Salmon and Trout Diseases

By Stephen G. Newman.

It is only in relatively recent times that coldwater fish, in particular salmon and trout, have been intensively reared, and the science of aquaculture came into existence. Along with the birth of this new area of agriculture came profit-limiting diseases. As with all other animals, these diseases are caused by various viruses, bacteria, protozoa and other parasites. The more important of these diseases are the subject of this section.

Viruses are extremely small infective agents capable of growth and multiplication only in living cells. Several of the viruses produce disease in epidemic proportions in hatchery-reared fish. They significantly impact commercial production and Government mitigation efforts.

Infectious Pancreatic Necrosis (IPN) is an acute, highly contagious disease, affecting primarily juvenile salmonids. Fish less than two inches long usually are the most susceptible. The disease has been reported in most species of trout, Atlantic and coho salmon, and several species of char. It has been reported in most trout-producing areas of the United States as well as Canada, Europe, and Japan.

Typically, affected fish whirl, become dark in color, and have bulging eyes, swollen abdomens, and hemorrhaging. Losses may occur at all water temperatures normally encountered in rearing trout.

IPN is transmitted from fish to fish by contaminated feces, urine, and ovarian or seminal fluid. There is no way to treat the disease once it occurs. IPN can be controlled by improving husbandry techniques including disinfecting eggs, eliminating carrier females, destroying infected animals, and possibly selective breeding.

Sockeye Disease

Infectious Hematopoietic Necrosis (IHN) is also an acute, highly contagious disease affecting primarily juvenile (less than two inches long) salmonids. Known variously as Oregon Sockeye Disease, Sacramento River Chinook Disease, Columbia River Sockeye Disease and Leavenworth Sockeye Disease, IHN is a disease of Pacific salmon affecting chi-nook, and sockeye salmon in addition to rainbow trout.

IHN has been reported throughout the United States, Canada and Japan. It causes massive destruction of the blood-forming organs. Affected fish hemorrhage under the skin and at the base of the fins. They have protruding eyes, swollen abdomens, and darkening of skin. Unlike IPN, IHN outbreaks occur mostly at 45 to 55 F (8 to 13 C) with some evidence that the incidence may be reduced at temperatures above 58 F (15 C).

IHN is transmitted from fish to fish by contaminated feces, urine, and ovarian or seminal fluid. As with IPN,survivors become carriers.

IHN cannot be controlled once the disease occurs. Control is best by avoidance. Disinfecting eggs, separating brood fish from fry, disinfecting contaminated facilities, destroying infected animals all are possible means of control.

Several other viral diseases including Viral Hemorrhagic Septicemia (VHS), Viral Erythrocytic Necrosis (VEN), and Herpesvirus salmonis (HS) also may significantly impact coldwater fish. VHS is a severe problem in Europe but has never been found in the United States, Canada or the Far East. Both VEN and HS, though responsible for severe disease, are of minor impact compared to IHN and IPN.

Bacteria are microscopic organisms living in soil, water, organic matter, plants or animals responsible for a wide variety of essential functions. They also are responsible for a wide range of diseases in all living creatures. Fish are no exception. In most cases, stress plays a role in predisposing fish to these bacterial diseases. Five of the most severe bacterial pathogens affecting coldwater fish in the United States are briefly discussed.

Vibriosis

Vibriosis is a severe disease caused by strains of Vibrio anguillarum and Vibrio ordalii affecting net pen reared salmon. All species of salmon reared in salt water are subject to infection. Vibriosis is worldwide, reported in the Americas, Europe, and the Far East. Large bloody lesions appear in skin and musculature. Hemorrhaged gills, eyes and fins also may occur.

Losses can occur at a wide range of temperatures, though the greatest generally take place at temperatures over 50 F (10 C). The disease is spread through the water column by infected fish or sediments, or by ingestion of infected materials.

Vibriosis can be prevented by immunization with commercially available bacterins (a type of vaccine). Bacterins have substantially reduced impact of this once devastating disease. The disease, when it does occur, usually responds to antibiotic treatment, although antibiotic resistance and failure of ill fish to eat are problems.

Furunculosis is another acute bacterial disease of cultured salmonid populations. Most species of salmonids and many species of non-salmonids reared in fresh, brackish and salt water are susceptible.

Furunculosis has been reported in most areas where fish are reared. As with other bacterial infections, symptoms vary somewhat with the fish's age and the severity of the disease. Chronically infected fish often display characteristic furuncles dark, raised fluid-filled bumps which ulcerate. Other signs include darkening, and loss of appetite.

The disease occurs at a wide range of temperatures and is transmitted through the water by diseased and carrier fish. Preventing contamination by carrier fish, and conscientious husbandry techniques, will help prevent spread of the disease. Sick animals may respond to treatment with antibiotics.

Rainbow Trout

Enteric Redmouth Disease (ERM) is a severe disease of commercially reared rainbow trout. The disease occurs largely in rainbow trout and has been reported throughout the United States and recently in Europe. Symptoms are like those in other bacterial diseases with a notable lack of lesions and a characteristic hemorrhage around the mouth and along the lateral line.