Salmonid Rickettsial Septicaemia (SRS)

Physical Signs:

• Darkened Coloration: Affected fish exhibit an overall darkening of the skin 1 16.
• Anemia and Pale Gills: Severe anemia leading to pale gills is a hallmark sign 6.
• Hemorrhages and Skin Lesions: Petechial hemorrhages, vesicles with scale elevations, and skin ulcers with underlying muscle hemorrhages16.
• Swollen Organs: Enlarged kidneys and spleen upon internal examination 17
• Liver Lesions: Ring-shaped, cream-colored, and occasionally hemorrhagic liver lesions in chronic cases. Affected livers are often pale, mottled, and in some cases may appear yellowish 1731.
• Scale Loss and Fin Fraying: External degradation of scales and fins 16.
• Fluid in abdomen (ascites): Accumulation of fluid in the abdominal cavity may cause visible abdominal swelling in severely affected fish 31.
• Internal Hemorrhages: Hemorrhages may occur in internal organs, particularly in the liver, intestine, pyloric caeca, peritoneal fat, and swim bladder, reflecting the systemic septicemic nature of the disease 31.

Behavioral Changes:

• Lethargy: Reduced activity and slow swimming 1.
• Loss of Appetite: Decreased feeding behavior and significant weight loss 1.
• Respiratory Distress: Increased gill movement and labored breathing1.
• Surface Swimming: Fish congregating near the water surface 1.

Causative Agent: Piscirickettsia salmonis, a facultative intracellular gram-negative bacterium, typically 0.5-1.5 μm in diameter. Originally described as obligate intracellular, it is now recognized as a facultative intracellular pathogen capable of growth in cell-free media. It was the first "rickettsia-like" bacterium to be recognized as a pathogen of fish 2 18. Genotypic variation within Piscirickettsia salmonis has been reported across different geographic regions, including Chile, Scotland, and Ireland, and mixed infections involving multiple strains may occur. Evidence suggests that different variants differ in virulence, disease severity, and associated mortality 31.

Transmission Methods: The natural reservoir of Piscirickettsia salmonis remains unclear 31.

• Horizontal Transmission: Direct water-borne transmission; the bacterium can survive several weeks at 5-20°C in saltwater. Experimentally, entry occurs through intact skin and gills 1. Transmission is enhanced under high stocking densities in experimental studies 31.
• Vertical Transmission: Possible transmission through eggs via an adhesion complex that allows pathogen entry 1.
• Vector Transmission: In Chile, parasitic organisms such as the isopod Ceratothoa gaudichaudii and sea lice of the genus Caligus have been associated with enhanced transmission and worsened disease outcomes 2 31.

Distinguishing SRS from Other Diseases: It is crucial to differentiate SRS from other conditions with similar clinical signs, including Infectious Salmon Anemia (ISA) which causes similar hemorrhaging, Bacterial Kidney Disease (BKD) which also involves kidney pathology, furunculosis, and Tenacibaculosis which generates similar skin lesions 6.

Chilean Regulatory Requirement (Sernapesca Resolution 1606/2021): In Chile, PCR-based discrimination between SRS and Tenacibaculosis is mandatory. The updated Tenacibaculosis definition requires Tenacibaculum spp. identification by PCR, with external signs (fin erosion, oral/rostral lesions with yellow pigmentation, gill ulcers with yellowish coloration), and explicitly excludes cases showing evident lesions of Piscirickettsiosis or Renibacteriosis. This regulatory requirement addresses the high co-occurrence of these pathogens, ensuring proper epidemiological classification 23.

Differentiating Clinical Signs:

• Piscirickettsiosis: Primarily a systemic disease characterized by ring-shaped liver lesions (cream/yellow), internal granulomatous lesions in viscera, kidney and spleen enlargement. Skin ulcers are secondary to systemic infection 16 17.
• Tenacibaculosis: Primarily an external bacterial disease characterized by external ulcerative lesions with yellow pigmentation, mouth erosion, fin necrosis, tail rot. Body and caudal fin lesions (T. maritimum) or head/snout lesions (T. dicentrarchi). Unlike SRS, tenacibaculosis does not typically produce characteristic systemic or internal lesions 24.

Core Pathobiome: Research has identified a complex polymicrobial community referred to as a core pathobiome, in Atlantic salmon skin ulcers during P. salmonis infection. The primary pathobiome consists of P. salmonis, Tenacibaculum dicentrarchi, and Aliivibrio wodanis, with Vibrio spp. enriched on infected gills. Four bacterial families are overrepresented in ulcerated tissue: Vibrionaceae, Flavobacteriaceae, Piscirickettsiaceae, and Pseudomonadaceae 25.

