Yersiniosis

Physical Signs:
**Rainbow Trout — Classic/Acute Form - Enteric Red Mouth (ERM)
Most commonly associated with the "Hagerman" strain (Serotype O1, Biotype 1)

• Red/bleeding mouth — bleeding under the skin around the mouth, gums, and throat 3
• Pop-eye (exophthalmia) — bulging eyes, often with blood spots inside the eye 1
• Dark skin — the fish becomes noticeably darker in colour 3
• Bleeding fins — reddened, congested fins 4
• Pale gills and swollen vent (anus)
• Internal bleeding — small haemorrhages on the liver, pancreas, swim bladder, and muscles 3
• Enlarged, darkened spleen - can be almost black in colour 3
• Inflamed intestine — reddened lower gut with a build-up of opaque yellow fluid 3

Atlantic Salmon and Chinook Salmon — Milder/Atypical Form
Associated with Serotype O1b / Norwegian virulent genetic variant

• The classic red mouth sign is often absent or subtle otherwise same signs as trout 2
• The most commonly detected sign is pop-eye and blood spots in the eye
• Most outbreaks in salmon occur during the juvenile (freshwater) phase or shortly after transfer to the sea 1

Behavioral Changes:

• Changes in farmed salmon are often non-specific and typical of a general blood infection, including lethargy, difficulty breathing, and abnormal swimming behaviour 2
• Lethargy: Reduced activity and slow swimming.
• Loss of Appetite: Decreased feeding behavior.
• Swimming Near the Surface: fish may swim near the surface or at the edges 3

Early Stages: Subtle signs such as reduced feed intake and slight lethargy, swimming near the surface, darkening of the skin and increase in mortality 4

Advanced Stages: More pronounced physical symptoms such as bulging eyes and hemorrhages (reddened areas) on the mouth, anus, base of fins and internal organs 5. Mortality starts low for ERM but then can increase rapidly 6

Impact on Fish Health: Yersiniosis severely compromises immune function and overall vitality, making fish susceptible to secondary infections.

Causative Agent: Yersinia ruckeri bacterium. The Yersinia genus have species causing animal-origin food outbreaks in humans 4. It is gram-negative, rod-shaped bacterium and the cells can survive in anaerobic and aerobic environments and are generally uniform in morphology 6.

Y. ruckeri is classified using three methods: O-serotyping based on surface antigen differences (O1–O8), serovar typing which groups strains into three broader groups — meaning two strains of the same O-serotype can still belong to different serovars — and biotyping which distinguishes motile (biotype 1) from non-motile (biotype 2) strains 7

It can also be classified by genetic diversity into clonal complexes (CC) using Multilocus Variable-Number Tandem-Repeat Analysis (MLVA) 7

Transmission Methods:

• Direct Contact: The main transmission, spreading between infected and healthy fish through close proximity 4. Infected fish can carry the disease for several months, especially in the lower intestine 8. Y. ruckeri enters fish primarily through the gills, and up to 25% of a rainbow trout population can carry the bacteria in their intestines, making fecal transmission possible 3. Transmission is believed to take place primarily during the freshwater phase 2

• Waterborne: The bacterium can spread through water, especially under conditions of poor water quality and high organic load 1, 3

  • Reported that it is more contagious when the water temperature varies between 15 to 20 °C 5
  • Incubation period (time between exposure and first clinical signs) is 5-10 days 5

Species Affected: Susceptible hosts include: Atlantic salmon (Salmo salar), brook trout (Salvelinus fontinalis), brown trout (Salmo trutta), Chinook salmon (Oncorhynchus tshawytscha), coho salmon (Oncorhynchus kisutch), rainbow trout (Oncorhynchus mykiss), eel (Anguilla anguilla), goldfish (Carassius auratus), perch (Perca fluviatilis), channel catfish (Ictalurus punctatus), sole (Solea solea), sturgeon (Acipenser baeri and A. schrencki) and turbot (Scophthalmus maximus) 1. Salmonids appear more sensitive to this bacterium and prone to suffer disease outbreaks 4

Environmental Factors: Poor water quality, high stocking densities, and stress conditions can increase the severity of an outbreak in chronically infected fish 9

Farm Management Practices: Ineffective biosecurity measures and lack of routine health monitoring can facilitate the spread of the bacterium 1. Delousing of latently infected fish (= infected but showing no signs, until triggered) can result in shedding and because of stress likely trigger a clinical outbreak 10

