Invasive Species Compendium

Detailed coverage of invasive species threatening livelihoods and the environment worldwide


enteric redmouth disease



enteric redmouth disease


  • Last modified
  • 14 July 2018
  • Datasheet Type(s)
  • Animal Disease
  • Preferred Scientific Name
  • enteric redmouth disease
  • Overview
  • Enteric redmouth (ERM) is an acute or chronic infection of fish caused by Yersinia ruckeri, a Gram-negative, enteric bacterium. It is present in a carrier state in many species of fish and remains undetected unti...

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Performance of Yersinia ruckeri vaccine in rainbow trout, Oncorhynchus mykiss.
TitleYersinia ruckeri vaccine in rainbow trout
CaptionPerformance of Yersinia ruckeri vaccine in rainbow trout, Oncorhynchus mykiss.
CopyrightM. T. Horne & A. C. Barnes
Performance of Yersinia ruckeri vaccine in rainbow trout, Oncorhynchus mykiss.
Yersinia ruckeri vaccine in rainbow troutPerformance of Yersinia ruckeri vaccine in rainbow trout, Oncorhynchus mykiss.M. T. Horne & A. C. Barnes


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Preferred Scientific Name

  • enteric redmouth disease

International Common Names

  • English: bacterial septicaemia; enteric red mouth disease; enteric redmouth disease of salmonids; ERM disease; red mouth disease; red throat; redmouth; redmouth disease; redthroat; Yersinia ruckeri infections; yersiniosis

English acronym

  • ERM


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Enteric redmouth (ERM) is an acute or chronic infection of fish caused by Yersinia ruckeri, a Gram-negative, enteric bacterium. It is present in a carrier state in many species of fish and remains undetected until stress, particularly associated with intensive culture and poor water quality, may result in heavy losses requiring immediate intervention.

Yersinia ruckeri was first isolated in the Hagerman Valley, Idaho, USA, in the early 1950s and was described fully by Ross et al. (1966) and Rucker (1966). The early terminology used to describe the disease, such as redmouth, redthroat or bacterial septicaemia, was derived from the gross clinical signs, which in many cases were caused by mixed infections of Gram-negative organisms. In other cases, where Y. ruckeri was isolated and almost certainly the causative agent, they were absent. Standardization as ‘enteric redmouth’ was made by the Fish Health Section of the American Fisheries Society in 1975 and this, with ‘yersiniosis’, remains the commonest name.

The placement of the causative agent in the genus Yersinia (Ewing et al., 1978) has also proved problematic, but this remains the accepted identifier. Since its isolation in the Hagerman Valley, ERM has either spread or been recognized in most areas of Europe and the USA where freshwater fish are cultured intensively. It may be controlled to acceptable levels by good husbandry, particularly high water quality, and it responds well to antibiotics and vaccines. Even so, it presents a constant threat, requiring management procedures and treatments which erode the profitability of the farms on which it is endemic. Losses of 10-15% over a growth cycle are not uncommon and individual outbreaks may result in much higher mortalities. In small scale, artificial challenge trials, where no intervention takes place, over 90% mortality may occur.

Future Direction

In spite of significant progress in certain areas, many questions concerning Y. ruckeri remain unanswered. Its taxonomic position is unclear and requires further study, probably at the molecular level. The serology is vastly complex, and consensus between different countries regarding nomenclature should be achieved. Furthermore, the methodologies on which serotyping is based need to be standardized, such that results from different laboratories may be reliably compared.

More is now known of the pathogenesis of Y. ruckeri with identification of a major temperature regulated protease (Yrp1) and catecholaate siderophore iron uptake system. A model for investigating in vivo-expressed genes has been reported and further advances are now expected in this area of understanding. A type III secretion system similar to that found in Y. enetercolitica has also been discovered, though no role for it in pathogenicity has yet been identified. Clearly with the advent of these new techniques and findings, further understanding of the processes involved in disease progression will rapidly follow.

Perhaps most importantly, the complacency resulting from the generally successful commercial vaccines must be overcome, as their mode of action and the specific antigens involved in protection are unknown. Whilst the Yrp1 protease appears to be protective, this does not seem to explain the general efficacy of commercial bacterins. Further work needs to be undertaken to determine the mechanisms of protection and cross-protection against this pathogen in salmonids and, in particular, the nature of the antigens involved. Without this vital knowledge, aquaculturists may find themselves helpless if vaccines begin to fail.