Co-infection Implications: The presence of this bacterial consortium may enhance P. salmonis pathogenicity through cooperative interactions. When diagnosing skin ulcers, clinicians should consider that multiple pathogens may be present simultaneously, requiring molecular methods (multiplex PCR) to identify primary vs. opportunistic agents 25.

Current Treatments:

• Antibiotics: Florfenicol is the primary antibiotic used, accounting for 87-97% of antimicrobial use for SRS in Chile. Oxytetracycline is also employed. In 2024, 351.1 tons of antibiotics were used in Chilean salmon aquaculture (9.7% increase from 2023), with over 96% directed at SRS control. The first half of 2024 showed a 22.6% increase compared to the same period in 2023, highlighting piscirickettsiosis as the major driver of antibiotic use in the Chilean salmon industry 7 8.
• PROA-Salmon Certification: This voluntary program, managed by Chile's National Fisheries Service (SERNAPESCA), recognizes farms that implement responsible antibiotic management. As of 2024, PROA-Salmon certified harvests represent 28.7% of production volume, with 34.5% of certified production being antibiotic-free in Chile. The 2019 CSARP (Chilean Salmon Antibiotic Reduction Program) aimed for a 50% reduction by 2025. While it fell short of this goal, it spurred significant voluntary improvements. 5 32.
• Treatment Protocol: Florfenicol at 20 mg/kg body weight for 15 days is the optimized dosing regimen 9.
• In Vitro Sensitivity and resistance: P. salmonis shows sensitivity to streptomycin, gentamicin, erythromycin, chloramphenicol, and oxytetracycline, but shows resistance to penicillin, penicillin G and spectinomycin in laboratory conditions 1.

Treatment Challenges: Because the bacterium partially survives and replicates inside the salmon's cells, antibiotic treatment often yields poor results 31. .

• Medicated feed to control intracellular pathogens has proven largely unsuccessful, possibly due to difficulty achieving sufficient intracellular antibiotic concentrations 1 10.
• Treatment failure is common; farms with lower mortality when therapy is initiated have better outcomes 10.

Vaccines:

• Approximately 34 commercial vaccines are licensed in Chile, with 94% injectable, 2% oral, and 1% immersion formulations 2.
• The best-performing vaccine reduced mortality by only 22% compared to reference vaccination regimens 2.
• Vaccine protection peaks at 600-800 degree-days post-vaccination and declines to pre-vaccination levels by 1800-1900 degree-days 2.

Why Vaccines Fail:

• P. salmonis employs sophisticated immune evasion mechanisms including intracellular survival within host cell vacuoles, cytokine manipulation (inhibiting IL-12 while inducing IL-10), and Type IV secretion system that inhibits phagosome-lysosome fusion 2.
• Current vaccines do not activate the cellular-mediated immune responses necessary to control intracellular pathogens 2.
• The protection is usually short-lived, where fish are often left unprotected during the final "fattening" stage at sea (3000+ degree-days) when they are most vulnerable to outbreaks 2.

Biosecurity Protocols:

• Early removal of mortalities and diseased fish to reduce infection pressure. Chile's General Sanitary Program for Mortality Management (PSGM) standardizes procedures for classification, handling, and safe disposal of fish mortalities 27 1.
• Broodstock screening and rejection of positive eggs 2 1.
• Individual egg batch incubation to prevent cross-contamination 2 1.
• Surveillance and genotyping of bacterial isolates are important for tracking circulating variants and supporting assessment of antimicrobial susceptibility for potential antibiotic use and control strategies 31.

Farm Management Practices:

• Reduced stocking densities to decrease stress and transmission 2 1.
• Site fallowing periods to break the infection cycle 2 1.
• Control of sea lice coinfection which overrides vaccine protection. While Caligus infestation levels increased significantly over the years, SRS mortality risk remained constant, suggesting complex interactions 27 2.
• Broodstock injection with antibiotics before seawater departure 2 1.
• Knowledge gaps regarding reservoirs and transmission mechanisms have hindered the development of effective long-term control strategies 31.

Notable Outbreaks:

• Chile (1989): The first identified outbreak occurred in coho salmon, killing approximately 1.5 million fish and causing $10 million USD in economic losses. This was the first recognition of P. salmonis as a fish pathogen 1 3.
• Chile (1990): The disease spread to Atlantic salmon with mortality rates reaching up to 90% on some farms 1.
• Chile (2007-2009): SRS contributed to a major aquaculture crisis alongside ISA outbreaks, devastating the Chilean salmon industry 4.
• Chile (2024): Environmental events including low oxygen levels and harmful algal blooms in Los Lagos and Aysén regions caused mortality peaks in January, with stable overall levels compared to previous years 5.

