Figure 1. Strawberry disease in rainbow trout in freshwater showing loss of scales, and accompanying haemorrhage into the scale pockets. Note the severity of the dermal inflammation, which is mostly mononuclear, and largely avoids the epidermis.

Strawberry Disease in Rainbow Trout– Histopathology

Strawberry Disease (SD) is a chronic, nonlethal skin condition that affects Rainbow Trout (Oncorhynchus mykiss) in the United States, several European countries (Scotland, England, Switzerland, Germany, France, among others), Chile and Peru (Sandoval 2017, unpublished report). In Europe it is also known as Red Mark Syndrome (RMS) or Cold-Water Strawberry Disease (CWSD).

Figure 1. Strawberry disease in rainbow trout in freshwater showing loss of scales, and accompanying haemorrhage into the scale pockets. Note the severity of the dermal inflammation, which is mostly mononuclear, and largely avoids the epidermis.
Figure 1. Strawberry disease in rainbow trout in freshwater showing loss of scales, and accompanying haemorrhage into the scale pockets. Note the severity of the dermal inflammation, which is mostly mononuclear, and largely avoids the epidermis.

SD is characterized by raised, sometimes bright red inflammatory lesions that can occur in almost any size of fish bigger than fingerlings, but usually in growing or market-sized fish. The disease presents as a severe, full-thickness, largely non-ulcerating, dermatitis centred on the dermis, with an inflammatory response dominated by small mononuclear cells. The severity of the response is such as to suggest a hypersensitivity reaction.

Figure 2. Compare and contrast this example of severe dermatitis with Figure 1. Although there is similarly severe inflammation, and loss of scales, the epidermis is involved to the same extent as the dermis, and there are many more neutrophils. Within the epidermis can be seen an aggregate of neutrophils, forming a “micro-abscess” (arrow).
Figure 2. Compare and contrast this example of severe dermatitis with Figure 1. Although there is similarly severe inflammation, and loss of scales, the epidermis is involved to the same extent as the dermis, and there are many more neutrophils. Within the epidermis can be seen an aggregate of neutrophils, forming a “micro-abscess” (arrow).











In both moderate and severe cases, scale resorption is observed concomitant with osteoclastic activity and oedema-induced distention of the scale pocket. In mild lesions, lymphocyte infiltration is observed in the stratum spongiosum and around the scale pocket; in moderate cases, marked oedema can be observed, often accompanied by mild neutrophil infiltration. In advanced cases, scales are usually absent, having been completely resorbed by osteoclasts, but the inflammatory response can extend down into the sub-cutis, or even the muscle layer, sometimes accompanied by myonecrosis.

 Figure 3. Early stage strawberry disease, prior to activation of osteoclasts and loss (resorption) of scales. Neutrophils dominate the inflammatory response in and around the scales.
Figure 3. Early stage strawberry disease, prior to activation of osteoclasts and loss (resorption) of scales. Neutrophils dominate the inflammatory response in and around the scales.


The epidermis is largely unaffected in mild or moderate cases, but erosion and ulceration of the epidermis are observed in severe cases, along with mild lymphocytic intra-epithelial infiltration. If the skin is indeed ulcerated, interpretation of the lesions can be hampered by secondary infection. In some severe cases, mild mucous and epidermal hyperplasia are also seen. In moderate to severe cases, collagen fibers of the dermis are infiltrated by lymphocytes and some plasmacytes.










REFERENCES

  • Lloyd, S. J., LaPatra, S. E., Snekvik, K. R., St-Hilaire, S., Cain, K. D., & Call, D. R. (2008). Strawberry disease lesions in rainbow trout from southern Idaho are associated with DNA from a Rickettsia-like organism. Diseases of aquatic organisms82(2), 111-118.
  • Sandoval, C., Infante, J., Abad, J., Ferguson, H. W., Paredes, E., Valdebenito, S., … & Avendaño-Herrera, R. (2016). Case report: Strawberry disease in farmed Chilean rainbow trout. Journal of aquatic animal health28(1), 1-10.
  • Verner-Jeffreys, D. W., Pond, M. J., Peeler, E. J., Rimmer, G. S. E., Oidtmann, B., Way, K., … & Feist, S. W. (2008). Emergence of cold water strawberry disease of rainbow trout Oncorynchus mykiss in England and Wales: outbreak investigations and transmission studies. Diseases of aquatic organisms79(3), 207-218.
  • Ferguson, H. W., Girons, A., Rizgalla, G., LaPatra, S., Branson, E. J., Mackenzie, K., … & Crumlish, M. (2006). Strawberry disease in rainbow trout in Scotland: pathology and association with Flavobacterium psychrophilum.

By: Hugh Ferguson

Dr Ferguson earned his veterinary degree from the Royal (Dick) School of Veterinary Studies, Edinburgh, Scotland and held a Wellcome Research Fellowship at the Institute of Aquaculture, Stirling University where he obtained his PhD. He subsequently worked for 4 years as a diagnostic pathologist at the Veterinary Research Laboratories, Belfast, Northern Ireland, prior to moving to Canada. He is board-certified in the American college of veterinary pathology (ACVP), and is a Fellow of the Royal College of Pathologists (FRCPath, London). Dr Ferguson is currently professor of veterinary pathology, and Senior Research Fellow in Windward Islands Research and Education Foundation (WINDREF), St George’s University (SGU), Grenada, West Indies.

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