[Based upon material originally published in Woo PTK, Bruno DW, eds., 1999. Fish diseases and disorders, Vol. 3 Viral, bacterial and fungal infections. Wallingford, UK: CABI Publishing.]

Hosts/Species Affected

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The initial isolation and description of ERM was from farmed rainbow trout, Oncorhynchus mykiss (Ross et al., 1966; Rucker, 1966), but has since been isolated from many species of fish, marine and freshwater, and from other animals. Environmental samples from river water, from sewage and from dairy situations have also been found to be positive (Pritchard et al., 1995). The majority of clinical disease conditions occur in intensively cultured salmonids and it is considered primarily a disease of salmonids (Busch, 1982), but a number of other families of fish have also been found to be infected (Berc et al., 1999; Carlson et al., 2002; Danley et al., 1999; Popovic et al., 2001; Valtonen et al., 1992; Xu et al., 1991). In those species where isolations have been made without the presence of clinical signs, it is probable that such species would succumb to infection if sufficient husbandry stress were applied.

Farmed species and young age were shown to be primary risk factors in ERM outbreaks in Canada (Good et al., 2001). Brook trout (Salvelinus fontinalis) proved to be more susceptible than other species, and detection of the pathogen in the genus Salvelinus was generally more likely than in other genera included in the study (Good et al., 2001). There was a significant increase in likelihood of detection in the 1-5 month age group compared with 6-19 month or in broodstock (>20 month) (Good et al., 2001).


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Following the first recognition of Y. ruckeri in Idaho, other workers began to identify the pathogen elsewhere. McDaniel (1971) recorded its presence in sites covering most of the Rocky Mountain area of the western USA, including Alaska; Wobeser (1973) showed its presence in Canada. Much of this early apparent spread was probably genuine and resulted from the dissemination of fish throughout this area, in the absence of strict controls and monitoring schemes. Its introduction to Europe in 1983 and its subsequent spread probably occurred in the same way, and it is now known to be present over large areas of the USA and Europe (Denmark, France, Germany, Italy, Norway, Hungary, Poland, Croatia and UK). It is also present in Australia (Bullock et al., 1978; Green and Austin, 1982), South Africa (Bragg and Henton, 1986) and Chile, all of which received imported salmonid eggs and fry. However, there is some variation in the strains and serotypes within the genus associated with geographical areas and it cannot be ruled out that it is of a more widespread occurrence in a natural manner than was at first thought.

Distribution Table

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The distribution in this summary table is based on all the information available. When several references are cited, they may give conflicting information on the status. Further details may be available for individual references in the Distribution Table Details section which can be selected by going to Generate Report.

Continent/Country/RegionDistributionLast ReportedOriginFirst ReportedInvasiveReferenceNotes

Sea Areas

Mediterranean and Black SeaPresentKaratas et al., 2004


IranPresentSoltani et al., 1999
TurkeyPresentKaratas et al., 2004


South AfricaPresentBragg and Henton , 1986

North America

CanadaPresentWobeser , 1973
USAPresentPresent based on regional distribution.
-AlaskaPresentMcDaniel , 1971
-ArkansasPresentDanley et al., 1999
-ColoradoPresentMcDaniel , 1971
-IdahoPresentRoss and et al. , 1966; Rucker , 1966
-MontanaPresentMcDaniel , 1971
-UtahPresentMcDaniel , 1971
-WyomingPresentMcDaniel , 1971

South America

ChilePresentEnríquez and Zamora, 1987; Horne and Barnes , 1999
PeruPresentBravo and Kojagura, 2004


CroatiaPresentOraic et al., 2002
Czechoslovakia (former)PresentVladik and Prouza, 1990
DenmarkPresentHorne and Barnes , 1999; Dalsgaard and Madsen, 2000
FinlandPresentValtonen et al., 1992
FrancePresentHorne and Barnes , 1999
GermanyPresentHorne and Barnes , 1999
HungaryPresentCsaba et al., 1991
ItalyPresentHorne and Barnes , 1999; Lucangeli et al., 2000
NorwayPresentHorne and Barnes , 1999
PolandPresentPopovic et al., 2001; Kozinska and Pekala, 2004
PortugalPresentSousa et al., 1996; Sousa et al., 2001
SpainPresentRomalde and et al. , 1993; Rodríguez et al., 1999; Romalde et al., 2003
UKPresentHorne and Barnes , 1999
Yugoslavia (former)PresentJeremic et al., 1997


