Amoebic Gill Disease (AGD)

Amoebic Gill Disease (AGD) is a parasitic disease affecting various fish species in aquaculture, including Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), and other marine fish. It is caused by the amoeba Neoparamoeba perurans, leading to gill inflammation and impaired respiratory function 1 4. AGD was first identified in Tasmania in the 1980s and has since been reported in several other countries, including Norway, Scotland, Ireland, and Chile 1 2 3.

• 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.

Progression of Symptoms

• 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.

Etiology

• 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.

Risk Factors

• 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.

Diagnostic Methods

• 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.

Differential Diagnosis

• Distinguishing AGD from Other Gill Diseases: It is crucial to differentiate AGD from other gill diseases, such as bacterial gill disease (BGD) and parasitic infections (e.g., Ichthyobodo necator). 3

Treatment Options

• 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.

Preventive Measures

• 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.

Real-World Examples

• 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.

Recent studies on Amoebic Gill Disease (AGD) have focused on various aspects of the disease, including its biology, epidemiology, diagnosis, and treatment. Here are some notable recent research findings:

1. "Amoebic gill disease (AGD) in Atlantic salmon (Salmo salar L.): a complex host-parasite interaction"
   Authors: Marcos-López, M., & Rodger, H. D.
   Reference: Marcos-López, M., & Rodger, H. D. (2020). Amoebic gill disease (AGD) in Atlantic salmon (Salmo salar L.): a complex host-parasite interaction. Diseases of Aquatic Organisms, 142, 101-113.
   Link to study
2. "Development and validation of a TaqMan® MGB real-time PCR for the detection of Neoparamoeba perurans, causative agent of amoebic gill disease"
   Authors: Downes, J. K., Henshilwood, K., Collins, E. M., Ryan, A., O'Connor, I., Rodger, H. D., MacCarthy, E., & Ruane, N. M.
   Reference: Downes, J. K., et al. (2015). Development and validation of a TaqMan® MGB real-time PCR for the detection of Neoparamoeba perurans, causative agent of amoebic gill disease. Journal of Fish Diseases, 38(12), 1085-1095.
   Link to study
3. "Transcriptome profiling of the Atlantic salmon (Salmo salar) gill during amoebic gill disease"
   Authors: Benedicenti, O., Secombes, C. J., & Collins, C.
   Reference: Benedicenti, O., Secombes, C. J., & Collins, C. (2019). Transcriptome profiling of the Atlantic salmon (Salmo salar) gill during amoebic gill disease. Frontiers in Immunology, 10, 2218.
   Link to study
4. "The effect of temperature on Neoparamoeba perurans growth and survival"
   Authors: Collins, C., Hall, M., Bruno, D., Sokolowska, J., Duncan, L., Yuecel, R., McCarthy, U., Fordyce, M. J., Pert, C. C., McIntosh, R., & MacKenzie, K.
   Reference: Collins, C., et al. (2017). The effect of temperature on Neoparamoeba perurans growth and survival. Journal of Fish Diseases, 40(9), 1213-1223.
   Link to study
5. "Hydrogen peroxide treatment in Atlantic salmon induces stress and detoxification response in the absence of any impact on growth or amoebic gill disease challenge"
   Authors: Vera, L. M., & Migaud, H.
   Reference: Vera, L. M., & Migaud, H. (2016). Hydrogen peroxide treatment in Atlantic salmon induces stress and detoxification response in the absence of any impact on growth or amoebic gill disease challenge. Fish & Shellfish Immunology, 55, 73-83.
   Link to study

These studies represent advancements in understanding the biology and epidemiology of Neoparamoeba perurans, development of diagnostic tools for early and accurate detection, and investigations into potential treatments and preventive measures for AGD.

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Last Modified: 2024-08-15

Tags: #AmoebicGillDisease, #Salmonoids, #Salmon, #AtlanticSalmon, #RainbowTrout , #Trout , #Salmon, #Norway, #Scotland, #parasiticdisease, #parasite