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Furunculosis (Aeromonas salmonicida)

Overview​

What is Furunculosis (Aeromonas salmonicida)?​

Furunculosis is a bacterial disease of salmonid fish caused by Aeromonas salmonicida, a Gram-negative, non-motile, facultatively anaerobic rod belonging to the family Aeromonadaceae. The disease is named after its most visually distinctive chronic presentation: raised, fluid-filled boil-like lesions (furuncles) in the musculature that ulcerate and rupture. Classical or "typical" furunculosis is caused by A. salmonicida subsp. salmonicida and primarily affects salmonid species including Atlantic salmon (Salmo salar), rainbow trout (Oncorhynchus mykiss), brown trout (Salmo trutta), brook trout (Salvelinus fontinalis), and Arctic char (Salvelinus alpinus) 1 17.

"Atypical" A. salmonicida strains (subspecies achromogenes, masoucida, smithia, and pectinolytica) affect a broader range of non-salmonid species, including marine species such as Atlantic cod (Gadus morhua), lumpfish (Cyclopterus lumpus), turbot (Scophthalmus maximus) and wrasse species, including ballan wrasse (Labrus bergylta). In cleaner fish, used for biological salmon-lice control in Norwegian aquaculture, atypical furunculosis is an important cause of disease, reduced welfare, and mortality in both hatchery and sea-cage production systems 1 17 18

Aeromonas salmonicida subsp. salmonicida was first isolated in 1894 from brown trout in Germany, making furunculosis one of the earliest described bacterial diseases of fish. The pathogen can cause disease in salmonids in both freshwater and seawater environments and is virtually ubiquitous in farmed salmonid populations globally, with both clinical disease and subclinical carrier states common. The organism can persist for weeks to months in water and sediment, and covert (latent) carriers — fish that harbor the pathogen without clinical signs — represent the most significant and difficult-to-control reservoir in aquaculture systems 1.

Furunculosis is historically one of the most economically important bacterial diseases in Atlantic salmon farming, with major impacts in Norway, Scotland, Chile, Canada, and the Faroe Islands. The disease drove massive antibiotic use in Norwegian salmon farming during the 1980s and early 1990s before the introduction of effective oil-adjuvanted injectable vaccines transformed disease management. The Norwegian vaccine success story — in which antibiotic use dropped by over 99% following vaccine deployment — is one of the most frequently cited examples of how vaccination can revolutionize antibiotic stewardship in aquaculture. Today, classical furunculosis is rare and well controlled in Norway. However, it can still pose a challenge where vaccine coverage is incomplete, in stressed fish during the post-smolt transition, and in wild salmonid populations where vaccination is not feasible. In addition it remains uncertain whether vaccine protection will be equally effective in a warmer marine environment as experimental and field evidence suggests that the pathogen may become more virulent at higher temperatures- meaning that an increase in disease outbreaks under climate-change scenarios cannot be ruled out 1 2 18.

Clinical Signs of Furunculosis (Aeromonas salmonicida)​

Common Signs​

  • Physical Signs:

    • Furuncles (Muscular Boils): Characteristic raised, haemorrhagic swellings in the musculature that rupture to form deep, necrotic ulcers — most prominent in the subacute or chronic form of the disease, and in older/bigger fish; may be absent in peracute or acute presentations 1 5.
    • Skin haemorrhages: Petechial and ecchymotic haemorrhages may be observed on the ventral surface, at the bases of the pectoral fins, along the opercular margins, and around the vent. These findings are consistent with systemic bacteraemia/septicaemia 1 5.
    • Exophthalmia: Unilateral or bilateral protrusion of the eyes due to retrobulbar fluid accumulation is commonly observed 5.
    • Bloody Discharge from Vent: Haemorrhagic enteritis can produce bloody mucoid discharge from the anus, a sign associated with acute and subacute disease 1.
    • Splenomegaly and Hepatic Necrosis: Post-mortem examination typically reveals a markedly enlarged, congested spleen and a friable liver with focal areas of necrosis 5.
    • Renal Oedema and Congestion: The kidneys show oedema with petechial haemorrhages and congestion; renal softening and discoloration are common post-mortem findings in conjunction with hepatic and splenic pathology 5.
    • Internal haemorrhages: Petechial to ecchymotic haemorrhages may be present in internal organs, particularly the liver, spleen, kidney and intestine; consistent with systemic septicaemia 17.
    • Ascites: Serous or blood-tinged fluid may accumulate in the abdominal cavity. This finding can accompany severe systemic disease and septicaemia 17

  • Behavioural Changes:

    • Lethargy: Reduced swimming activity; affected fish isolate from the shoal and swim near the surface or cage edges 1.
    • Loss of Appetite: Rapid and marked decline in feed intake is often the first sign noticed by farm staff 1.
    • Increased respiratory rate: Affected fish may show respiratory distress; rapid opercular movements and increased ventilation rate, particularly during peracute and acute disease 1.
    • Darkening of Skin: Affected fish frequently display abnormal melanization (darkening) of body coloration 1.