AustraliaPresentBullock and et al. , 1978; Green and Austin , 1983


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Internally, there is congestion of the blood-vessels throughout the peritoneum, and petechial haemorrhages, affecting liver, pancreas, swim-bladder, lateral muscles and adipose tissues associated with the pyloric caecae (Wobeser, 1973). The kidney and spleen may be swollen and there may be fluid in both the stomach and the intestine, where it has a yellowish, opaque, mucoid appearance (Busch, 1982). Light microscopy shows bacteria in virtually all tissues, particularly the kidney, heart, liver and gills. There may be severe necrosis of the haemopoietic tissues of the kidney.

Wobeser (1973), Quentel and Aldrin (1986) and Lehman et al. (1987) observed acute anaemia, with an average haematocrit as low as 23% and total serum-protein values of 2.8 g 100 ml-1. Miller (1983) attributed this to the effects of endotoxin on coagulation ultimately producing thrombosis in the capillaries and generalized haemorrhaging.


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The early signs of yersiniosis in the acute phases of infection are typical of many other Gram-negative bacterial septicaemias, and anorexia, darkening of the skin and lethargy almost always precede the disease, except in small fry, where virtually asymptomatic deaths may occur (Kawula et al., 1996). The reddening of the throat and mouth, caused by subcutaneous haemorrhaging and from which the disease received its name, are commonly, but not invariably, present. If the disease progresses without treatment, erosion of the jaw and palate may occur. Haemorrhage occurs on the body surface, at the gill tips, at the base of the fins and around the lateral line. Rarely, these may progress to ulcers. The vent area may also become inflamed, both externally and internally, at the distal end of the intestine. Exophthalmia has been reported in some cases in the later stages of infection, with haemorrhaging of the ocular cavity and iris (Fuhrmann et al., 1983). This has been referred to as a separate condition in Australian trout (Llewellyn, 1980).

Diagnostic Methods

Provisional diagnosis of yersiniosis in fish is often made on the basis of experience at a particular site, combined with the clinical signs seen in the fish. However, as indicated above, these are typical of many other Gram-negative, septicaemial infections; even the frequently cited reddening of the mouth may be absent and, in any case, is seen in other types of infection. Additionally, small fish may die virtually asymptomatically.

Confirmation of yersiniosis is most easily made by culture from tissues, particularly the spleen, heart and kidney. Widely available media, such as tryptone soya agar (TSA), sometimes supplemented with 5% blood, are the most effective. The use of a preculture procedure, where the tissue suspected to be infected is incubated in liquid media (tryptone soya broth (TSB) or TSB + blood) at 18°C for up to 48 h may increase the number of positives obtained from a population (Daly and Stevenson, 1985). If required, confirmation can be obtained by non-lethal sampling (Noga et al., 1988). The regular cyclical shedding of Y. ruckeri which has been reported to occur from the intestinal tract of carrier fish, however, may hinder isolation (Busch and Lingg, 1975). Furthermore, the populations sampled should be designed to take into account the higher prevalence in age blocks between 1-5 months (Good et al., 2001).

Attempts have been made to produce diagnostic media to aid culture (Waltman and Shotts, 1984; Rodgers, 1991b), but, as with most diagnostic selective media, growth of the target organism may be reduced in addition to the desirable reduction in growth of other species. Additionally, the requirement of this medium that Tween 80 should be hydrolysed is not met by many isolates (Austin and Austin, 1993), complicating the concept of its use. More recently, differentiation based on virulence characteristics has been used successfully (Furones et al., 1993), but, in general, standard TSA is effective and preferred for isolation (Hastings and Bruno, 1985; Rodgers and Hudson, 1985).

Identification by the determination of attributes uses traditional methods, and laboratory minikits, such as API 20E, are also used successfully (Santos et al., 1993). However, additional tests may be required here if an exact definition of the species is required, since closely related species, such as H. alvei, would not be differentiated by this method.