• Microbiology: Isolation and identification of Yersinia ruckeri from tissue samples. Microbiology cultures identify the serotype, support future research, and enable custom vaccine development 7. It can be easily cultured from the head kidney of infected fish on standard media such as blood agar at 20°C 2.
• PCR (Polymerase Chain Reaction): Detects bacterial DNA in tissue samples. PCR is the fastest and most reliable diagnostic method for identifying Y. ruckeri 5, 3. However it doesn't identify whether it is pathological, so PCR should be followed up with microbiology 7.
  • qPCR of spleen DNA ought to be considered the preferred standard for detection of carriers of Y. ruckeri 8
  • Environmental or eDNA swabs are possible to see presence on surfaces or in post lice treatment water 7.
• Histopathology: findings are typically non-specific but histopathology can be used to assess the extent of damage and infection 2
• LAMP Assay : Loop-mediated isothermal amplification (LAMP) is a field-deployable molecular test that detects a target gene of Y. ruckeri directly from water samples. This test can be done within an hour and has significantly higher sensitivity than conventional PCR 11, 12

• Early signs of can closely resemble other bacterial infections, so a clinical examination alone is not enough to confirm Y. ruckeri, laboratory testing is needed alongside the clinical exam to confirm the disease 3
• Other bacterial infections such as Aeromonas salmonicida and Vibrio spp. can produce similar clinical signs to Y. ruckeri, while its biochemical similarity to other Yersinia species and Hafnia alvei also makes laboratory misidentification possible 3

• Antibiotics: Antimicrobials can be administered through medicated feed or water, though their use requires caution due to the risk of developing antimicrobial resistance 2
• Supportive Care: Improving nutrition, improving water quality and reducing stress to support 13. A commercial bacteriophage CUSTUS®YRS performed in a well boat by STIM is also available, which reduces bacteria load in the water.

• Vaccines against Y. ruckeri have proven effective especially when administered by injection, with both commercial and autogenous vaccines offering comparable levels of protection 6.
• Monovalent vaccines targeting specific strains have been developed based on epidemiological studies of bacterial populations in fish farms 4
• Vaccination reduces disease and mortality but does not eliminate carrier status, as Y. ruckeri can still be detected in surviving vaccinated fish 3
• Vaccines against O1a may not be fully protective against O1b, it’s the lipopolysaccharide (LPS) which is the decisive antigen for vaccine protection 14.

• Effective immune defences in salmonids develop around first feeding, when fish simultaneously begin ingesting exogenous food and become exposed to waterborne pathogens — including Y. ruckeri. This creates a critical window of vulnerability, emphasising the importance of early biosecurity and vaccination strategies 15
• Disinfection of equipment and surfaces using UV treatment or chemical disinfectants is essential, with contact time being critical for effectiveness 6.
• Stock management measures include strict quarantine, egg disinfection, traffic control, and proper disposal of mortalities 16
• Reducing stocking density and maintaining good water quality are also important, as stress is a key trigger for disease outbreaks 3

• Stressful handling procedures such as delousing should be managed carefully, beginning with the healthiest cages and replacing treatment water regularly to reduce bacterial accumulation 10
• Reducing stocking densities to decrease the risk of bacterial transmission.

• United States (2007-2009): A series of outbreaks of Yersinia ruckeri occurred in farmed rainbow trout in the United States, particularly in the Midwest and Idaho regions, during 2007-2009. These outbreaks were associated with high mortality rates and were linked to the emergence of more virulent strains of Yersinia ruckeri. The outbreaks led to significant economic losses and prompted research into the development of more effective vaccines 14.

• Norway — Marine outbreaks (2015 onwards) There has been a rapid increase in yersiniosis cases in the seawater life stages of Atlantic salmon in Norway, particularly since 2015, with outbreaks increasingly occurring in larger fish well beyond the early seawater stage 3, 10

• UK and Europe — Rainbow trout (1980s onwards) The same biotype 2, serotype O1 clone has been responsible for the majority of ERM outbreaks in rainbow trout within the United Kingdom since the 1980s, with the disease first described in Europe in 1981 in France, Germany, and the United Kingdom 9. High incidence of ERM in rainbow trout cultured in Portugal 5

• Scotland — Atlantic salmon (2001–2014) A 14-year study of Atlantic salmon in Scotland identified 19 distinct Y. ruckeri clones, with a new serotype O8 emerging as the most common between 2006 and 2014, highlighting increasing serological diversity and vaccine challenges 13

• New Zealand and Australia — Atlantic salmon Associated with serotype O1b, representing distinct strains from those found in European rainbow trout 13

• Peru — Rainbow trout Y. ruckeri causes significant economic losses in rainbow trout farms in Peru, where no commercial vaccine is nationally available, and both biotype 1 and biotype 2 strains have been identified 3

• Chile - Cases have been reported over the years, and in 2008 there were outbreaks of Y. ruckeri O1b, including in vaccinated fish 17. Yet there is limited data available but expected to increase considering global trends 18

• China - Mass mortality at a sturgeon (Acipenser sinensis) farm (to establish an artificial population for the endangered fish) in 2022, with fish showing a red mouth and intestinal inflammation 19

• Global trend The number of outbreaks caused by Y. ruckeri has substantially increased in recent years globally, with the emergence of new serotypes reducing the effectiveness of existing vaccines 6.

