culture - an intensive polyculture of three Indian major carps and three Chinese carps including phytoplanktophagus silver carp, Hypophthalmichthys molitrix (Val.) - has revolutionised freshwater aquaculture industry in India and silver carp is gradually becoming one of the major component of pond fish culture. This species is relatively more active and is very sensitive to various environmental and physical stresses and thus more susceptible to bacterial invasions resulting in disease outbreak outbreak and mortality. However, there is very little published information regarding bacterial diseases of this species. Pal and Tripathi (1978) have encountered a frequently occurring disease in composite fish culture ponds among silver carp in India causing red blotches over the body. Growth among such diseased fish was extremely poor followed by descaling and mortality resulting in considerable losses to farmers. Although the causative organism could not be detected by the authors, the disease was successfully controlled by using terramycin in feed thus hinting towards bacterial nature of the etiological organism. Bejerano et al. (1979) have described mass mortalities in silver carp associated with bacterial infection induced by handling. Csaba et al. (1981) have investigated the bacterial septicaemia in silver carp and bighead caused by Pseudomonas fluorescens. The authors have also experienced frequent disease outbreak and mortality in silver carp in rural fish culture ponds after netting, handling, transport etc., with symptoms similar to the present description. In view of its widespread nature and frequent occurrence, one such case of disease outbreak in silver carp has been investigated and reported upon in the present communication.

MATERIALS AND METHODS

Moribund specimens of silver carp) (average weight 1340 g, average length 517 mm) with signs of typical lesions were collected twice within a week from a 2.5 ha pond of the Freshwater Aquaculture Research and Training Centre (NACA Lead Centre on Carp Culture), Bhubaneswar, showing incidence of disease outbreak and mortality of silver carp. The samples were dcarefully examined and the nature and position of the body lesions recorded. Haematocrit values were measured by a IEC MB centrifuge and bacterial samples obtained according to standard procedures from the body lesions, kidney, heart, liver and coelomic fluid for isolation and study of bacterial pathogen using diagnostic scheme of shotts and Bullock (1975). Further characterization was done according to Bergey's Manual of Determinative Bacteriology (Buchanan and Gibbson, 1974).

Difco antibiotic disc series was used for antibiotic sensitivity detection with seven commonly used antibiotics. Antibiotic resistance was tested on yeast extract - free agar nutrient medium. Bacteria were suspended in physiological saline after 24–48 hours culture on slant agar and then layered on agar plate and incubated for 24 hours - 48 hours at 30°C. On the basis of inhibition zone resistant and sensitive marks were used.

Routine parasitological and pathoanatomical observations were also made. Fluid smears from the lesions, blood and coelomic fluid, and squash preparations of kidney, spleen, liver and gills were examined immediately. For histopathological studies, organs were fixed in Bouin's fluid, embedded in paraffin and processed for routine histology and haemosiderin test.

RESULTS

All the specimens of silver carp were highly anaemic and emaciated. The most obvious clinical symptom of the disease was the occurrence of haemorrhagic lesions and blotches all over the body especially concentrated towards caudal region. Head and opercular portions had bony appearance due to thinned overlying integument. Examination of gills revealed decolouration and haemorrhagic nature of few lamellae. Average haematocrit value in diseased specimens was 27.63 as compared to 34.5 recorded in healthy specimens. Slightly opaque exudate was found to have accumulated in the abdominal cavity. Intestine was empty in all the seven specimens. Liver was brick red in colour and showed brownish patches. No parasite or parasitic cysts were observed on the body and in the squash preparations of internal organs.

Pure culture of Aeromanas hydrophila sub-species hydrophila was isolated from kidney, heart, liver and coelomic fluid. In some cases they were mixed with Bacillus. Isolated strains of Aeromonas hydrophila could be regarded as identical ones and formed yellow colonies on Rimler-Shotts agar (Shotts and Rimler, 1973). The results of biochemical tests are presented in Table I and anthibiogram in Table II.

