Bacterial gill disease – Gross Pathology

This rainbow trout fingerling with BGD shows the typical flared opercula of respiratory distress. Note that the edge of the flared operculum is eroded. This erosion is typical of intensively-reared fish; its cause is unknown, but might be the result of higher-than-optimal bacterial enzymes in and around the branchial cavity. Whatever the cause, the consequence is a huge reduction in the ability of the animal to pump water due to elimination of the water-tight seal between the trailing edge of the operculum and the body of the fish (the cleithrum). Thus the negative pressure of the opercular pump is greatly reduced. And this in an animal that is trying to pump larger-than-normal volumes of water over its gills due to the hypoxaemia of BGD!

Bacterial gill disease (BGD) occurs in several species, but it is an especially serious problem of intensive salmonid culture, and in some parts of the world it is the most common disease, especially in fry and fingerlings. BGD is characterized by explosive morbidity and mortality rates, attributable to bacterial colonization of gill surfaces. Despite such pronounced clinical signs and high mortality, pathological changes are surprisingly few and hard to find in the acute disease. It is only when (if) the fish survive for a few...

Jellyfish Lesions in Fish – Histopathology

Figure 5. Phialella quadrata attached to gill raker of Atlantic salmon and showing necrosis of epithelium, loss of basement membrane, and underlying dermal haemorrhage. A close inspection of the interface between the jellyfish and the gill epithelium shows tube-like extensions reaching down through the epithelium – nematocysts?

The negative interactions between jellyfish and fish in aquaculture appear to be an increasing problem. This is partly due to increased numbers of jellyfish, associated with global warming, reduced numbers of their predators, and to the intensification of aquaculture operations in many coastal areas worldwide. Most reported problems have occurred in marine-farmed salmonids in northwest Europe. Nevertheless, aquaculture operations in other regions such as Asia, North America, and Australia have also been affected. Jellyfish involved are primarily cnidarians i.e. those species with stinging cells –...

Gastric dilation & Air sacculitis or “Bloat” in salmonids – Gross Pathology

Chinook salmon with “Bloat”. Note the severe abdominal distention.

Bloat is a non-infectious condition of salmonids where the abdomen is abnormally distended by an enlarged, fluid-filled stomach. The wall of the enlarged stomach wall is thin and flaccid. Occasionally the swimbladder is also affected.   The condition is seen in salmonids reared in sea water and fed fishmeal-based pelleted rations. While it has been reported occasionally in Atlantic salmon (Salmo salar L.) members of the genus Oncorhynchus are more susceptible. These include Rainbow trout (O. mykiss), Chinook salmon (O. tshawytscha), and Coho salmon (O....

Proliferative Kidney Disease in Salmonids (PKD) – Gross Pathology

Rainbow trout with PKD. There is a marked hyperplasia of interstitium with the kidney thrown into bulbous ridges. The reddening is tge result of secondary yersinia infection.

Proliferative kidney disease (PKD) is an endoparasitic disease of salmonid fish caused by Tetracapsuloides bryosalmonae (Myxozoa: Malacosporea).  This chronic, largely renal interstitial disease is caused by the extraporogonic but intracellular stages of the parasite, which cause a severe granulomatous host response. The severity of the disease is linked to water temperature, with roughly 15 degrees °C as the cut-off: below that temperature, lesions and clinical disease are minimal. Above that temperature, however, lesions can be severe and mortality high. Inevitably, global warming has resulted in...

Cardiomyopathy syndrome (CMS) – Gross Pathology

Figure 2. Salmon with CMS. Necrosis in the atrium of this fish has been so severe that it has ruptured. Note that the ventricle appears normal, due only to the fact that the compact layer remains largely unaffected.

Cardiomyopathy syndrome (CMS), is a severe heart disease of Atlantic salmon.  This transmissible condition has been diagnosed in several countries, especially Norway, where it was first reported, but also Scotland, Ireland and the Faroes. It is causally linked to the Piscine Myocarditis Virus (PMCV), closely allied to the Totiviridae. Typically, CMS occurs in the biggest, fastest-growing fish that never go off their feed – indeed their stomach is usually full of pellets when they suddenly die! Brood stock are also susceptible. This disease has an...

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