Rainbow trout with SD, showing characteristic bright-red lesions on the skin, and scale loss.
Skin

Strawberry Disease in Rainbow trout – Gross Pathology

by Hugh Ferguson

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European countries (Scotland, England, Switzerland, Germany, France, among others), Chile and Peru (Sandoval 2017, unpublished report).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.

Acute haemorrhagic dermatitis in rainbow trout, typical of what is called “summer strawberry disease”. This mild condition usually responds well to increased levels of dietary vitamin C. It should not be confused with the more serious colder water condition.
Figure 1. Acute haemorrhagic dermatitis in rainbow trout, typical of what is called “summer strawberry disease”. This mild condition usually responds well to increased levels of dietary vitamin C. It should not be confused with the more serious colder water condition.


The severity of the skin lesions suggests a hypersensitivity response, but hypersensitivity to what?  Affected fish respond well to immune suppressors including corticosteroids, but also to a wide range of antibiotic treatments, suggesting that bacteria are the root cause.

Rainbow trout with SD, showing characteristic bright-red lesions on the skin, and scale loss.
Figure 2. Rainbow trout with SD, showing characteristic bright-red lesions on the skin, and scale loss.













Several potential aetiological agents have been proposed, including a Rickettsia-like organism (RLO) in SD in the USA and Chile, and Flavobacterium psychrophilum in the UK.

Rainbow trout with characteristic bright-red gross lesions on the skin, and scale loss.
Figure 3. Rainbow trout with characteristic bright-red gross lesions on the skin, and scale loss.


The disease is generally observed in fish that weigh more than 100 g and that are held at water temperatures below 15°C.

Rainbow trout with characteristic bright-red gross lesions on the skin, and scale loss.
Figure 4. Rainbow trout with characteristic bright-red gross lesions on the skin, and scale loss.















Grossly, the condition is characterized by scale loss (resorption), prior to the emergence of the characteristic external lesions, bright-red, usually raised, non-ulcerative lesions that range from 2 mm to several centimeters in diameter and that are normally found on the flank of the fish. There are no systemic signs of infection (i.e. no effect on appetite, growth or mortality).

Rainbow trout with characteristic bright-red gross lesions on the skin, and scale loss. (Picture courtesy of Luis Montoya VM)
Figure 5. Rainbow trout with characteristic bright-red gross lesions on the skin, and scale loss (Picture courtesy of Luis Montoya VM).


















REFERENCES

  • Ferguson, HW., Girons, A., Rizgalla, G., LaPatra, S., Branson, EJ., MacKenzie, K., Davies, M., Collins, RO., Diab, A., Crumlish, M. (2006) Strawberry disease in rainbow trout in Scotland: pathology and association with Flavobacterium psychrophilum. Veterinary Record 158, 630-632.
  • Metselaar, M. (2012). Aetiology of red mark syndrome in rainbow trout (Oncorhynchus mykiss).
  • 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 health, 28(1), 1-10.

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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.

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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 then worked for 4 years as a diagnostic pathologist at the Veterinary Research Laboratories, Belfast, Northern Ireland, prior to moving to Canada. He left Ontario Veterinary College after 19 years as a full professor of veterinary pathology, to return to Scotland to become head of diagnostic pathology in Stirling. During all this time he became board-certified in the American college of veterinary pathology (ACVP), and a Fellow of the Royal College of Pathologists (FRCPath, London). After Scotland he moved to become chair of veterinary pathology, and Senior Research Fellow in Windward Islands Research and Education Foundation (WINDREF), St George’s University (SGU), Grenada, West Indies. He has published more than 230 papers in refereed journals.

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