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 –...


Figure 1. Multiple giant cells in the peritoneal cavity of salmon, a response to vaccine, its bright red character apparent (arrows). Similar to so-called Splendore-Hoeppli reactions (asteroid bodies).

Multinucleated cells, often simply called giant cells, are found in a variety of situations in teleost fish. They are not uncommon in granulomatous inflammatory responses, such as bacterial kidney disease, proliferative kidney disease, or as a response to vaccine, and are a result of fusion of macrophages or epithelioid cells. They are also found in virus infections, so-called syncytial giant cells, in which a number of non-macrophage cells, such as hepatocytes, fuse together. But giant cells are also found in normal fish as osteoclasts. Although...

Haemorrhagic Smolt Syndrome (HSS) – Gross Pathology

Atlantic salmon, with visceral fat haemorrhage and pale liver.

HSS is a condition of unknown aetiology, which affects Salmo salar leading to dramatic pathological changes. The disease occurs in pre smolts, smolts and post smolts. Affected fish typically have good body condition, without obvious weight loss; they show lethargy, superficial swimming and dark colouration. Bilateral exophthalmia and increased respiratory rate can sometimes be observed The external gross findings include pale gills, with haemorrhages in the branchial arches and lamellae, plus petechiae and ecchymoses ventrally and at the base of the fins. Internally, generalized visceral...