Gram-negative rods were seen in the smear preparations from body lesions. Focal necrosis and severe oedéma were seen in the liver with disruption of hepatocyte lines (Fig. 1). Significantly high level of haemosiderin pigment was detected in liver and ovary sections. Acute tubular necrosis in focal areas was found in the kidney with pyknotic nuclei and cellular debris in the lumen (Fig. 2).

DISCUSSION

Aeromonas hydrophila has been identified as a well-known facultative pathogen of freshwater fish, and even reptiles with widespread occurrence all over the world (bullock and McLaughlin, 1970, Meyer, 1970; Shotts et al., 1972. Hazen et al., 1978; Farkas and Olah, 1982). Gopalakrishnan (1961 a, 1961 b), Fijan (1983) and Kumar, Mishra and Dey (1983) have also described the presence of Aeromonads in Indian freshwater culture systems infecting the Indian major carps. In the present communication a case of bacterial septicaemia in silver carp caused by Aeromonas hydrophila is discussed. The strain has been further classified as Aeromonas hydrophila ssp. hydrophila according to Heuschmann-Brunner (1978) though the butanediol test was negative. Bejerano et al. (1979) isolated and characterized the bacterial pathogen as Proteus rettgeri, a Gramnegative bacterium normally found in the gut of poultry birds. However, poultry faeces were used extensively to fertilize the pond. Csaba et. al. (1981) have detected Pseudomonas fluorescens as aetiogogical agen for the bacterial septicaemia of silver carp.

Histopathological changes were typical of those associated with an acute septicaemia in all the three cases exhibiting focal necrosis and severe oedema in the liver with marked chordal disarry and degeneration and necrosis of renal tubules in the kidney. The toxin produced by Aeromonas hydrophila damages kidney and liver (Huizinga et al., 1979). In addition, excessive haemosiderin pigment accumulation has been found in the liver parenchyma and ovary.

External clinical lesions over the body were similar to those reported by Bejerano et al. (1979) and Csaba et al. (1981). However, there were considerable amount of variations. While larger nature of integumental losions and deep ulcers were typical of Proteus rettgeri infection, petechae and bleedings in the mouth cavity were associated with Pseudomonas infection. In the present case of septicaemia caused by A. hydrophila spp. hydrophila, the lesions were smaller and superficial, and concentrated more towards caudal region and fin bases. Such lesions were always associated with hydrops, anaemia and emaciation.

Drug sensitivity tests have shown that the bacterial strain in the present observation is more sensitive to tetracycline. Erythromycin and Chloramphenical. Pal and Tripathi (1978) had also been successful in controlling the disease in silver carp exhibiting similar clinical features by tetracycline. However, the causative organisms of the other two septicaemic diseases of silver carp discussed earlier were resistant to chloramphenicol and tetracycline. The findings of the present investigation will be helpful to quicken field diagnosis of the disease and immediate planning of proper prophylactic and treatment measures to combat this commonly occurring disease of silver carp in India.

ACKNOWLEDGEMENTS

The authors express their deep sense of gratitude to Dr. A.V. Natarajan, Director Central Inland Fisheries Research Institute, and Dr. V.R.P. Sinha. National Project Director, FAO/UNDP Projects and Head, Freshwater Aquaculture Research and Training Centre for their keen interest in this work and encouragement.

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Buchanan, R.E. and Gibbson, N.E., 1974. Bergey's mannual of determinative bacteriology. 8th ed. Williams and Willkins, Baltimore.

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Csaba, Gy., Felgli, M., Bekesi, L., Kovacs-Gayer, E., Bajmocy. and Fazekas, B., 1981. Septicaemia in silver carp (Hypophthalmichthys molitrix val.) and bighead (Aristichyys nobilis Rich) caused by Pseudomonas fluorescens. Proc. Int. Seminar on Fish Pathogens and Environment in European Polyculture, Szarvas, Hungary. 111–133.

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Table I. Characteristics of isolated bacteria.

Table II. Antibiotic sensitivity of the bacterial isolate.