Amoebic gill disease (AGD) is a potentially fatal disease of some marine fish. It is caused by '' Neoparamoeba perurans'', the most important amoeba in cultured fish. It primarily affects farm raised fish of the family

Salmonidae Salmonidae is a family of ray-finned fish that constitutes the only currently extant family in the order Salmoniformes . It includes salmon (both Atlantic and Pacific species), trout (both ocean-going and landlocked), chars, freshwater whitefis ...

, most notably affecting the

Tasmanian ) , nickname = , image_map = Tasmania in Australia.svg , map_caption = Location of Tasmania in AustraliaCoordinates: , subdivision_type = Country , subdi ...

Atlantic salmon (''Salmo salar'') industry, costing the A$20 million a year in treatments and lost productivity.

Turbot The turbot (''Scophthalmus maximus'') is a relatively large species of flatfish in the family Scophthalmidae. It is a demersal fish native to marine or brackish waters of the Northeast Atlantic, Baltic Sea and the Mediterranean Sea. It is an ...

, bass,

bream Bream ( ) are species of freshwater and marine fish belonging to a variety of genera including '' Abramis'' (e.g., ''A. brama'', the common bream), '' Acanthopagrus'', ''Argyrops'', '' Blicca'', '' Brama'', '' Chilotilapia'', ''Etelis'', '' L ...

, sea urchins 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.EAFP Search
/ref> It was first diagnosed in the summer of 1984/1985 in populations of Atlantic salmon off the east coast of Tasmania and was found to be caused by ''N. 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 that many aquatic organisms use to extract dissolved oxygen from water and to excrete carbon dioxide. The gills of some species, such as hermit crabs, have adapted to allow respiration on land provided they are ...

s of infected fish and hyper-plasic lesions, causing white spots and eventual deterioration of the gill tissue. Fish will show signs of

dyspnoea Shortness of breath (SOB), also medically known as dyspnea (in AmE) or dyspnoea (in BrE), is an uncomfortable feeling of not being able to breathe well enough. The American Thoracic Society defines it as "a subjective experience of breathing dis ...

such as rapid opercular movements and lethargy. Although usually recognised by hyperplasic and proliferative gill lesions, the effects of AGD occur before oxygen transfer across the gill is severely compromised. AGD affected fish show a significant increase in vascular resistance contributing to cardiovascular collapse. Such effects result in compensatory changes in heart shape to improve its efficiency at pumping blood. 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 commonly performed by a surgeon, interventional radiologist, or an interventional cardiologist. The process involves extraction of sample cells or tissues for examination to determine the presence or extent of a dise ...

can be observed under the microscope for amoebas, or tested using fluorescent antibody testing.

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, sold under the brand name Ergamisol among others, is a medication used to treat parasitic worm infections, specifically ascariasis and hookworm infections. It is taken by mouth. Side effects may include abdominal pain, vomiting, ...

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 refer to derivatives of ammonia and organic amines wherein one or more N-H bonds have been replaced by N-Cl bonds. Two classes of compounds are considered: inorganic chloramines and organic chloramines. Inorganic chloramines Inorgan ...

and

chlorine dioxide Chlorine dioxide is a chemical compound with the formula ClO2 that exists as yellowish-green gas above 11 °C, a reddish-brown liquid between 11 °C and −59 °C, and as bright orange crystals below −59 °C. It is usually ...

have also been used. Other potential in-feed treatments such as immunosupportive-based feeds, mucolytic compounds such as L-cysteine ethyl ester and the parasiticide bithionol have been tested with some success although not developed for commercial use.

Notes

References

Amoebic Gill Disease
''Wikivet''. Retrieved 6 October 2011. {{salmon Fish diseases