Lessons Learned: The importance of integrated disease management combining biosecurity, stress reduction, sea lice control, and early intervention. The limited efficacy of current vaccines highlights the need for continued research into cellular immune responses and novel vaccine approaches 2 4.

SRS Incidence in Chile:

• SRS is the most significant disease in Chilean salmon aquaculture and consistently accounts for the largest proportion of infectious disease-related mortality.
• In 2020, SRS was responsible for 47.8% and 67.3% of mortality related to infectious diseases and 11.7% and 10.7% of total mortality in Atlantic salmon and rainbow trout production, respectively 4.
• Over recent years, SRS has remained the leading infectious cause of mortality, although its relative contribution has fluctuated. It accounted for 52.8% of infectious mortality in 2022, 44.7% in 2023, and 44% in the first half of 2024 13 4 14.
• In the first half of 2024, 15.3 million salmonids died at marine sites. A harmful algal bloom (HAB) event in early January 2024 caused 2,854 tons of mortality 5.
• In the first half of 2025, SRS represented 60.1% of infectious mortality in Atlantic salmon 29 13.
• Total marine site mortality remains substantial, with 13.6 million salmonids lost in the first half of 2025 and an average monthly seawater mortality of 0.91% (decreased from 1.15% in 1S 2024) 29.

Geographical Spread:

• The disease is prevalent throughout Chilean salmon farming regions. In 1S 2025: Los Lagos region reported 1.8% of production sites classified as Centros de Alta Diseminación- High-Dissemination Centers (CAD), while Aysén had 2% CAD (weekly average, increased from 0.7% in 1S 2024). A total of 22 CAD centers were registered in 1S 2025 29.
• In 1S 2024: Los Lagos had 12% of sites classified as alert centers and 1.8% CAD; Aysén had 10.9% alert centers and 0.7% CAD (weekly average). PSEVC-Piscirickettsiosis showed 73% of CAD centers in T3 stage (final production cycle). No confirmed SRS cases were reported in Magallanes region 4 13.

Economic Impact:

• Annual economic losses exceed $700 million USD, equivalent to approximately 10.5% of Chile's salmon exports (2022) 4.
• In 1989, losses were $10 million USD; by 1995, $49 million USD; and in some years, losses exceeded $100 million USD 1.
• Chile harvested 1,035,307 tons of salmon in 2024, making disease control critical to the industry 5.

Treatment & Management:

• 231.19 tons of antibiotics were used in Chilean salmon aquaculture in the first half of 2025, a 6.34% increase compared to 1S 2024 (217.38 tons). 94.82% of antimicrobial use at closed cycle level was directed at Piscirickettsiosis treatment 30.
• The PROA-Salmón certification program has grown to cover 123,435 tons (27% of national production) in 1S 2025, promoting antimicrobial stewardship 30.
• 351.1 tons of antibiotics were used in Chilean salmon aquaculture in 2024, with over 96% directed at SRS treatment 5 11 15.
• Despite a downward trend in antibiotic use, there was a 3.6% increase in seawater use during the first half of 2024 due to environmental stressors 5 15.
• The First Congress on Sustainable Management of Aquaculture Bacterial Diseases was held in November 2024 in Puerto Varas, Chile, addressing SRS challenges 4.

SRS Incidence in Norway:

• SRS has been reported in Norway since 1988 but with considerably lower incidence than Chile and economic impact compared to Chile 131.
• The disease was detected at multiple locations in Norway in the late 1980s and again in 2002 31 33.
• Over the past ~20 years, it has only been detected sporadically. However, several detections were reported in Nordland and Troms in 2024, although with limited reported mortality 31 33.
• Genomic sequencing of Norwegian isolates indicates that they are genetically distinct and cluster separately from known Chilean isolates 33.
• Sporadic outbreaks occur but do not reach the severity seen in Chilean aquaculture 1 31.

Treatment & Management:

• Norwegian farms employ similar biosecurity measures and treatment protocols as used globally 1.

• SRS has been reported in Canadian salmon farms, particularly in British Columbia 1.
• The disease impact is significantly lower than in Chile 1.

• Occasional outbreaks have been reported in both Scotland and Ireland 1.
• Incidence and losses are much lower than in Chile 1.