Amoebic gill disease


Amoebic gill disease (AGD) is a potentially fatal disease of some marine fish.

It is caused by Neoparamoeba pemaguidensis and Neoparamoeba perurans, some of the most important amoebas in cultured fish.

It primarily affects farm raised fish of the Salmonidae
Salmonidae is a family of ray-finned fish, the only living family currently placed in the order Salmoniformes. It includes salmon, trout, chars, freshwater whitefishes and graylings...

 family, most notably affecting the Tasmanian Atlantic Salmon
Atlantic salmon
The Atlantic salmon is a species of fish in the family Salmonidae, which is found in the northern Atlantic Ocean and in rivers that flow into the north Atlantic and the north Pacific....

 (Salmo salar) industry, costing the AU$230 million a year in treatments and lost productivity.

The turbot is a species of flatfish in the family Scophthalmidae. It is native to marine or brackish waters of the North Atlantic, Baltic Sea and the Mediterranean Sea.-Etymology:...

, bass
Bass (fish)
Bass is a name shared by many different species of popular gamefish. The term encompasses both freshwater and marine species. All belong to the large order Perciformes, or perch-like fishes, and in fact the word bass comes from Middle English bars, meaning "perch."-Types of basses:*The temperate...

, bream
Common bream
The common bream, freshwater bream, bream, bronze bream or carp bream, Abramis brama, is a European species of freshwater fish in the family Cyprinidae....

, sea urchins
Sea urchin
Sea urchins or urchins are small, spiny, globular animals which, with their close kin, such as sand dollars, constitute the class Echinoidea of the echinoderm phylum. They inhabit all oceans. Their shell, or "test", is round and spiny, typically from across. Common colors include black and dull...

 and crabs have also been infected.

The disease has also been reported affecting the commercial salmon fisheries of the United States, Australia, New Zealand, France, Spain, Ireland and Chile.

It was first diagnosed in the summer of 1984/1985 in populations of Atlantic Salmon
Atlantic salmon
The Atlantic salmon is a species of fish in the family Salmonidae, which is found in the northern Atlantic Ocean and in rivers that flow into the north Atlantic and the north Pacific....

 off the east coast of Tasmania and was found to be caused by the Neoparamoeba perurans n.sp.

Clinical Signs and Diagnosis

Symptoms typically begin to appear two months after the fish are transferred from freshwater hatcheries to open net sea cages.

Symptoms include mucus build-up on the gill
A gill is a respiratory organ found in many aquatic organisms that extracts dissolved oxygen from water, afterward excreting carbon dioxide. The gills of some species such as hermit crabs have adapted to allow respiration on land provided they are kept moist...

s of infected fish and hyper-plastic lesions, causing white spots and eventual deterioration of the gill tissue. Fish will show signs of dyspnoea such as rapid opercular
Operculum (fish)
The operculum of a bony fish is the hard bony flap covering and protecting the gills. In most fish, the rear edge of the operculum roughly marks the division between the head and the body....

 movements and lethargy.

Contributing factors are an ambient water temperature above 16 degrees Celsius, crowding and poor water circulation inside the sea pens. Clinical cases are more common in the Summer.

The lesions on the gills are highly suggestive of infection. Gill biopsies
A biopsy is a medical test involving sampling of cells or tissues for examination. It is the medical removal of tissue from a living subject to determine the presence or extent of a disease. The tissue is generally examined under a microscope by a pathologist, and can also be analyzed chemically...

 can be observed under the mciroscope for amoebas, or tested using fluorescent antibody testing
Immunofluorescence is a technique used for light microscopy with a fluorescence microscope and is used primarily on biological samples. This technique uses the specificity of antibodies to their antigen to target fluorescent dyes to specific biomolecule targets within a cell, and therefore allows...


Treatment and Control

Currently, the most effective treatment is transferring the affected fish to a freshwater bath for a period of 2 to 3 hours. This is achieved by towing the sea cages into fresh water, or pumping the fish from the sea cage to a tarp filled with fresh water.

Mortality rates have been lowered by adding Levamisole
Levamisole is an anthelminthic and immunomodulator belonging to a class of synthetic imidazothiazole derivatives. It was discovered at Janssen Pharmaceutica in 1966...

 to the water until the saturation is above 10ppm. Due to the difficulty and expense of treatment, the productivity of salmon aquaculture is limited by access to a source of fresh water. Chloramine
Chloramines are derivatives of ammonia by substitution of one, two or three hydrogen atoms with chlorine atoms. Monochloramine is an inorganic compound with the formula NH2Cl. It is an unstable colourless liquid at its melting point of -66° temperature, but it is usually handled as a dilute...

 and chlorine dioxide
Chlorine dioxide
Chlorine dioxide is a chemical compound with the formula ClO2. This yellowish-green gas crystallizes as bright orange crystals at −59 °C. As one of several oxides of chlorine, it is a potent and useful oxidizing agent used in water treatment and in bleaching....

have also been used.
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