Renibacterium salmoninarum

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Renibacterium salmoninarum
Kingdom Bacteria
Phylum Actinobacteria
Class Actinobacteria
Sub-class Actinobacteridae
Order Actinomycetales
Sub-order Micrococcineae
Family Micrococcaceae
Genus Renibacterium
Species R. salmoninarum

Also Known As: Bacterial Kidney Disease –- BKD –- Corynebacterial Kidney Disease -– Dee Disease –- Salmonid Kidney Disease -– White Boil Disease

Introduction

Renibacterium salmoninarum is a gram positive, rod shaped, non spore forming intracellular bacterium that causes Bacterial Kidney Disease (BKD) in salmonid fish.

BKD is usually a chronic disease and causes mortality in juvenile fish and pre-spawning adults. Due to its prevalence, it has a significant impact upon both farmed fish and conservation efforts in wild endangered species.[1] Its production of chronic granulomata within the tissues is similar to that caused by Mycobacteria spp. in mammals.

This disease is not zoonotic.

Distribution

North America, UK, Continental Europe, Japan, Chile and Scandinavia.

Disease can be spread horizontally via shared water[2] and vertically[3]. This transmission via the egg makes it unusual as a bacterial pathogen.

Most outbreaks have occurred in fish culture facilities. As the bacteria are often enzootic in wild populations and waterborne, transmission to captive fish is often a concern.

Signalment

Salmonids of the Onchorhynchus, Salmo and Salvelinus genera appear to be the primary hosts and many other species can acquire natural infections. The organism is highly adapted to both infectivity and persistence in these hosts. Its intracellular nature provides it with both a nutrient source and method of evasion from the host immune response.

Pacific salmon species are the most susceptible with losses as high as 80% recorded,[4] while Atlantic salmon and rainbow trout are thought to be more resistant.[5]

Clinical Signs

Some fish show no, few or very subtle external signs of BKD.

Affected fish may be lethargic, pale around the gills and may have exopthalmos. Haemorrhagic lesions may be visible around the vent or on the skin, fins and musculature. Cystic cavities may be visible within the musculature.

Fish with BKD are often found to be anaemic with a 49-66% decrease in circulating erythrocytes, which are of a smaller size and have increased sedimentation rates. Bilirubin, Blood Urea Nitrogen and Potassium also increase. [6]

Diagnosis

Diagnosis most commonly relies on direct detection of bacteria or antigens rather than antibodies as the latter are both difficult to measure and an unreliable indicator of infection status.

This can be achieved using culture, ELISA, and PCR. The organism can be cultured on cysteine blood enriched medium. Isolation can take 2-19 weeks and is often contaminated as the organism is very slow growing. Kidney tissue should not be used for culture as it is inhibitory to in-vitro growth of R. salmoninarum.[7] The bacterium can be located in tissue sections using periodic acid-Schiff stain, but FAT is more sensitive and therefore can allow visualisation of bacterial cells in subclinical infections.[8] Several methods can then be used to attempt to quantify the degree of infection.

The most widely used antigen detecting test used in large surveys are dot-blot and ELISA. ELISA allows precise quantitative detection while dot-blot is qualitative.[9] ELISA is thought to be as sensitive as culture of IFAT, if not more so. It may however be affected by cross-reactivity with other antigens and this is poorly understood at present. Its sensitivity and specificity also rely on the quality of the antisera used. It is therefore recommended that diagnosis is supported by another diagnostic test.

PCR is often used as a confirmatory test and is both rapid and sensitive, but equipment and sampling criteria make it impractical for use on a large scale.

Antigens can also be detected by immunodiffusion or counterimmunophoresis, from kidney, liver, spleen and blood. The former two tissues are most useful. Counterimmunphoresis is rapid, taking only 1.5h and is also capable of detecting low levels of R. salmoninarum such as in subclinical infections.

Necropsy findings usually include white-grey necrotic patches and granulomatous inflammation of most commonly the kidneys, also the spleen and liver, and sometimes other viscera. The kidneys may also be swollen and oedematous. Bloody fluid may have accumulated in the body cavities. Musculature is also often necrotic. Granulomas vary slightly in their nature according to the species of fish affected.

Viable R. salmoninarum cam be detected in fresh and salt water by Fluorescent Antibody Testing (FAT) and culture.[10]

Treatment

Erythromycin and Oxytetracycline are the antibiotics of choice, both can be injected and oxytetracycline is also available in an oral formulation. Erythromycin resistance is increasing however. Both can be used in fry and broodstock.