Progression of Clinical Signs

  • Early Stages (Peracute): Fish may die suddenly with minimal external signs — peracute disease presents as rapid systemic bacteraemia with internal haemorrhages and enlarged spleen but no furuncle formation. Mortality rises steeply over a 1–3 day period. The peracute form is usually seen at the parr-stage in hatcheries for salmon 1 5 19.
  • Acute/subacute stage: A septicaemic form with a more prolonged course and lower mortality than peracute disease. In acute disease, fish show lethargy, loss of appetite, and fin-base haemorrhages. Furuncles may form beneath the skin or deep in the musculature that may progress to open ulcers. Fin rot, external ulcers, and focal necrosis in internal organs may occur, typically in larger fish; mortality usually develops within a few days 19.
  • Advanced Stages (Chronic): In chronic infections the clinical course is more protracted, with fish surviving longer but serving as ongoing sources of pathogen shedding. Chronic furunculosis is typically associated with bloody discolouration of the intestine and anal prolapse, and generally occurs in older fish with relatively low mortality. 119.
  • Latent infection (carrier state): Latently infected fish show no mortality or clinically apparent pathological changes, yet Aeromonas salmonicida can be isolated from internal organs. The prevalence of latent carriers may be high in some populations, creating an important reservoir for persistence and transmission of the pathogen. Stress events — such as seawater transfer (smoltification), handling, crowding, temperature change, or concurrent parasitic infestations — commonly trigger transition from latent carrier to clinically diseased states 19.
  • Impact on Fish Health: Beyond direct mortality, furunculosis causes significant production losses through reduced growth, increased feed conversion ratio, carcass downgrades due to furuncle damage, and costs associated with treatment and mortality removal. Fish that survive clinical episodes may remain chronic carriers that re-shed the pathogen under subsequent stress events 1.
  • Atypical and Related-Agent Presentations: Re-emergent atypical A. salmonicida outbreaks in Chilean Atlantic salmon have presented with periorbital haemorrhages, fin-base and abdominal haemorrhages, furuncles, skin ulcers, hepatic/splenic/renal petechiae, and haemorrhagic enteritis, a picture largely indistinguishable from typical disease on gross examination 25. Aeromonas piscicola, a distinct species causing furunculosis-like disease in Chilean S. salar, produces a comparable disease picture under experimental challenge: focal cutaneous ulcerative lesions, marked visceral congestion, and septicaemia with mortality exceeding 80% by 4 days post-infection for the more virulent isolates 24.

