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Enteric septicaemia
Introduction/Aetiology
Classification of the Causative Agent
Eubacteria, family Enterobacteriaceae, genus Edwardsiella, species E. ictaluri. It is a Gram-negative, motile, facultatively aerobic, nonsporulating bacterium that is stereotypically homogeneous.
Resistance to Physical and Chemical Action
| Temperature: |
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Optimum temperature for culture: 25o-30oC. Preserved by refrigeration, freezing, freeze-drying. Sensitive to 60oC for 1 hour. |
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| Chemicals: |
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Sensitive to oxidising agents, detergents and lipid solvents. |
| Disinfectants: |
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Killed by alcohols (70% ethanol or isopropanol), quaternary ammonia, aldehyde, phenolic, strong acid and base (muriatic acid/hydrochloric acid, lye-sodium hydroxide), and halide disinfectants (chlorine 540 mg/litre and iodine compounds 250 ppm) for 30 minutes. Caution: phenolic compounds are extremely toxic to fish and any trace should be removed from equipment before the equipment is used. |
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| Survival: |
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Survives 3-4 months in pond water, mud, and vegetation. |
Epidemiology
- Morbidity and mortality can be high in affected heavily stocked ponds.
Hosts
- Enteric septicaemia of catfish (ESC) is a highly infectious bacterial disease of catfish especially ictalurids. The causative agent has also been associated with disease in Clarias, and ornamental fish.
Transmission
- Faecal-oral, cannibalism and direct contact
- Indirect transmission by water, and contaminated materials used in handling.
Sources of the Agent
- Intestinal tract is the primary site of infection.
- Faeces are the main source of shedding and dissemination, although dead fish and inanimate vectors (fishing nets, sorting devices) may be involved in contamination.
- Carriers: in addition to catfish recovered from the disease, the bacterium seems to be able to survive in the intestinal tract of different fish species.
Occurrence
Endemic in the southern part of the United States of America, where channel catfish are intensively farmed, but occasional cases have been described in other parts of the United States of America (USA) and in South-East Asia. Disease occurs primarily at water temperatures between 18C and 28C. Stress is often a predisposing factor.
For detailed information on occurrence, see recent issues of World Animal Health and the OIE Web site.
Diagnosis
Incubation period may be long, as environmental factors are likely to modulate the clinical expression of the disease.
Clinical Diagnosis
Acute septicaemic form
- Starts as an enteritic infection. The vent may appear congested, with mucoid and haemorrhagic intestinal exudate.
- In advanced cases, fish become lethargic.
Chronic meningo-encephalitis
- Progresses slowly from the olfactory sacs, through the nasal route, causing a granulomatous inflammation in brain tissues.
- Listlessness and erratic swimming are resultant behavioural signs. Affected fish may display rigor or uncoordinated muscle twitching when handled.
- Many cases of meningo-encephalitis occur after an acute septicaemic form has occurred in the population.
Lesions
Acute septicaemia form
- Petechia and haemorrhages around the mouth, on the throat, abdominal region and fin bases.
- Anaemia, exophthalmia, abdominal distension or dropsy in advanced cases.
- Gills often appear irritated with petechia.
- 1-2 mm raised lesions scattered on the body. These lesions progress in to shallow ulcers.
- Internally, haemorrhages and necrotic foci in the liver and other internal organs. Enteritis, systemic oedema, accumulation of yellow- or blood-tinged ascitic fluid in the body cavity and enlargement of the spleen.
- Concurrent infection with Flavobacter columnare is common.
Chronic meningo-encephalitis
- A raised lesion on the dorso-cranial and the frontal parts of the head. This may ulcerate, exposing the brain ('holes in the head').
- Many cases of meningo-encephalitis display no external lesions
Although the 1-2 mm lesions on the body in the systemic form and the cranial lesion of the meningo-encephalitis are strongly suggestive of ESC, they are not pathonomonic. Final diagnosis requires bacterial isolation and identification or direct serological identification
Differential Diagnosis
- Channel catfish virus disease
- Motile Aeromonas septicaemia (MAS)
- Edwardsiella tarda infection
- Reported to be similar to Yersina ruckeri or Hafnia alvei infection in catfish although these infections are believed to be rare.
Laboratory Diagnosis
Procedures
Identification of the agent
- Isolation and culture on nutrient-rich medium at 28C, completed by biochemical bacteriological differentiation techniques. A selective medium has been described.
- Agglutination.
- Immunofluorescence (direct/indirect fluorescent antibody test [FAT and IFAT]).
- Enzyme-linked immunosorbent assay (ELISA).
Serological tests
- Direct agglutination and passive haemagglutination
- ELISA
Samples
Identification of the agent
- For isolation: posterior kidney, spleen, and liver from diseased fish. In suspected meningo-encephalitis cases, brain tissue should be cultured because the fish may not be systemically infected.
- Tissues prints or smears.
Serological tests
Disease Prevention and Management
Sanitary Prophlaxis
- Removal and destruction of dead fish.
- For isolated facilities, removal of infected fish populations and maintenance of ESC-free environment.
- Disinfection of premises, materials and ponds.
- Control of water sources.
- Control of newly introduced stocks, in accordance with quarantine measures.
Treatment
References
Chapter 2.2.7. in the OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.
Chapter 2.2.7. in the OIE International Aquatic Animal Health Code, OIE (World Organisation for Animal Health), Paris, France.
PLUMB J.A. (1999). Health Maintenance and Principal Microbial Diseases of Cultured Fishes. Iowa State University Press, Ames, Iowa, USA, 187
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