Riemerella anatipestifer: A Comprehensive Veterinary Reference on Pathogenesis, Epidemiology, Diagnostics, and Antimicrobial Resistance

Introduction

Riemerella anatipestifer is a Gram-negative, non-spore-forming, rod-shaped bacterium belonging to the family Weeksellaceae within the order Flavobacteriales [1]. This pathogen is the etiological agent of epizootic infectious polyserositis, a septicemic disease primarily affecting domestic waterfowl including ducks, geese, and turkeys [1, 11]. The infection causes substantial economic losses to the global poultry industry due to high morbidity and mortality rates, particularly in duck flocks [1, 28]. In recent years, R. anatipestifer has emerged as a significant threat to chicken populations, demonstrating cross-species adaptation and expanding its host range [2, 4, 5, 8]. The bacterium is characterized by a high degree of serotypic diversity and a pronounced capacity for multidrug resistance, which complicates both prevention and treatment strategies [1, 19, 25].

Taxonomy and Morphology

Riemerella anatipestifer was historically classified within the genus Moraxella and later Pasteurella before being reclassified into its current genus based on phylogenetic analyses [1]. The bacterium is a non-motile, non-flagellated, facultatively anaerobic rod that typically measures 0.2 to 0.4 micrometers in width and 1.0 to 5.0 micrometers in length [1]. It stains Gram-negative and exhibits a characteristic bipolar staining pattern when examined with appropriate stains [1]. The organism does not form spores and is catalase-positive and oxidase-positive [1]. The complete genome of chicken-derived strain JN01 was determined to be 2,284,590 base pairs, as determined by third-generation sequencing [5].

Epidemiology and Host Range

Riemerella anatipestifer has a global distribution and is considered one of the most important bacterial pathogens in duck production systems worldwide [1, 11]. The disease is most commonly reported in ducklings and goslings between 2 and 7 weeks of age, although infections in older birds are also documented [1]. The bacterium has been isolated from a wide range of avian hosts including domestic ducks, geese, turkeys, wild waterfowl, and, increasingly, chickens [1, 2, 4, 5, 8, 31].

A large-scale epidemiological investigation conducted across 29 provinces in China between 2021 and 2024 demonstrated the rapid geographical expansion of R. anatipestifer in chicken populations [2]. The isolation rate increased from 1.03% to 4.56% over the study period, with infections peaking during winter and spring seasons [2]. The study revealed a shift toward infection in younger chickens, with isolation rates in 3 to 6-week-old white-feathered broilers increasing dramatically from 0.99% to 13.24% [2]. Tissue-specific colonization patterns showed high bacterial loads in oviducts (21.68%) and hocks (10.89%) [2]. The detection of R. anatipestifer in 20.18% of early "jelly-like embryos" suggested potential vertical transmission, although the bacterium was not found in semen or ovarian follicles [2].

In Assam, India, a study of 40 samples from sick and ailing ducks across organized and unorganized farms yielded 26 isolates tentatively identified as R. anatipestifer, with the highest prevalence of PCR positivity detected in pharyngeal swabs (73.33%) followed by ocular swabs (63.00%) [26]. This confirmed the emerging endemic nature of the infection in duck populations of northeastern India [26].

Serotype Classification

Riemerella anatipestifer exhibits extensive serotypic diversity, with at least 21 serotypes having been described globally [1, 19]. The predominant serotypes vary by geographic region and host species. In a comprehensive study of 51 R. anatipestifer strains, serotyping using rabbit antisera revealed 6 distinct serotypes with two untypable strains [19]. Among chicken isolates from China, serotype 1 (70.82%) and serotype 10 (15.86%) were dominant, with 2.83% of strains remaining untypable [2]. In a separate analysis of 92 strains isolated from 13 provinces between 2008 and 2023, serotypes 2 (29.4%), 7 (25.0%), and 1 (21.7%) were predominant [25]. In southern China, analysis of 195 isolates from ducks revealed serotypes 1, 2, 7, and 10, with serotypes 1 and 2 accounting for 82% of isolates [28].

The molecular basis of serotype diversity has been investigated through pan-genomic approaches [25]. The accessory genome was found to be enriched in mobile elements and O-antigen-related genes, and multiple serotype-specific marker genes (e.g., pgIA, wbpI) were identified [25]. Insertion sequences (IS1595) flanking capsular polysaccharide (CPS) gene clusters suggested that horizontal gene transfer and recombination events play pivotal roles in serotype variation [25].

Pathogenesis and Virulence Factors

The pathogenesis of R. anatipestifer infection involves multiple virulence determinants that facilitate adhesion, invasion, immune evasion, and iron acquisition [1, 20, 32]. The bacterium primarily targets the respiratory tract and gains access to the bloodstream, leading to septicemia and subsequent localization in serosal surfaces, joints, and internal organs [1].

Adhesion and Invasion

Adhesion to host cells is a critical initial step in R. anatipestifer infection. Affinity chromatography-based surface proteomics identified the PorV and PosF proteins, along with the previously described OMP71 protein, as important mediators of adhesion and invasion of duck embryo fibroblast (DEF) cells [20]. Knockout of porV or posF reduced adhesion and invasion capabilities, and the pathogenicity of mutant strains was significantly attenuated [20]. The PorV protein is a key component of the type IX secretion system (T9SS), while PosF belongs to the porin superfamily of barrel-shaped transmembrane proteins [20]. This was the first description of PorV functioning as an adhesin involved in host-microbial interactions [20].