Causes of Furunculosis (Aeromonas salmonicida)​

Etiology

  • Causative Agent: Classical furunculosis is caused by Aeromonas salmonicida subsp. salmonicida, and is a Gram-negative, non-motile, non-spore-forming, facultatively anaerobic rod (1.0–1.5 × 0.8–1.0 μm). A hallmark of typical strains is the production of a brown, water-soluble diffusible pigment (a hydroxamate-type melanin-like compound) on tryptic soy agar (TSA), which serves as a useful phenotypic identifier. The organism grows optimally at 22–25°C and poorly above 32°C. Isolates possess an ordered crystalline surface protein layer (A-layer or S-layer), composed of a 49 kDa protein subunit encoded by the vapA gene, which is a critical virulence determinant and a key target for diagnostic and vaccine development 1 3. Typical and atypical A. salmonicida isolates possess both the LPS O-antigen biosynthesis cluster and the A-layer. In contrast, subsp. pectinolytica lacks these features and, unlike the other recognised subspecies, has not been associated with disease in fish 3
  • Atypical strains: Atypical furunculosis is caused by Aeromonas salmonicida subspecies other than subsp. salmonicida: subsp. achromogenes, masoucida, smithia, and pectinolytica. These strains have a broader host range than classical A. salmonicida and affect numerous farmed and wild fish species. They are highly heterogeneous in phenotype, genotype, pigment production, and expression of haemolysins and other extracellular virulence factors, which contributes to variable clinical presentations and makes serotyping unreliable 20.
  • Key Virulence Factors: The pathogenicity of A. salmonicida is multifactorial. Key virulence determinants include: the A-layer surface protein (VapA), which confers resistance to phagocytosis and serum killing; a Type III Secretion System (T3SS) that injects effector proteins directly into host cells to induce a transient immunosuppressive state, facilitating systemic spread 1; siderophores (amonabactin and acinetobactin) enabling iron acquisition in host tissues; a Type II Secretion System (T2SS) that exports extracellular enzymes including serine protease AsaP1, lipase, and glycerophospholipid-cholesterol acyltransferase (GCAT) — the latter responsible for erythrocyte lysis and tissue destruction 1. Cold shock proteins CspB and CspD also contribute to virulence by acting as transcriptional regulators of key virulence loci in the chromosome and the large pASal virulence plasmid 4.
  • Transmission Methods:
    • Horizontal Transmission: Fish-to-fish transmission is the primary route of spread. Infected and carrier fish shed A. salmonicida into the surrounding water via feces, mucus, and from ulcer sites. Infection occurs via the gills, skin abrasions, or the oral route. Waterborne transmission within a cage or net pen is extremely efficient at typical aquaculture stocking densities. Genomic epidemiological analysis of outbreak isolates from multiple aquaculture facilities has confirmed that spatially dispersed farms can share a single epidemiological unit, consistent with waterborne inter-site spread 10.
    • Vertical Transmission: Not definitively established for A. salmonicida subsp. salmonicida. Contamination of egg surfaces at spawning from carrier parents is possible, but true transovarial vertical transmission has not been demonstrated 1.
    • Vector/Environmental Transmission: The pathogen can persist in freshwater sediment and river biofilm for weeks. Wild salmonids and sea-run fish may introduce the pathogen to previously unaffected farms. Ectoparasites (e.g., Lepeophtheirus salmonis, sea lice) may compromise skin integrity and facilitate bacterial entry, though they are not established as direct vectors 1.

Risk Factors

  • Environmental Factors: Disease expression caused by Aeromonas salmonicida is influenced by temperature, host susceptibility, management conditions, and other environmental stressors. Classical furunculosis is generally more likely at moderate-to-warm water temperatures, between 12°C and 20°C; outbreaks in sea-cage Atlantic salmon commonly occur in spring and early autumn when sea temperatures are in this range. Rapid temperature changes and acute warming events can precipitate outbreaks in carrier populations. Poor water quality and reduced dissolved oxygen further compromise immune competence 1.
  • Farm Management Practices: Seawater transfer (smoltification stress) is a major trigger for clinical outbreaks in previously subclinical carrier fish. High stocking density, handling, crowding, sorting, and the concurrent presence of sea lice infestations or other parasites all elevate susceptibility. Incomplete vaccination coverage or inadequate vaccine storage and administration practices reduce immune protection 1.

