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.

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.

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.

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.

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.

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.

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.

PGD Incidence in Norway:

• Proliferative gill disease (PGD) is an emerging problem in Norwegian culture of Atlantic salmon (Salmo salar) 18.
• A new poxvirus, salmon gill poxvirus (SGPV), was discovered on the gills of salmon suffering from PGD in fresh water in northern Norway in 2006 18.

Geographical Spread:

• SGPV was found on gills of salmon at two marine sites in western Norway later in 2006 8.
• PGD is particularly common in farmed Atlantic salmon in Norway 12.

Economic Impact:

• All farms affected by SGPV suffered high losses associated with the presence of this virus 8.
• PGD can lead to elevated mortality rates, reduced growth rates, and reduced food conversion in salmon 2.

Treatment & Management:

• Management focuses on identifying and monitoring potential pathogens associated with gill disease 6.