Strawberry Disease in Rainbow trout – Gross Pathology

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

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European countries (Scotland, England, Switzerland, Germany, France, among others), Chile and Peru (Sandoval 2017, unpublished report).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

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Spawning Rash – Gross Pathology

Skin
by Hugh Ferguson
Rainbow trout, multi-focal granulomatous dermatitis

The bacterium Renibacterium salmoninarum is a small (~ 1.0 μm), intracellular, non-motile diplobacillus, gram +, that is slow growing and a fastidious pathogen. The external gross findings on the skin could be a good example of chronic-active dermal o subdermal lesion associated with R. salmoninarum. “Spawning rash” can be observed in adults, particularly at or around spawning time. Lesions may be largely dermal, often centering round scale-pockets to produce multifocal chronic-active dermatitis. Alternatively the response, mainly in the superficial muscle, may be so severe as...

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Caligus Rogercresseyi – Gross Pathology

Skin
by Carlos Sandoval
Atlantic salmon with adult females (C. rogercresseyi). Note the lesions on the skin surface. Note also the paired trailing egg sacs.

Caligus rogercresseyi is a predominant parasitic problem in the salmonid industry in Chile. It mainly affects Atlantic salmon and Rainbow trout. Coho salmon is not considered susceptible due to a well-developed inflammatory response. The life cycle of C. rogercresseyi is composed of eight stages of development, two nauplius stages I and II, an infested copepodite stage, four chalimus stages and an adult stage (young adult and mature adult). The amount of time it takes for a louse to develop depends greatly on the temperature. Under...

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Syncytial Hepatitis of Tilapia – Transmission Electron Microscopy

Gastrointestinal Tract, Liver
by Hugh Ferguson
Transmission electron micrograph from liver of fingerling tilapia with SHT. Virus particles, typical of orthomyxovirus, can be seen between and within cells (arrows).

Tilapia are considered to be relatively resistant to many of the common diseases that beset other farmed fish. Viral diseases, however, are not common, and there are only a few reports in the literature. Syncytial Hepatitis of Tilapia (SHT) is a newly described viral disease reported from several countries where tilapia farming is present. This virus is a member of the Orthomyxoviridae family. It has been demonstrated within hepatocytes of affected fish. The described ultrastructural changes provide further evidence to support the previous suggestion of...

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Heart and Skeletal Muscle Inflammation (HSMI) – Histopathology

Cardiovascular System, Musculoskeletal System
by Carlos Sandoval
Heart and Skeletal Muscle Inflammation (HSMI)

Heart and skeletal muscle inflammation (HSMI) is an emerging disease of marine-farmed Atlantic salmon (Salmo salar L), first recognized in 1999 in Norway, and later also reported in Scotland, Chile and Canada. HSMI is transmissible and, the weight of recent evidence supports the contention that the cause is an Orthoreovirus with the proposed name Piscine orthoreovirus (PRV). This virus has not yet been cultured in vitro, thereby hampering traditional experimental reproduction of the disease. As the name of the disease suggests, major changes in HSMI...

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