Diagnosis​

Diagnostic Methods

  • Clinical Examination: A presumptive diagnosis of furunculosis can be made from characteristic gross pathology in salmonids, particularly the combination of furuncles, fin-base haemorrhages, exophthalmia, bloody vent discharge, and splenomegaly. However, peracute and acute presentations may lack furuncles entirely, and clinical signs overlap significantly with those of other bacterial septicaemias. Isolation of the causative agent is required for definitive diagnosis, while clinical and histopathological findings provide important supporting evidence for the diagnosis 117 .
  • Laboratory Tests:
    • Bacterial Culture: A. salmonicida grows readily on standard bacteriological media (TSA, blood agar) at 22°C. Incubation at temperatures above 28°C should be avoided as it may inhibit growth. Typical subsp. salmonicida colonies produce a characteristic diffuse brown pigment visible after 48–72 hours of incubation — the most practical rapid phenotypic identifier in field-oriented laboratories. Oxidase-positive, glucose-fermentative, and negative for motility 1.
    • PCR-Based Methods: PCR targeting the vapA gene (encoding the A-layer VapA protein) is the standard molecular confirmation tool for typical A. salmonicida. Quantitative PCR (qPCR) assays allow load estimation and are useful for surveillance of subclinical carrier fish without clinical signs. Multiplex PCR assays can simultaneously detect and differentiate A. salmonicida subsp. salmonicida from Flavobacterium psychrophilum and Yersinia ruckeri in a single reaction, with detection limits of 30 CFU for A. salmonicida in pure culture and 250 CFU in the presence of tissue debris 6. Importantly, vapA-based PCR targets may fail to amplify in emerging vapA-absent strains now circulating in Chilean salmon farms, underscoring the need to complement standard assays with culture or broader-spectrum molecular screening 2. The same limitation affects A. piscicola: isolates causing furunculosis-like disease in Chilean salmon farms also fail to amplify both vapA and fstA qPCR markers, so colonies with an A. salmonicida-like appearance that test negative on these assays require whole-genome sequencing or phylogenetic analysis for definitive species identification rather than being dismissed as non-pathogenic 24 25. For broader subtyping of atypical and typical isolates alike, vapA A-layer genotyping provides a standardized epidemiological marker that has been applied across hundreds of isolates internationally 26.
    • Serological Methods: Indirect fluorescent antibody test (IFAT) and ELISA-based assays using A. salmonicida-specific antisera have been used for pathogen detection and for monitoring vaccine-induced antibody responses. Serum agglutination titers can serve as a proxy measure of immune protection post-vaccination 1.
    • Histopathology: Histological sections from muscle, liver, spleen, heart, and kidney reveal necrosis and bacterial aggregates. Muscle tissue around furuncles shows extensive myonecrosis with few inflammatory cells, reflecting the immunoevasive properties of the A-layer. Hepatic and renal necrosis with bacteria in sinusoids and tubular lumina are consistent acute systemic findings, with significant organ dysfunction and persistent pathological changes documented even after the resolution of peak bacteraemia 5.

Differential Diagnosis

  • Distinguishing Furunculosis from Other Diseases:
    • Clinical signs of furunculosis overlap substantially with those of other bacterial fish diseases causing septicaemia, including vibriosis (Vibrio spp.), yersiniosis (Yersinia ruckeri), bacterial kidney disease (BKD, Renibacterium salmoninarum), winter ulcer disease (Moritella viscosa), and haemorrhagic septicaemia caused by Aeromonas hydrophila (motile aeromonads).
    • The non-motility of A. salmonicida subsp. salmonicida and the characteristic brown pigment on agar distinguish it from motile aeromonads; however, pigment production is not universal for all subspecies, so definitive identification requires laboratory testing (like qPCR).
    • In Atlantic cod, atypical furunculosis should be differentiated from francisellosis, caused by Francisella noatunensis subsp. noatunensis. Both diseases can produce chronic disease and internal nodules or granulomatous lesions. Histopathology together with culture and pathogen-specific PCR is therefore important for distinguishing these infections in cod 21.
    • Multiplex PCR can rapidly and simultaneously detect A. salmonicida subsp. salmonicida, Flavobacterium psychrophilum, and Y. ruckeri in a single reaction. It is therefore useful for targeted differential diagnosis, but additional culture, histopathology, and pathogen-specific molecular assays are needed to investigate other potential causes of disease 6.

Treatment and Prevention​

Treatment Options

  • Current Treatments:
    • Antibiotics: Antimicrobial treatment of furunculosis is prescribed by a veterinarian and is generally administered orally in medicated feed. Florfenicol is the primary licensed treatment in most major salmon-producing countries. Oxytetracycline is also approved in several jurisdictions. Historically, sulfonamide-trimethoprim combinations were widely used. Antibiotic treatment does not eliminate the carrier state and is used primarily to reduce acute mortality and bacterial shedding during an active outbreak 1.
    • Treatment Challenges: Antimicrobial resistance (AMR) in A. salmonicida is a growing concern. Resistance to tetracyclines (mediated by tetA and tetB genes on large resistance plasmids) and sulfonamides (sul1) is encoded predominantly on conjugative multiresistance plasmids, notably pSN254b — the dominant AMR plasmid in Canadian isolates — and variants of the pAsa4 plasmid carrying chloramphenicol, sulfonamide, and tetracycline resistance genes whose mosaic architecture is shaped by mobile genetic elements 7 8.
    • Antibiotic use in fish farms creates selection pressure that increases the occurrence and abundance of multiresistant Aeromonas strains in both fish and their surrounding water environments 9. A critical challenge is that antibiotic therapy does not eliminate latent carrier fish, which remain a reservoir for re-introduction of the pathogen into clean populations after treatment ends 1.
    • Treatment of atypical furunculosis in lumpfish remains challenging. A Norwegian FHF-funded study found that none of the tested antibacterials (florfenicol, oxolinic acid, or flumequine) provided satisfactory efficacy against A. salmonicida. Florfenicol (20 mg/kg/day for 10 days) was considered the best available option, although mortality increased substantially after treatment ended, suggesting that antimicrobial therapy may provide only limited control of the disease 22