The use of immunodiagnostic methods has matured to the point where these may also be described as traditional. Choice may be made between monoclonal and polyclonal reagents in the protocols of these methods, although, for commercial purposes, monospecific polyclonals are more robust and problemfree. Antigen detection, using specific antibodies in enzyme-linked immunosorbent assay (ELISA) formats, has been used in both research and commercially in plate and rapid, ‘on-stick’ formats. Antibody detection by latexagglutination tests has been used to determine subclinical infections, both experimentally (Johnson et al., 1974; Hansen and Lingg, 1976) and commercially (Romalde et al., 1995).

The most recent method of detection and identification, based on the development of specific molecular probes and polymerase chain reaction (PCR), is capable of detecting extremely low levels of Y. ruckeri (Argenton et al., 1996). However, this laboratory method is not used routinely and its sensitivity generally restricts its use to special, screening situations. Problems arise in validation of this technique and in judging the relevance of the positives obtained to farm husbandry situations (Hiney and Smith, 1998).

Refined methods of molecular diagnosis have been reported (Wilson and Carson, 2003). These can detect rRNA from <10 cfu cultured Y.ruckeri and have been adapted for high throughput screening. The relevance of these assays in clinical samples has not yet been demonstrated and they are likely to suffer from the same limitations as other PCR-based methodologies.

Safest diagnoses will always be made based on combinations of observed symptoms, histology, culture along with immunological and molecular methods.

List of Symptoms/Signs

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SignLife StagesType
Finfish / Build up of bloody fluids - Body Cavity and Muscle Aquatic:Fry Sign
Finfish / Cessation of feeding - Behavioural Signs Aquatic:Adult,Aquatic:Broodstock,Aquatic:Larval Sign
Finfish / Darkened coloration - Skin and Fins Aquatic:Adult,Aquatic:Broodstock,Aquatic:Larval Sign
Finfish / 'Dropsy' - distended abdomen, 'pot belly' appearance - Body Aquatic:Fry Sign
Finfish / 'Dropsy' - distended abdomen, 'pot belly' appearance - Body Aquatic:Fry Sign
Finfish / Fish swimming near surface - Behavioural Signs Aquatic:Fry Sign
Finfish / Fish swimming near surface - Behavioural Signs Aquatic:Fry Sign
Finfish / Generalised lethargy - Behavioural Signs Aquatic:Adult,Aquatic:Broodstock,Aquatic:Larval Sign
Finfish / Haemorrhagic lesions - Skin and Fins Aquatic:Adult,Aquatic:Broodstock,Aquatic:Larval Sign
Finfish / Liver - white / grey patches (haemorrhage / necrosis / tissue damage) - Organs Aquatic:Fry Sign
Finfish / Liver - white / grey patches (haemorrhage / necrosis / tissue damage) - Organs Aquatic:Fry Sign
Finfish / Mortalities -Miscellaneous Aquatic:Fry Sign
Finfish / Paleness - Gills Aquatic:Fry Sign
Finfish / Paleness - Gills Aquatic:Fry Sign
Finfish / Periorbital oedema - Eyes Aquatic:Adult Sign
Finfish / Pop-eye - Eyes Aquatic:Adult Sign

Disease Course

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Many potentially pathogenic Gram-negative organisms persist in small numbers in the intestine of apparently healthy fish and remain undetected in standard health checks. Viable bacteria detected in the lymphoid tissues of fish, however, indicate that the health of the fish has been compromised, even though no external indicators of poor condition may be noticeable. Yersinia ruckeri behaves in this manner and is sometimes described as establishing a ‘carrier state’ although, in the strictest definition of this phrase, this is arguable.

Throughout the farm cycle, periodic stress factors may move the balance of advantage towards the pathogen, as the performance of the immune system and other physiological factors are reduced to suboptimal levels. Fish carrying Y. ruckeri become sluggish in behaviour, reduce diet intake and may have a darkening of the skin. Factors known to promote subclinical and clinical infection by Y. ruckeri are, predictably, handling, grading and excessive stocking densities. When infection occurs in the absence of these factors, suspended organic matter in the water, coupled with high temperatures and consequentially low oxygen, along with exposure to low levels of chemicals in the water-supply, may promote clinical disease (Bullock and Snieszko, 1975; Knittel, 1981). In conditions where Y. ruckeri is endemic, therefore, the farm cycle is characterized by rapid changes in the external appearance of the stock and persistent low mortalities from ERM. Correction of the environmental factor(s) responsible may restore the fish to health, although an antibiotic may be needed to reduce the level of loss if condition has deteriorated too far. Acute infections, if not rapidly checked, can vary between 30 and 70% of the stock. As the fish become larger, chronic, slower infections are more characteristic, but these too may reach epizootic proportions if mishandled.


