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Salmon Rickettsial Disease
Aetiology
Classification of the Causative Agent
Piscirickettsia salmonis is a Gram-negative obligate intracellular pathogen of salmonids and perhaps other species of fish. It is fastidious and does not grow on artificial bacteriological media, but can be grown on fish cell culture lines. It is sensitive to antibiotics in vitro. This organism is pleiomorphic, occurring in coccoid or ring forms, frequently in pairs, with a diameter of 0.5-1.5 mm.
Resistance to Physical and Chemical Action
| Temperature: |
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Optimal temperature range for culture: 15o-18oC. |
| pH: |
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Unknown. |
| Chemicals: |
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500 mg/litre chlorine for 20 minutes. |
| Disinfectants: |
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100 ppm Ido-Sept 11 for 15 minutes. |
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| Survival: |
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Titre diminished 99% after a single cycle of freeze-thaw at -70oC. |
Epidemiology
- Causes 10-90% mortality in seawater net-pen-raised coho salmon (Oncorynchus kisutch) in Chile.
- Mortality considerably less in other geographical areas.
Hosts
- Piscirickettsia salmonis has been detected in coho salmon (Oncorynchus kisutch), chinook salmon (O. tshawytscha), sakura salmon (O. masou), rainbow trout (O. mykiss), pink salmon (O. gorbuscha) and Atlantic salmon (Salmo salar) and may occur in other species of fish.
- Coho salmon appear to be the most susceptible to this bacterium.
Transmission
- The mechanisms of transmission are not completely understood.
Horizontal transmission in salt and fresh water has been demonstrated.
- The role of carriers and vectors has not been established.
Sources of the Agent
- Carrier and infected fish
- Water-borne infection
- Other equipment contaminated with P. salmonis
Occurrence
The disease has been described in Chile, Ireland, Norway, and both the west and east coasts of Canada. Piscirickettsiosis has been primarily reported in marine fish farms, and has also been observed in freshwater facilities.
For detailed information on occurrence, see recent issues of World Animal Health and the OIE Web site.
Diagnosis
Clinical Diagnosis
- Increased mortality
- Anorexia
- Pale gills and lowered haematocrits
- Swollen abdomens
- Affected fish appear dark and lethargic, swimming at the sides of the net-pens
Lesions
- The first evidence of disease may be the appearance of small white lesions or shallow haemorrhagic ulcers on the skin.
- The major gross pathological changes are peritonitis, ascites, enlarged spleen, swollen grey kidney, and liver with large pale necrotic lesions.
Differential Diagnosis
- Furunculosis
- Virbiosis
- Septaemia caused by other Gram-negative bacteria that infect fish
Laboratory Diagnosis
Procedures
Isolation of the agent
- Inoculation of susceptible cell lines such as BF?2 or RTG-2 followed by microscopic examination
Identification of the agent
- Detection in Giemsa-stained tissue smears or tissue sections, followed by the indirect fluorescent antibody test (IFAT).
Isolation and dentification of the agent
- Isolation in CHSE-214 cell culture (ATCC CRL 1681)
- Must use antibiotic-free media
- Incubate at 15oC-18oC for 28 days
- Cytopathic effect (CPE) consists of plaque-like clusters or rounded cells
- Follow by confirmation using serological test
Serological tests
- IFAT
- Immunohistochemistry of tissue sections
Samples
Identification and isolation of the agent
- Kidney, liver and blood suitable for virological testing are collected from diseased fish during either overt or covert infections.
- Smears or impressions of the kidney, liver, and spleen should be prepared, air-dried, and fixed for 5 minutes in absolute methanol.
- Tissues smears to be examined by IFAT must be freshly prepared or stored at -20oC or below.
Disease Prevention and Treatment
- No specific successful treatment is currently available.
- Implementation of hygienic measures and management policy are the only methods of control currently available.
- Antibiotics have been used, however, their value is questionable.
- Oxalinic acid presently appears to be the drug of choice.
- Intensive efforts are underway by various groups to develop an effective vaccine.
Sanitary Prophylaxis
- Sanitize equipment and facilities.
- Restock with healthy stocks of fish.
- Use 6 ml/litre Ido-Sept 11 for wipe down of counter surfaces and nets.
Treatment
References
Chapter 2.2.8. in the OIE Diagnostic Manual for Aquatic Animal Diseases, OIE (World Organisation for Animal Health), Paris, France.
Chapter 2.2.8. in the OIE International Aquatic Animal Health Code, OIE (World Organisation for Animal Health), Paris, France.
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