Preventive Measures

  • Vaccines:

    • Commercial Vaccines: Several oil-adjuvanted injectable furunculosis vaccines (administered intraperitoneally to juvenile salmon prior to seawater transfer) have been commercially available since the early 1990s. These vaccines, combined with multivalent formulations covering other key pathogens (e.g., ISA virus, Vibrio spp., M. viscosa), are the cornerstone of disease prevention in Norwegian, Scottish, and Chilean salmon farming. Deployment of effective multivalent vaccines, extensive vaccination coverage, and improved biosecurity and disease-management practices in Norway in the early 1990s resulted in a reduction in antibiotic use of approximately 99% and is the defining example of how vaccination can transform antibiotic stewardship in aquaculture 1.
    • Vaccine Implementation: Current oil-adjuvanted vaccines are formulated on the basis of standard A. salmonicida antigenic profiles, including the VapA protein as a key immunogen. A newly emerged vapA-absent strain in Chilean salmon farms elicits a reduced serological response in vaccinated fish, suggesting that a growing proportion of recent furunculosis cases may be caused by a strain that partially escapes existing vaccine-induced immunity 2. Additionally, while oil-adjuvanted bacterin vaccines provide significant protection, they are associated with intraperitoneal granulomatous reactions, peritoneal adhesions, pigmentation and spinal deformities. In experimental rainbow-trout studies, alternative adjuvant formulations — including CpG oligodeoxynucleotides and the liposomal CAF01 system — reduce the severity and frequency of these adverse effects while maintaining protective efficacy comparable to the mineral oil benchmark 13. Autogenous vaccine strategies, supported by whole-genome sequencing evidence that outbreak isolates from geographically dispersed farms often form a single epidemiological unit, offer a complementary prophylactic tool where commercial vaccine coverage is imperfect 10.
    • Vaccination against atypical furunculosis: Vaccination against atypical A. salmonicida is more challenging than vaccination against classical furunculosis because atypical isolates are antigenically diverse and may show host specificity. Commercially available and authorised vaccine options are limited. In Norway, vaccines containing atypical A. salmonicida antigens have been used in Atlantic cod and lumpfish farming 23 . Autogenous vaccines manufactured from bacterial isolates recovered from actual outbreak sites may be used where disease remains a challenge and appropriate regulatory approval has been obtained.

  • Biosecurity Protocols:

    • All juvenile salmon should receive a full vaccination course with an oil-adjuvanted intraperitoneal injection of a licensed furunculosis vaccine prior to seawater transfer, with correct fish size at vaccination, injection technique, and sufficient time for immune development are necessary for effective protection 1.
    • Certified health screening of broodstock and incoming smolts using culture and PCR to detect covert carriers before transfer into sea cages — noting that vapA PCR screening should be supplemented with culture in regions where the vapA-absent strain has been detected 2.
    • Fallowing and disinfection of net pen sites between production cycles to reduce environmental bacterial load 1.
    • Use all-in/all-out production where feasible; restrict movement of fish, personnel, boats, and equipment between epidemiologically separate sites. 1.
    • Autogenous vaccines may be considered when commercial vaccines are unavailable or provide insufficient protection. Their use requires the appropriate regulatory approvals and should be based on characterisation of the outbreak isolate. Genomic epidemiological characterization of outbreak isolates supports the application of site-specific autogenous vaccines to aquaculture facilities sharing a confirmed epidemiological link 10.

  • Farm Management Practices:

    • Minimizing handling, crowding, and other stressors during the peri-smoltification period, when carrier fish are most likely to transition to clinical disease 1.
    • Integrated parasite management (particularly sea lice control) to maintain skin integrity and reduce entry points for A. salmonicida 1.
    • Surveillance of wild salmonid populations in proximity to farms where they may act as a reservoir and re-introduction source 1.

