Amoebic Gill Disease (AGD)

Physical Signs:

• Gill Lesions: White mucoid patches and hyperplasia on the gill filaments 1 5.
• Gill Inflammation: Swollen, reddened, and thickened gill tissues 1.
• Increased Mucus Production: Excessive mucus covering the gills 1 5.
• Respiratory Distress: Rapid or labored breathing due to impaired gill function 1 2.

Behavioral Changes:

• Lethargy: Reduced activity and slow swimming 1 2.
• Loss of Appetite: Decreased feeding behavior and subsequent weight loss 1.
• Abnormal Swimming Patterns: Fish may swim near the surface or close to aeration points to compensate for reduced oxygen uptake 1 2.

Early Stages: Mild gill lesions and slight increase in mucus production 1.

Advanced Stages: Severe gill inflammation, extensive mucus production, respiratory distress, and increased mortality if not treated 1 2.

Impact on Fish Health: AGD severely compromises gill function, leading to stress, reduced growth rates, and higher susceptibility to secondary infections 1 2 4.

Causative Agent: The amoeba Neoparamoeba perurans 1 2 3 4.

Transmission Methods:

• Direct Contact: Spread between infected and healthy fish through close proximity 2.
• Waterborne: Amoeba can spread through water, especially under conditions of high organic matter and poor water quality 2.

Environmental Factors: High water temperatures (above 16°C), high salinity levels, and organic matter can increase susceptibility to AGD 1 2 4.

Farm Management Practices: High stocking densities and poor biosecurity measures can facilitate the spread of the amoeba 2.

Clinical Examination: Observation of physical symptoms, such as gill lesions and increased mucus production 1 2 5.

Laboratory Tests:

• Histopathology: Microscopic examination of gill tissues to identify characteristic lesions of AGD and the presence of amoeba 2 4.
• PCR (Polymerase Chain Reaction): Detects Neoparamoeba perurans genetic material in gill tissues 3.
• Gill Smears: Examination of gill smears under a microscope to identify amoebic cells 2 4.

Current Treatments:

• Freshwater Baths: Immersion of fish in freshwater for 2 to 3 hours to reduce amoeba load on the gills 1 2 4.
• Hydrogen Peroxide Baths: Use of hydrogen peroxide to treat affected fish 13.
• Other Treatments: Chloramine, chlorine dioxide, and levamisole have also been used 4

Experimental Treatments: Research is ongoing into potential chemotherapeutants and novel treatment methods, including immunosupportive-based feeds, mucolytic compounds, and parasiticides 4.

Biosecurity Protocols:

• Implementing strict biosecurity measures to prevent the introduction and spread of Neoparamoeba perurans 2.
• Regular monitoring of water quality and gill health to detect and address early signs of AGD 1 2.

Farm Management Practices:

• Reducing stocking densities to decrease the risk of amoeba transmission 2.
• Using aeration and water flow management to maintain optimal oxygen levels and reduce organic matter 2.
• Implementing fallowing periods and site rotations to break the infection cycle 2.

Notable Outbreaks:

• Tasmania (1984/1985): The first identified outbreaks led to significant losses and increased awareness of AGD 4.
• Norway: Major outbreaks affected both wild and farmed salmon populations 4.
• Scotland: Severe outbreaks caused substantial economic impact, costing the industry up to A$230 million a year 1.

Lessons Learned: The importance of early detection, robust biosecurity, and coordinated response efforts in controlling AGD outbreaks 1 2 4.

AGD Incidence in Norway:

• Growing concerns of AGD with a few outbreaks occurring in south west Norway in 2012 9.
• By 2015-2016, AGD had become established but was not as severe as initially feared 10

Geographical Spread:

• In 2016, Paramoeba perurans was detected by real-time (RT)-PCR from the county Vest-Agder to Nord-Trøndelag 10.
• Outbreaks of AGD were not detected north of Nord-Trøndelag 10.

Treatment & Management:

• The Norwegian industry has become more experienced in handling AGD over time 10.
• Control measures include gill scoring and early treatment with freshwater or hydrogen peroxide 10 12.
• In 2016, the number of treatments increased at sites on the southwest coast of Norway during autumn months 10.
• Recent cases in 2023 have been managed with freshwater treatments, demonstrating the continued use of this method 11 12.

AGD Incidence in Scotland:

• Up to 2011: Rare/intermittent cases with negligible impact.
• Mid to late 2011: First reports of gill challenges attributed to AGD 6.
• 2024: Mowi Scotland reported severe biological issues, including a high level of AGD, linked to El Niño cyclical weather patterns 8.

Economic Impact:

• In Scotland, AGD was estimated to cost the industry up to £30 million annually in 2011 7.
• AGD management can account for up to 20% of production costs for some companies in affected regions 7.

Treatment Regimes:

• Scotland and Ireland have been using hydrogen peroxide treatments for AGD 7.