An intraperitoneal vaccine is also available and can be used as a treatment in the face of an outbreak of BKD.

Control

Avoidance is key – this can be difficult due to exposure of captive fish from waterborne bacteria shed by feral fish upstream of hatcheries.[11]

Segregation or culling of broodstock is now often used to ensure a source of egg lots and thus juvenile fish [12] with no or very low levels of R. salmoninarum. It is poorly understood what other impacts segregation may have at this point. Contaminated premises may need to be emptied and restocked with BKD-free fish.

Regular fallowing may help to break the disease cycle.[13]

An intra-peritoneal vaccine is also available but can only be used in fish >10g in weight.

References

  1. ↑ Flagg, T. A., Mahnken, C. V. M., Johnson, K. A (1995) Captive broodstocks for recovery of Snake River sockeye salmon. In: Schramm ,H. L., Piper, R. G., eds. Uses and effects of cultured fishes in aquatic ecosystems. Bethesda, MD: American Fisheries Society, Symposium 15, 81-90
  2. ↑ Bell, G. R., Higgs, D. A., Traxler, G. S (1984) The effect of dietary ascorbate, zinc, and manganese on the development of experimentally induced bacterial kidney disease in sockeye salmon (Oncorhynchus nerka). Aquaculture, 36(4):293-311
  3. ↑ Pascho, R. J., Elliott, D. G., Streufert, J. M (1991) Brood stock segregation of spring chinook salmon Oncorhynchus tshawytscha by use of the enzyme-linked immunosorbent assay (ELISA) and the fluorescent antibody technique (FAT) affects the prevalence and levels of Renibacterium salmoni. Diseases of Aquatic Organisms, 12(1):25-40
  4. ↑ Evenden, A. J., Grayson, T. H., Gilpin, M. L., Munn, C. B (1993) Renibacterium salmoninarum and bacterial kidney disease - the unfinished jigsaw. Annual Review of Fish Diseases, 3:87-104
  5. ↑ Evelyn, T. P. T (1993) Bacterial kidney disease - BKD. In: Inglis, V., Roberts, R. J., Bromage, N.R. eds. Bacterial Diseases of Fish. New York, USA: Halsted Press, 177-195
  6. ↑ Turaga, P., Wiens, G., Kaattari, S (1987) Bacterial kidney disease: the potential role of soluble protein antigen(s). Journal of Fish Biology, 31(Suppl. A):191-194
  7. ↑ Daly, J. G., Stevenson, R. M (1988) Inhibitory effects of salmonid tissue on the growth of Renibacterium salmoninarum. Diseases of Aquatic Organisms, 4:169-171
  8. ↑ Laidler, L. A (1980) Detection and identification of the bacterial kidney disease (BKD) organism by the indirect fluorescent antibody technique. Journal of Fish Diseases, 3(1):67-69
  9. ↑ Olea, I., Bruno, D. W., Hastings, T. S (1993) Detection of Renibacterium salmoninarum in naturally infected Atlantic salmon, Salmo salar L., and rainbow trout, Oncorhynchus mykiss (Walbaum) using an enzyme-linked immunosorbent assay. Aquaculture, 116(2/3):99-110
  10. ↑ McKibben, C. L., Pascho, R. J (1999) Shedding of Renibacterium salmoninarum by infected chinook salmon Oncorhynchus tschawytscha [tshawytscha]. Diseases of Aquatic Organisms, 38(1):75-79
  11. ↑ Håstein, T., Lindstad, T (1991) Diseases in wild and cultured salmon: possible interaction. Aquaculture, 98(1/3):277-288
  12. ↑ Meyers, T. R., Korn, D., Glass, K., Burton, T., Short, S., Lipson, K., Starkey, N (2003) Retrospective analysis of antigen prevalences of Renibacterium salmoninarum (Rs) detected by enzyme-linked immunosorbent assay in Alaskan Pacific salmon and trout from 1988 to 2000 and management of Rs in hatchery Chinook and coho salmon. Journal of Aquatic Animal Health, 15(2):101-110
  13. ↑ Bruno, D. W (2004) Prevalence and diagnosis of bacterial kidney disease (BKD) in Scotland between 1990 and 2002. Diseases of Aquatic Organisms, 59:125-130


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