Virulence Factors

It is somewhat ironic that, in spite of numerous effective vaccination regimes, little is known of the virulence determinants associated with Y. ruckeri. Perhaps this may in part be explained by the complexity of the different serogroups and the fact that relatively few studies have compared virulent and avirulent isolates within single serogroups, concentrating predominantly on comparisons between serogroups. The lack of research in this area may also reflect the early successes with vaccination against this disease, reported as early as 1965 (Ross and Klontz, 1965). Nevertheless, the data available suggest that the virulence of Y. ruckeri, as with most pathogens, is multifactorial.

Iron uptake and in vivo induced genes

Further light has been shed on the virulence mechanisms of this group of pathogens using novel molecular techniques to investigate in vivo-induced genes (Fernandez et al., 2004). Using a promoterless reporter system Fernandez et al (2004) showed that a number of genes associated with the expression and activity of the catecholate siderophore Ruckerbactin were expressed during infection and critical for high virulence; isogenic mutants lacking rucC were less virulent the wild type. Various putative transport proteins were also expressed in vivo during infection in rainbow trout (Fernandez et al., 2004).

Yrp1 serralysin metalloprotease

The Yrp 1 protease of Y. ruckeri was identified and characterized by Secades and Guijarro (1999). The gene encoding Yrp1 forms part of an operon which is regulated by temperature and osmolarity (Fernandez et al., 2003). Expression of the protease is optimal at 18°C. Interestingly, inactivated Yrp1 was protective when delivered as an immunogen by intraperitoneal injection, suggesting a key role in pathogenesis and virulence (Fernandez et al., 2003).

Type III secretion system

Type III secretion systems (TTSS) are involved in subversion of host cellular systems for direct secretion of bacterial products into host cells. Systems such as these form important components of colonization strategies for human and mammalian pathogens such as Y. pestis, Y. entercolitica, Shigella flexneri and Salmonella enterica var typhimurium, with the plasmid-mediated Yop system of Yersinia pestis being the most thoroughly studied. Components of a TTSS system with some homology to the Y. enteroclitica system have been identified in Y. ruckeri (Gunasena et al., 2003). However, the role it plays in the pathology of ERM has not been explored.


In the human pathogenic Yersinia species, Y. pestis, Y. enterocolitica and Y. pseudotuberculosis, virulence is associated with a 40-50 MDa plasmid (Skurnik, 1985). Plasmid profiling of Y. ruckeri has identified a large plasmid of 70-88 kb carried in many isolates of serovar I (de Grandis and Stevenson, 1982; Toranzo et al., 1983, Stave et al., 1987). However, this plasmid was not homologous with virulence plasmids from other yersiniae (Stave et al., 1987), although strains of serovar I carrying the plasmid decreased the chemiluminescent response of striped bass macrophages (Stave et al., 1987).

Heat-sensitive factor

Furones et al. (1990) compared virulent and avirulent isolates of Y. ruckeri from serovar I. They reported a strong correlation between the production of a heatsensitive factor (HSF), probably lipid in nature, and virulence. Indeed when trout were infected by bath or intraperitoneal injection, only HSF+ isolates resulted in mortality. However, mortalities during bath challenge were variable and the study concluded that other factors must also be involved.

Resistance to Non-Specific Immune Mechanisms

Strains of Y. pestis carrying the 40-50 MDa plasmid produce antiphagocytic factors, which reduce the oxidative microbicidal activity of mouse macrophages (Charnetzky and Shuford, 1985), as determined by chemiluminescent (CL) response. Stave et al. (1987) reported that serovar I isolates of Y. ruckeri harbouring a 70 MDa plasmid also depleted the CL response of striped bass, Morone saxatilis, macrophages, although the factors expressed by the plasmid were not determined. Furthermore, one of the serovar II isolates, which did not carry the plasmid, also showed a decreased CL response in striped bass macrophages. The decreased CL response may reflect the method used.

