Amoebic Gill Disease (AGD) in Farmed Salmon: A Disease Guide
Amoebic gill disease, caused by Neoparamoeba perurans, damages the gills of farmed salmon. Unlike most listed diseases, it is treatable with freshwater or hydrogen-peroxide baths.
Overview
Amoebic gill disease (AGD) is a parasitic gill disease of sea-farmed salmonids. It is caused by the marine amoeba Neoparamoeba perurans (also written Paramoeba perurans), which colonizes the gill surface. The parasite triggers proliferation of the gill epithelium and excess mucus, impairing gas exchange. AGD was first recognized in Atlantic salmon off Tasmania in the mid-1980s and is a growing problem as sea temperatures rise. Importantly, unlike many notifiable aquaculture diseases, AGD is treatable.
Affected species and risk factors
The main host is sea-farmed Atlantic salmon (Salmo salar), and rainbow trout (Oncorhynchus mykiss), turbot, and other marine fish can also be affected. Clinical AGD is favored by warmer seawater, higher salinity, crowding, and poor water circulation in sea pens, so cases are most common in summer.
Clinical signs
- Raised white to grey mucoid patches on the gills
- Gill epithelial hyperplasia and fusion of the gill lamellae
- Excess gill mucus and deterioration of gill tissue
- Respiratory distress, rapid opercular (gill cover) movements, and flared opercula
- Lethargy and congregation near the surface
- Mortality, largely from impaired gas exchange
Diagnosis
On farms, AGD is assessed by gross gill scoring, in which the white mucoid patches are scored to grade severity. Diagnosis is supported by wet-mount examination of gill mucus, histopathology showing the characteristic gill lesions, and qPCR detection of Neoparamoeba perurans.
Treatment and control
AGD is treated by bathing affected fish to reduce the amoeba burden on the gills, and treatment is repeated as needed based on gill scoring.
- Freshwater bath, typically for about 2 to 3 hours at low salinity, which osmotically shocks the amoebae and thins gill mucus
- Hydrogen peroxide bath, used in cooler regions, typically around 1,000-1,400 mg/L for roughly 18-30 minutes
- Both freshwater and hydrogen peroxide baths are highly amoebicidal but work by different mechanisms
- Monitor gills by regular gill scoring and re-treat when scores rise
- Avoid hydrogen peroxide at higher water temperatures, where it becomes unsafe for the fish