Proliferative Gill Disease

• Physical Signs:

  • Gill Proliferation: Thickened, swollen, and pale gills with excessive mucus production 24.
  • Gill Lesions: White or greyish nodules and lesions on the gills 4.
  • Hemorrhages: Reddened areas in the gill tissues due to bleeding 4.
  • Respiratory Distress: Rapid or labored breathing due to impaired gill function 2.
• Behavioral Changes:

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

Progression of Symptoms

• Early Stages: Mild signs such as reduced feed intake and slight lethargy 2.

• Advanced Stages: Pronounced physical symptoms including gill proliferation, lesions, and respiratory distress, leading to high mortality if not managed 24.

• Impact on Fish Health: PGD severely compromises respiratory function, leading to stress, reduced growth rates, and higher susceptibility to secondary infections 25.

Etiology

• Causative Agents: Multiple pathogens can be associated with PGD in salmonids, including:

  • Parasites: Ichthyobodo spp., Neoparamoeba sp. 12
  • Bacteria: Flexibacter/Flavobacterium, 'Candidatus Branchiomonas cysticola' 12
  • Viruses: Salmon gill poxvirus (SGPV) 13
• Transmission Methods:

  • Waterborne: Pathogens spread through water, particularly in conditions of high fish density and poor water quality 2.
  • Direct Contact: Spread between infected and healthy fish through close proximity 2.

Risk Factors

• Environmental Factors: Poor water quality, high stocking densities, and stress conditions can increase susceptibility to PGD 25.

• Farm Management Practices: Ineffective biosecurity measures and lack of routine health monitoring can facilitate the spread of pathogens 25.

Diagnostic Methods

• Clinical Examination: Observation of physical and behavioral symptoms indicative of PGD 24.

• Laboratory Tests:

  • Histopathology: Microscopic examination of gill tissues to identify characteristic lesions and pathogens 14.
  • PCR (Polymerase Chain Reaction): Detects specific pathogens associated with PGD, such as SGPV 13.
  • Gross Pathological Scoring: A novel total gill scoring system has been developed to assess gross pathological changes 4.

Differential Diagnosis

• Distinguishing PGD from Other Gill Diseases: It is crucial to differentiate PGD from other gill diseases, such as amoebic gill disease (AGD) and complex gill disease (CGD) 245.

Treatment Options

• Current Treatments: There are no specific treatments for PGD. Management focuses on preventing the spread and mitigating the impact through biosecurity measures and supportive care 25.
• Experimental Treatments: Research is ongoing into potential antiparasitic agents and other therapeutic approaches 2.

Preventive Measures

• Biosecurity Protocols:

  • Implementing strict quarantine measures for new stock to prevent the introduction of pathogens 25.
  • Regular disinfection of equipment and facilities to minimize the risk of contamination 25.
• Farm Management Practices:

  • Reducing stocking densities to decrease the risk of pathogen transmission 25.
  • Regular health monitoring and early detection programs to identify and manage outbreaks promptly 245.
  • Implementing all-in/all-out stocking practices to reduce the risk of spreading infections 25.

Real-World Examples

• Notable Outbreaks:

  • Norway (2006): Discovery of salmon gill poxvirus (SGPV) in Atlantic salmon suffering from PGD in freshwater and marine sites 1.
  • Norway and Ireland: Identification of 'Candidatus Branchiomonas cysticola' as a common agent of epitheliocysts in seawater-farmed Atlantic salmon 2.
  • Global: Increasing reports of complex gill disease (CGD) in Atlantic salmon aquaculture 25.
• Response Strategies and Outcomes:

  • Norway: Development of genotyping tools for SGPV to better understand its epidemiology and spread 9.
  • Chile: Implementation of improved diagnostic methods, including gross pathological scoring systems 4.
  • Industry-wide: Increased focus on multifactorial approaches to understanding and managing gill diseases in salmon aquaculture 25.

• Lessons Learned: The importance of considering multiple pathogens and environmental factors in the etiology and management of PGD in salmonids 25.

Proliferative Gill Disease (PGD) remains a significant challenge for aquaculture, necessitating diligent management and preventive measures. By implementing robust biosecurity protocols, investing in ongoing research, and fostering industry-wide cooperation, fish health managers and veterinarians can mitigate the impact of PGD and ensure the sustainability of aquaculture operations. This comprehensive guide aims to provide the necessary information and resources to effectively manage PGD and support the health and welfare of farmed fish.