Chemiluminescent assays detect reactive oxygen species (ROS); it may be that the reduced CL response results from quenching of ROS by bacterial enzymes, such as superoxide dismutase (SOD) and catalase. Indeed, all serovar I isolates analysed contain at least one superoxide dismutase enzyme, depending on the growth conditions. These enzymes may in themselves be important virulence factors, increasing resistance to phagocytic killing. It remains to be determined whether they are plasmid- or chromosomally encoded.

In addition to resistance to macrophage bactericidal activity, virulent serovar I isolates of Y. ruckeri are reported to be resistant to killing by non-immune rainbow trout serum, while avirulent serovar I isolates are sensitive (Davies, 1991b). However, not all serum-resistant isolates are virulent, reaffirming the multifactorial nature of virulence.

Antimicrobial Activity

Yersinia ruckeri has been reported to produce water-soluble antimicrobial activity capable of inhibiting Vibrio anguillarum, Aeromonas hydrophila and Aeromonas salmonicida (Michel and Faivre, 1987). This is not without precedent, as strains of Y. pestis produce a potent murein-hydrolysing bacteriocin, pesticin (Ferber and Brubaker, 1979). However, the activity produced by Y. ruckeri does not inhibit other strains of Y. ruckeri and, as such, does not fulfil the criteria of a bacteriocin (Michel and Faivre, 1987), although it is important to state that it is unclear which serotypes were used in this study. Whether or not the production of antimicrobial activity confers a genuine selective advantage on Y. ruckeri remains contentious, as mixed infections with both A. salmonicida and V. anguillarum could be obtained experimentally in rainbow trout (Michel and Faivre, 1987).

Survival Under Starvation Conditions

Survival under conditions of starvation is important to pathogenesis in two ways. Firstly, the organism is at an advantage if it can survive outside the host for extended periods to facilitate transmission. In this respect, Y. ruckeri is capable of surviving in unsupplemented fresh or brackish water (salinity 0-20 p.p.t.) for at least 4 months, while survival is greatly reduced in water of salinity 35 ppt. (Thorsen et al., 1992). Furthermore, Romalde et al. (1994) reported that Y. ruckeri was capable of survival for over 100 days in river water, lake water, estuarine water and sediments. While acridine orange direct counts remained constant, viable counts decreased over the experimental period, with culturable cells persisting for longer periods in sediments than in water (Romalde et al., 1994). In all cases, dormant bacteria could be revived by addition of fresh TSB to the microcosms and virulence was maintained throughout the dormant state. Although non-culturable cells showed marked changes in shape, size and metabolic rate, no changes were reported in membrane proteins or plasmid profiles, while minor changes were detected in LPS (Romalde et al., 1994). Long-term survival may be facilitated by replication of the genome before the onset of starvation is complete, with starved cells being able to carry up to six copies of the genome (Thorsen et al., 1992). The second important factor is the ability to acquire iron during the early stages of infection. Availability of iron in the host is largely restricted by its association with iron-binding proteins, such as haemoglobin and transferrin. The ability of a pathogen to acquire iron is therefore of utmost importance. It has been reported that Y. ruckeri has a siderophore-mediated iron-uptake system, along with several outer membrane proteins associated with iron-restricted growth conditions (Romalde et al., 1991a).

Other Factors

Other putative virulence-associated factors which have been studied in Y. ruckeri include cell-surface hydrophobicity and haemagglutination. Santos et al. (1990) reported no haemagglutinating activity against human or trout erythrocytes for all serovar I and II strains tested. This study also reported a lack of hydrophobicity and no adherence to epithelioma papillosum cyprini (EPC) cells for the same strains. However, adherence would appear to be dependent upon the cell line used in the study, with Y. ruckeri strains showing moderate adhesion and invasiveness in the chinook salmon embryo cell line CHSE-214 (Romalde and Toranzo, 1993). In addition, Romalde and Toranzo (1993) reported that extracellular products from Y. ruckeri were strongly toxic for fish, and displayed haemolytic activities for trout, salmon, sheep and human erythrocytes.