Case Studies​

Real-World Examples

  • Notable Outbreaks:

    • Norway (1980s–Early 1990s) — The Pre-Vaccine Epidemic: The Norwegian salmon farming industry experienced devastating losses from furunculosis and other bacterial diseases during this period as sea-cage production expanded rapidly. Injectable oil-adjuvanted vaccines against furunculosis were introduced in the early 1990s and were rapidly incorporated into multivalent vaccination programmes that also protected against classical vibriosis and cold-water vibriosis. Extensive vaccination coverage, together with improved hygiene, biosecurity, and fish-health management, was associated with a reduction of more than 99% in antibiotic use within the Norwegian aquaculture sector — arguably the most dramatic demonstration of vaccine-mediated antibiotic stewardship in the history of aquaculture 1. The Norwegian experience remains the foundational case study for the role of vaccination in managing this disease at a national scale.
    • Chile (Ongoing) — Furunculosis in the World's Second-Largest Salmon Producer: Chile's salmon farming sector experienced a steep rise in antibiotic consumption through the mid-2010s, reaching 382.5 tonnes in 2016 (~0.53 kg per tonne of harvested salmon) 11. Sustained regulatory pressure, improved disease management, and expanded vaccination coverage have since driven a substantial reduction in treatment intensity: by 2025, total antimicrobial use reached 415.4 tonnes on a production base of 1.167 million tonnes of biomass, corresponding to an Antimicrobial Consumption Index (ICA) of 0.036% (360 g/tonne produced) — a 93% reduction in intensity relative to the 2016 peak despite a 60% increase in production volume 15 16. Piscirickettsia salmonis (SRS) remains the dominant driver of antibiotic use, accounting for 97.51% of antimicrobial consumption in closed sea-phase production cycles in 2025, with furunculosis contributing a further 0.25% of sea-phase use and 7.35% of freshwater antibiotic use 16. SERNAPESCA's passive surveillance detected A. salmonicida (typical form) in 284 positive diagnostic reports from freshwater facilities and 238 from marine farms in 2025, ranking it the fourth most frequently identified pathogen in Chilean freshwater salmon production 15. Both typical and atypical (vapA-absent) strains are circulating: atypical A. salmonicida generated a further 33 freshwater and 31 marine positive reports, with confirmed detections across Los Lagos, Aysén, and Magallanes 15. The vapA-absent strain characterized in 2025 shows profoundly altered outer membrane protein and LPS antigenic profiles, causing failure of standard vapA PCR diagnostics and a reduced serological response to current commercial vaccines — raising concern that a growing proportion of Chilean furunculosis outbreaks may be partially vaccine-escape events 2. Genomic analysis has identified A. piscicola as an additional cause of furunculosis-like disease in juvenile Atlantic salmon in Chile. The pathogen can cause systemic disease but may not be identified by routine A. salmonicida molecular assays. Field data also suggest that current A. salmonicida-based vaccines induce only a limited antibody response against A. piscicola, highlighting a potential diagnostic and vaccine-coverage gap in Chilean salmon farming 24.

  • Lessons Learned: The Norwegian experience demonstrates that investment in vaccination programs can achieve near-elimination of antibiotic reliance for a major bacterial pathogen. The emergence of the vapA-absent strain in Chile illustrates that even highly effective vaccines can face challenges as pathogen populations evolve antigenic variation — underscoring the ongoing need for molecular surveillance of circulating strains, adaptation of diagnostic targets, and updating of vaccine antigens 2. The additional identification of re-emergent atypical A. salmonicida lineages and A. piscicola as contributors to the same Chilean furunculosis resurgence reinforces that "furunculosis" in this context is better understood as a complex of related agents rather than a single strain, with each agent requiring its own diagnostic targets and vaccine antigens 25 24. Genomic analysis of outbreak isolates supports linking geographically separated facilities into shared epidemiological units, enabling autogenous vaccine strategies for farm clusters that face recurring furunculosis where commercial vaccines provide incomplete protection 10.

Data Insights​

Disease Impact by Country​

Chile​

  • Furunculosis Incidence:

