Hugh Ferguson has recently had a second edition published of his book, entitled "Systemic Pathology of Fish".
At a time when we are being constantly encouraged to eat more fish for the health of our own hearts, it is indeed ironic that the animals themselves are suffering from increasing levels of severe cardiovascular disease, particularly in farmed salmonids. Aside from involvement in some of the alphaviral diseases, examples include cardiomyopathy syndrome of salmon, a disease that targets the largest and fastest-growing fish, but for which no cause has yet been determined. Animals frequently die due to a ruptured atrium, a consequence of severe necrosis of the spongy myocardium. If the fish survives, the heart tissue will completely regenerate.
Bacterial diseases continue to dominate the daily diagnostic caseload. Examples include emerging chronic granulomatous diseases of warmwater fish, caused by Francisella sp. and Edwardiella ictaluri in tilapia in Central America and in Pangasius catfish in Vietnam, respectively. A range of syndromes associated with the yellow pigmented Flavobacterium psychrophilum can be found in trout in the UK; these include “coldwater disease” with tailrot, “necrotic myositis”, “dissolving head disease” with generalized necrosis of cartilages in the head and spinal column, and most recently “strawberry disease”, which causes severe lichenoid-type dermatitis with lysis of scales thereby rendering the skin fragile and easily damaged by even relatively minor management procedures such as grading. Consequent financial losses due to downgrading of the fish at slaughter is a major concern. Candidatus Arthromitus is a long filamentous spore-forming bacterium that is associated with an enteric disease of European trout, “summer enteritis syndrome”; mortality can be high, but we know very little about the pathogenesis of this disease, nor the relationship, if any, of these very distinctive organisms to the ensuing enteritis.
Nodaviruses cause a vacuolating encephalitis and retinitis in a range of marine species worldwide. Farmed cod are not immune, although until recently it was thought that disease was restricted to fry. We have recently seen the disease in on-growing cod, targeting the cerebellum and leading to loss of Purkinje cells; affected fish were easily spotted clinically due to their loss of balance and incoordinated swimming. Once again, if the fish survives, central nervous system tissue will regenerate.
Streptococcus iniae causes disease in farmed warmwater fish, but can also cause encephalitis in humans. This bacterium was originally described from Dolphin iniae the freshwater dolphin of the Amazon basin. It was shown to be responsible for a major mortality of reef fish in the Caribbean basin in 1999, almost completely wiping out the entire fish population of reefs round several of the Windward Islands. The source of the infection was never established, although an especially heavy outfall from the Orinoco River was suspected. At the time of the “Millenium” celebrations, there were almost no affected fish left on the beaches, and a potential source of human infection was thereby greatly reduced.