In conclusion, the pathogenicity of Y. ruckeri is poorly understood, predominantly as a result of the scarcity of good data. This lack of data may reflect the general view that yersiniosis represents ‘a problem solved’ through the early success of vaccination regimes. However, the limited data that are available suggest that pathogenesis is complex and multifactorial, and highlights the necessity for further study of this organism.


Although Y. ruckeri O-antigen has been used as a model antigen for the study of the kinetics of immune responses in rainbow trout (Anderson et al., 1979a,b,c) and carp (Lamers and Muiswinkel, 1984), little is known of the key antigenic components of Y. ruckeri to which the fish responds during infection or following vaccination.

The kinetics of the antibody response to O-antigen has been thoroughly documented in rainbow trout. Peak numbers of antibody-producing cells are detected in the spleen 14 days after antigen administration by flush. The titres of circulating antibody increase to a maximum after about 28 days, with the onset of detection coinciding with maximum numbers of antibody producing cells (Anderson et al., 1979c). Immunization of rainbow trout, using commercial Y. ruckeri bacterin, also demonstrated a significant secondary response to antigen injected intraperitoneally 146 days after first exposure (Cossarini- Dunier, 1986a). This demonstration of an anamnestic immune response to Y. ruckeri in rainbow trout is important. However, there is a lack of correlation between antibody titre and protection; Cipriano and Ruppenthal (1987) found that, although humoral agglutinins were detected in brook trout 24 h after passive immunization by intraperitoneal injection with immune sera, these antibodies conferred no resistance to disease. Furthermore, Lillehaug et al. (1996) demonstrated that protection in fry could not be achieved by passive transfer of antibodies maternally. Brood-stock females were vaccinated repeatedly by injection, and an increase in antibody titre, determined by ELISA and agglutination, was obtained against serovars I and II in the serum of vaccinated fish. Low levels were also detected in eggs and fry 1 week posthatching, although mortalities were not reduced in the offspring of vaccinated mothers compared with unvaccinated controls. This suggests that other immune mechanisms, such as a localized or cell-mediated response, must be involved in protection against yersiniosis.

Cell-mediated response to Y. ruckeri has been less thoroughly studied. Jones et al. (1993) reported that killed cells of serovar I and serovar II strains stimulated proliferation of naïve peripheral-blood leucocytes, and that the two strains were similarly mitogenic. However, the study concluded that this was irrespective of the immune status of the fish.

Antibodies produced by rainbow trout appear to be serotype-specific (Cipriano and Ruppenthal, 1987). Furthermore, antibodies against serovar I do not recognize LPS in immunoblot, while those raised against 0:2/serovar II do bind LPS (Stevenson et al., 1993). Additionally, a significantly higher dose of Oantigen from serovar I is required to elicit a response in rainbow trout than with O-antigen from serovar II (Anderson and Dixon, 1980).

In spite of the serological differences between Y. ruckeri serovars, there are numerous reports of cross protection. Indeed, Cipriano and Ruppenthal (1987) reported that, although immunization by intraperitoneal injection with bacterins against Y. ruckeri resulted in circulating antibody only to the specific serovar used in the bacterin, brook trout immunized in such a manner were protected during experimental challenge with both serovars I and II. Furthermore, a bacterin prepared from an avirulent serovar II isolate performed equally well compared with bacterins produced from virulent isolates (Cipriano and Ruppenthal, 1987). It is unlikely that this is a result of non-specific crossprotection, as Amend and Johnson (1984) reported that there was no antigenic competition between Y. ruckeri, A. salmonicida, V. anguillarum and Renibacterium salmoninarum when administered to rainbow trout. The antigens responsible for cross-protection have not yet been clearly identified; however, a recent review suggested that native flagellar antigens seem to be antigenically common among motile serogroups (Stevenson, 1997). In addition, Fernandez et al. (2003) reported high protective efficacy of heat inactivated Yrp1 protease from Y.ruckeri. However, most commercial vaccines are produced at temperatures where Yrp1 expression would be minimal to non-existent, and are formalin inactivated, so these observations do not explain the efficacy of commercial bacterins.