    • A. salmonicida is recognized as a persistent challenge in both freshwater smolt production and sea-phase grow-out in Chilean Atlantic salmon farms. Under SERNAPESCA's passive surveillance program, typical A. salmonicida generated 284 positive diagnostic reports from freshwater facilities and 238 from marine farms in 2025 — ranking it the fourth most frequently detected pathogen in Chilean freshwater salmon production (after PRV, Flavobacterium psychrophilum, and IPNV) 15. Detection is distributed across all major production regions, with the highest diagnostic burden in Los Lagos (154 freshwater reports), Magallanes (76 freshwater; 53 marine reports), and Aysén (42 freshwater; 103 marine reports) 15.
    • Atypical A. salmonicida strains (reported as "A. salmonicida atípico") generated a further 33 positive freshwater and 31 marine reports in 2025, with confirmed detections in Los Lagos, Aysén, and Magallanes 15. An emerging vapA-absent strain, first characterized in 2025, is responsible for a rising proportion of furunculosis diagnoses and causes diagnostic failure with standard vapA PCR-based screening protocols; its profoundly altered outer membrane protein and LPS profiles suggest it arose through a complex genomic rearrangement (indel) encompassing the entire vapA locus 2.
    • The atypical/non-classical furunculosis complex extends beyond this single vapA-absent strain. Whole-genome sequencing of additional Chilean isolates has characterized re-emergent atypical psychrophilic A. salmonicida lineages carrying a novel virulence/resistance plasmid not previously described in the species 25. Separately, a portion of A. salmonicida-like, vapA/fstA-negative colonies recovered from freshwater furunculosis outbreaks since 2022 has been reclassified — via whole-genome and phylogenetic analysis — as Aeromonas piscicola, a distinct species that, in challenge trials, produced focal cutaneous ulcerative lesions, visceral congestion, and septicaemia with mortality exceeding 80% by 4 days post-infection for the more virulent isolates, with one isolate showing a markedly elevated oxytetracycline MIC (>64 µg/mL) 24. Together, these findings suggest the true diagnostic burden of "atypical furunculosis" in Chile is likely underrepresented by counts based on vapA-dependent assays alone.

  • Economic Impact:

    • Chile consumed 382.5 tonnes of antibiotics in salmon farming in 2016 (~0.53 kg per tonne of harvested salmon), up from 143.2 t in 2010 11. By 2025, total consumption stood at 415.4 tonnes, but against a 60% larger production base (1.167 million tonnes biomass), the ICA had fallen to 0.036% (360 g/tonne produced) — a 93% reduction in treatment intensity from the 2016 peak 16. While Piscirickettsia salmonis drives ~97.5% of sea-cycle antibiotic use, furunculosis accounted for 7.35% of freshwater antibiotic use and 0.25% of sea-phase consumption in 2025, confirming its persistent but secondary role in the national antimicrobial burden 16.

  • Treatment & Management:

    • Florfenicol (82.5% of total antibiotic use in 2016) and oxytetracycline (16.8%) have historically dominated Chilean salmon farming therapeutics 11. By 2025, florfenicol accounted for 97.57% of antimicrobial use in closed sea cycles, administered almost exclusively via medicated feed; furunculosis treatment follows this pattern almost entirely 16. In the freshwater phase, oxytetracycline (60.63%) and florfenicol (39.15%) together represent 99.78% of antibiotic use, with furunculosis the third most treated disease after flavobacteriosis and renibacteriosis 16. SERNAPESCA's voluntary PROA-Salmón antimicrobial stewardship certification covered 262,416 tonnes (approximately 25% of 2025 national output), achieving an ICA of 13.47 g/tonne — 96% below the national sea-phase average — demonstrating that best-practice management can dramatically reduce treatment intensity 16. The limited scientific research on resistance gene epidemiology in Chilean A. salmonicida isolates and the emergence of a vapA-absent vaccine-escape strain represent significant knowledge gaps requiring urgent investigation 2 11.

Norway​

  • Furunculosis Incidence and Importance:
    • Classical furunculosis, caused by Aeromonas salmonicida subsp. salmonicida, is a notifiable disease in Norway. It is classified as a Category F national disease, meaning that suspected or confirmed cases must be reported to the Norwegian Food Safety Authority (Mattilsynet) 17.
    • The disease is now effectively controlled in Norwegian salmon aquaculture. It has not been detected in farmed fish since 2022, and no infections have been reported in wild Norwegian salmon since 2019 18.
    • Industry respondents did not identify furunculosis as a cause of mortality, impaired growth, reduced welfare, or an increasing problem in either smolt production or grow-out of Atlantic salmon. Nevertheless, occasional outbreaks remain possible, and warming seawater may increase future disease risk, supporting continued surveillance and routine prevention 18.
    • In contrast, atypical furunculosis—caused by non-salmonicida subspecies or strains of A. salmonicida—is not notifiable in Norway. Although it is rare in farmed Atlantic salmon, it is a clinically important disease in cleaner fish, including lumpfish and wrasse, where it compromises welfare and contributes to production losses. 18. .