Yersiniosis Incidence in Norway:

• The number of detections of Yersinia ruckeri, which causes yersiniosis, continued to increase in 2023 20.
• A high and increasing number of requested doses of injection vaccine against yersiniosis indicates significant problems with the disease 20.
• In Norway, all marine Atlantic salmon outbreaks have been caused by a single clonal complex (CC1) of serotype O1b, which is critical to consider when determining vaccination strategy 7, 10

Geographical Spread:

• Yersinia ruckeri clonal complex 1 (CC1) was found responsible for all major yersiniosis outbreaks diagnosed in Norwegian salmon farming 10
• Putatively avirulent Y. ruckeri strains were confirmed to be widespread in freshwater salmon hatcheries 21
• Mid-Norway appears to be the main geographical reservoir, with the strain spreading to and from this region through movement of infected fish 10

Economic Impact:

• No data currently available. Want to edit this section? Start here

Treatment & Management:

• Widespread use of intraperitoneal vaccination from around 2016–2017 significantly reduced outbreaks, though cases continue to occur mainly in unvaccinated fish 10
• Stressful management procedures may result in increased shedding of Y. ruckeri by sub-clinically infected fish 21.
• Thermal delousing procedures have been found to be effective for detection of Y. ruckeri in sub-clinically infected populations 21, 10

• Yersinia ruckeri is endemic in both countries, with broader strain diversity and distribution in Australia, while New Zealand is dominated by serotype O1b; the virulent O1a strain is absent 1

• Significant in Australia due to mortality and production losses in salmon farming, but relatively minor in New Zealand, where outbreaks are sporadic and less severe 1

• Managed mainly through vaccination, supported by good husbandry, biosecurity, and stress reduction, with antibiotics used when necessary 1

• Y. ruckeri is endemic in Canadian salmonid aquaculture, mainly affecting rainbow trout and Atlantic salmon in hatcheries and farms 22

• Present in major aquaculture regions, especially British Columbia, occurring in both freshwater and marine production systems 22

• Causes sporadic mortality and production losses, but overall impact is considered moderate compared with other salmonid diseases 22

• Controlled mainly through vaccination, biosecurity, and stress reduction, with antibiotics used during outbreaks when necessary 22

• Outbreaks have shifted from rare, freshwater-associated cases to more frequent seawater infections in larger fish, often without a freshwater link 7.

• Multiple strains circulate Atlantic salmon farms, particularly in freshwater hatcheries and smolt production systems 13

• Generally lower than major diseases such as Piscirickettsiosis, but still contributes to production losses through periodic outbreaks, especially in early life stages and during stress events 13

• Y. ruckeri is confirmed in rainbow trout aquaculture in Peru, with documented outbreaks in farms such as Huaraz causing high mortality 23, [24]((https://www.sciencedirect.com/science/article/abs/pii/S0044848611002754)

• The disease is mainly reported in high-altitude Andean trout farming regions (e.g., Huaraz and other freshwater systems in the central Andes), where intensive rainbow trout production is concentrated 23

• Outbreaks can cause very high mortality, making yersiniosis an important production constraint in Peruvian trout aquaculture 23

• Production is mainly land-based (flow-through tanks, ponds, and raceways rather than open sea pens), which facilitates disease control through biosecurity, antibiotic treatment during outbreaks, and improving husbandry practices, with increasing interest in vaccination and prevention strategies 23

• In October 2022, ERM caused mass mortality in artificially bred Chinese sturgeons at a farm in Hubei, with whole-genome sequencing confirming Y. ruckeri, carrying 135 drug-resistance genes and 443 virulence factor-related genes 19
• Chinese sturgeon farms have been experiencing ERM outbreaks for many years, with disease onset occurring at water temperatures below 22°C, affecting sturgeons weighing 1–2 kg 19

• Documented outbreaks span multiple provinces — including Hubei (Chinese sturgeon, 2022) and regions associated with silver carp, bighead carp, channel catfish, and sturgeon farming, indicating broad inland freshwater distribution 19

• In some Chinese sturgeon ponds, incidence rates have reached 80% and mortality rates 50%, representing severe production losses, particularly for an endangered and conservation-priority species 19

• Drug-susceptibility testing of the Chinese sturgeon isolate (zhx1) showed high sensitivity to chloramphenicol and florfenicol, but varying degrees of resistance to 18 other antimicrobial drugs, including commonly used aquaculture antibiotics such as doxycycline and neomycin 19
• No commercial Y. ruckeri vaccine is currently approved for use in China; management relies primarily on antibiotic treatment, biosecurity, and water temperature management, with disease risk highest below 22°C 19