The generally successful vaccination of salmonids against yersiniosis using bacterins based on serovar I and the numerous studies reporting cross-protection have perhaps led to a degree of complacency and a general consideration that other serovars are unnecessary in commercial vaccines (McCarthy and Johnson, 1982). With regard to the latter, as reported above, experience in Norway has suggested otherwise (Erdal, 1989).


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Transmission of the Disease

Early work and descriptions of the disease were made by those working in salmonid aquaculture, leading to the concept of yersiniosis as primarily a disease of salmonids. Confirmed clinical outbreaks were seen in both trout (rainbow, steelhead, cutthroat, brown and brook) and in salmon (coho, sockeye and Atlantic) (Busch, 1982). Early epizootics in these species have been described in detail by Ross et al. (1966), Rucker (1966) and Busch (1973). It has been recognized that the host range of Y. ruckeri is more diverse. Whether it is capable of a saprophytic existence as a part of the environmental flora outside a host is a matter of debate; Rucker (1966) suggested that this may be the case, but later authors, such as Klontz and Huddlestone (1976), argued against this. Their findings reflected those seen for many other Gram-negative coliforms, including E. coli and Aeromonas salmonicida, where the survival in clean water in a laboratory experiment may be a matter of hours, but may be prolonged for 2-3 months in sediments or where there is organic matter present (Romalde et al., 1994). This allows transmission from fish to fish under natural conditions without the need to propose that the organism has lived saprophytically outside an animal host.

Whilst birds have been implicated as vectors (Willumsen, 1989), the prime source of infection is generally considered to be the shedding of large numbers of bacteria from carrier or infected fish in the faeces. Such fish do not normally shed quantities capable of causing infection, unless stressed (Hunter et al., 1980). The level of carrier fish in a population depends on the criteria and methods used. Busch and Lingg (1975) demonstrated that, 45 days postinfection, 25% of a population of rainbow trout carried Y. ruckeri asymptomatically in the lower intestine. The preculture/culture method used in this case is sensitive and the data may reflect a situation in fish following the first serious epizootic in a population. However, where a detailed investigation is made, using culture, immunological and molecular methods together, in a population grown under normal farm conditions for several months, a much higher proportion of individuals is found to be positive and it is not improbable that all are actually carrying the pathogen. Once infected, therefore, a fish population can maintain the pathogen indefinitely.

Yersinia ruckeri has also been isolated from brood stock, which are frequently, if not always, infected, but transmission through eggs where disinfection has been properly carried out has not been reported (Dulin et al., 1976).

Conditions predisposing populations to clinical infection relate primarily to stress. Healthy laboratory populations can withstand exposure to high numbers of cells without succumbing to clinical disease (Ross et al., 1966). Infection may occur where fish are obese through poor feeding regimes, but poor water quality is the prime cause. Common causes are high ammonia, due to poor water flow or too high densities, low oxygen, due to poor flow and high temperatures, or the presence of a high level of suspended organic and siliceous matter (Bullock and Snieszko, 1975). When these conditions are marginal, handling stress may trigger infections where the fish would have remained healthy if left untouched. The expectation of trouble, therefore, is in summer conditions, where temperatures rise and water flows are reduced. The peak is considered to be 15-18°C, and monitoring oxygen and temperatures daily is an effective warning system where yersiniosis is endemic. Infections have not been reported below 10°C.

Impact Summary

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Biodiversity (generally) Negative
Fisheries / aquaculture Negative
Native fauna Negative

Impact: Economic

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The British Trout Association estimated losses of £1.3-1.5 million in 1998. However, vaccines against ERM are generally effective, so in regions where vaccination is routinely used, the impacts of ERM are limited. Elsewhere it can be managed to a degree through use of good biosecurity practices and antibiotic treatments.

Impact: Environmental

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Yersinia ruckeri is not widely reported to cause significant wild fish kills. Treatment of diseased farm stock with antibiotics clearly has serious environmental implications. Use of vaccines and best practice can eliminate this impact.

Zoonoses and Food Safety

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Yersinia ruckeri is not reported to be zoonotic and as any isolates are unable to initiate growth at 37°C (de Grandis et al., 1988) this would appear to be an unlikely scenario. There have not been any reports of food safety issues related to Y. ruckeri.


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Main Author
A Barnes
FRS Marine Laboratory, PO Box 101, Victoria Road, Aberdeen AB11 9DB, UK

Joint Author
AC Barnes

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