Research and References​

Latest Research Findings​

  1. "Major Antigenic Differences in Aeromonas salmonicida Isolates Correlate with the Emergence of a New Strain Causing Furunculosis in Chilean Salmon Farms" Authors: Mancilla, M., Ojeda, A., Yuivar, Y., Grandón, M., Grothusen, H., Oyarzún, M., Bisquertt, A., Ugalde, J. A., Fuentes, F., Ibarra, P., & Bustos, P. Reference: Frontiers in Cellular and Infection Microbiology, 2025; 15: 1508135. A vapA-absent A. salmonicida strain is responsible for a rising share of furunculosis cases in Chilean salmon farms and shows reduced detection by standard PCR and reduced response to current vaccines. DOI: 10.3389/fcimb.2025.1508135

  2. "Genomics of Re-Emergent Aeromonas salmonicida in Atlantic Salmon Outbreaks" Authors: Godoy, M., Montes de Oca, M., Suarez, R., Martinez, A., Pontigo, J. P., Caro, D., Kusch, K., Coca, Y., Bohle, H., Bayliss, S., Kibenge, M., & Kibenge, F. Reference: Microorganisms, 2024; 12(1): 64. Whole-genome sequencing of re-emergent Chilean isolates identified a novel plasmid in atypical psychrophilic lineages contributing to the recent furunculosis resurgence. DOI: 10.3390/microorganisms12010064 · Full text

  3. "Aeromonas piscicola in Chilean Salmon Farming: Genomic Insights, Phenotypic Traits, Virulence and Field Immune Response" Authors: Mancilla, M., Ojeda, A., Yuivar, Y., Grandón, M., Valderrama, S., Oyarzún, M., Grothusen, H., Ibarra, P., & Bustos, P. Reference: Pathogens, 2026; 15(4): 402. A. salmonicida-like, vapA/fstA-negative isolates from Chilean furunculosis outbreaks were reclassified as Aeromonas piscicola, a distinct species producing a comparable disease picture with only modest vaccine cross-protection. DOI: 10.3390/pathogens15040402 · Full text

  4. "Genomic Analysis of Aeromonas salmonicida ssp. salmonicida Isolates Collected During Multiple Clinical Outbreaks Supports Association with a Single Epidemiological Unit" Authors: Wojnarowski, K., Cholewińska, P., Steinbauer, P., Lautwein, T., Hussein, W., Streb, L.-M., & Palić, D. Reference: Pathogens, 2024; 13(10): 908. Whole-genome sequencing of outbreak isolates from geographically separate farms found they belonged to a single epidemiological unit, supporting site-cluster autogenous vaccine strategies. DOI: 10.3390/pathogens13100908

  5. "Dynamics of Pathomorphological and Pathophysiological Alterations in Rainbow Trout (Oncorhynchus mykiss) During Acute Aeromonas salmonicida Infection" Authors: Nikiforov-Nikishin, D., Kochetkov, N., Gavrilin, K., Gaffarova, V., Medvedev, K., Smorodinskaya, S., Klimuk, A., Kuchikhin, Y., Svinarev, I., Gladysh, N., Kudryavtseva, A., Shitikov, E., & Nikiforov-Nikishin, A. Reference: Biology, 2025; 14(10): 1330. Experimental infection of rainbow trout documented significant organ dysfunction and incomplete physiological recovery even after sublethal infection, highlighting the sustained production impact of non-lethal exposure. DOI: 10.3390/biology14101330

Conclusion​

Furunculosis remains one of the most consequential bacterial diseases in global salmonid aquaculture. The Norwegian vaccination success story of the early 1990s, which cut antibiotic use by roughly 99% remains a landmark example of how vaccination can transform disease management 1. Yet the disease persists because of A. salmonicida's ability to establish subclinical carrier states, its environmental persistence, incomplete protection from current vaccines against emerging antigenic variants, and the spread of plasmid-borne antimicrobial resistance genes 7 8 9. Management today rests on universal pre-smolt vaccination, covert carrier screening by culture and PCR, and stress reduction during smoltification, with antibiotics reserved for confirmed outbreaks under susceptibility testing 1 2.

Last Modified: 2026-08-12

Tags: #Furunculosis, #AeromonasSalmonicida, #BacterialDiseases, #Bacteria

Other Bacterial Diseases​

Bacterial Kidney Disease (BKD) Moritella Viscosa Pasteurellosis Salmonid Rickettsial Septicaemia (SRS) Tenacibaculosis (Tenacibaculum spp) Vibriosis Winter Wounds Yersiniosis (enteric redmouth disease)

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