Type IX Secretion System

The type IX secretion system (T9SS) is a recently discovered secretion apparatus found in Gram-negative bacteria of the FCB superphylum (Fibrobacteres, Chlorobi, and Bacteroidetes) [32]. In R. anatipestifer, at least seven T9SS component proteins have been identified, and a comprehensive search of the Yb2 genome revealed 19 T9SS proteins [32]. The T9SS plays a key role in bacterial virulence by transporting effector substrates, including adhesins and degradative enzymes, to the extracellular environment [20, 32].

Iron Acquisition

Iron acquisition is essential for bacterial growth within the host. Riemerella anatipestifer employs a unique haemophore, RhuH, to scavenge haem from host haemoproteins [12, 27]. RhuH is secreted as a component of outer membrane vesicles (OMVs) and exhibits a high binding affinity for haem (Kd of 3.44 x 10^-11 M) [12]. X-ray crystallography revealed the three-dimensional structure of RhuH at 2.85 angstroms resolution, showing a dimeric conformation with each monomer exhibiting a unique structure [12]. Haem is captured in a beta-barrel-like region displaying classic iron coordination [12]. RhuH homologues are predominantly distributed in Weeksellaceae and Flavobacteriaceae [12].

Other Virulence Determinants

Genomic analysis of hen-derived R. anatipestifer strains revealed 18 mutated virulence genes associated with diverse virulence determinants, including type IV secretion systems (T4SSs), hemolysin, yersiniabactin (Ybt), lipooligosaccharide (LOS), lipopolysaccharide (LPS), BrkA, capsule biosynthesis, flagella, caseinolytic protease C (ClpC), FeoAB, and Vi antigens [16]. The chaperone DnaK was shown to be critical for pathogenicity, as it is required for heat stress resistance inside ducks [6]. The MoxR protein, an ATPase regulated by the PhoPR two-component system, was demonstrated to play a significant role in anti-stress responses and pathogenicity by influencing the expression of the Bacteroides aerotolerance (Bat) operon [17]. Mutations in the clpS gene were found to affect stress response and bacterial virulence [22].

Host Immune Response

The host immune response to R. anatipestifer infection involves both innate and adaptive components, with significant variation observed between different host species and breeds [4, 18]. Comparative studies between chickens and ducks revealed that chickens exhibit relatively higher levels of IL-2, IL-8, IFN-gamma, TLR3, and STING expression during infection, reflecting the potential roles of these cytokines in mediating immune responses and bacterial clearance [4].

In ducks, breed-specific differences in immune responses and iron metabolism were identified [18]. Native Ji'an Red-feathered ducks (JR) exhibited lower morbidity (85.63% vs. 100%) and mortality (68.75% vs. 88.76%) compared to commercial White Kaiya ducks (WK) under identical infection conditions [18]. Susceptible individuals in both breeds showed severe weight loss, splenomegaly, and multiorgan histopathological damage [18]. Resistant JR ducks maintained stable IgA/Th1/Th2 cytokine levels with elevated IL-17A, while resistant WK ducks showed lower IgM with stable IgG/Th1/Th2/Th17 levels [18]. Pathological iron accumulation occurred in all infected WK ducks but not in resistant JR ducks, highlighting the importance of coordinated immune-iron homeostasis in disease resistance [18].

Clinical Signs and Pathology

The clinical presentation of R. anatipestifer infection varies depending on the host species, age, and virulence of the infecting strain [1, 4, 5]. In ducks, the disease is characterized by acute septicemia with high mortality, particularly in ducklings aged 2 to 7 weeks [1]. Clinical signs include depression, anorexia, ocular and nasal discharge, diarrhea, ataxia, tremors, and opisthotonos [1]. Postmortem examination typically reveals fibrinous pericarditis, perihepatitis, airsacculitis, and meningitis [1].

In chickens, the clinical presentation differs somewhat from that observed in ducks [4, 5]. Experimental infection of specific-pathogen-free (SPF) chickens with a challenge dose of 1 x 10^8 CFU per chicken successfully reproduced clinical symptoms, with high bacterial loads detected in joint cavities and brains at 10 days post-inoculation [5]. The pathogenicity of chicken-derived strains was lower in chickens than in ducks [4, 5]. In laying hens, infection was associated with decreased egg production and reduced hatching rates, with primary pathological findings observed in the oviducts [8, 16, 31]. Vertical transmission was suggested by the detection of R. anatipestifer in "jelly-like" lifeless embryos [2, 31]. In broilers, leg lesions were a prominent clinical feature, with infection rates in 3 to 6-week-old white-feathered broilers increasing dramatically [2].

Diagnostics

Accurate and timely diagnosis of R. anatipestifer infection is essential for implementing appropriate control measures [1, 3]. Diagnostic approaches include bacterial isolation, biochemical characterization, serological typing, and molecular detection methods.

Bacterial Isolation and Identification

Riemerella anatipestifer can be isolated from clinical specimens (brain, liver, heart blood, joint fluid) on blood agar or tryptic soy agar incubated under microaerophilic conditions at 37 degrees Celsius for 24 to 48 hours [1]. Colonies are typically smooth, circular, and non-hemolytic [1]. Biochemical identification is based on catalase and oxidase positivity, lack of motility, and specific carbohydrate fermentation patterns [1].

Molecular Detection

Polymerase chain reaction (PCR) assays targeting species-specific genes, such as the 16S rRNA gene or the Z gene, provide rapid and sensitive detection [3, 26]. A multiplex PCR assay was developed for simultaneous detection of R. anatipestifer serotype 1 and serotype 2 strains [3]. This assay amplifies a 505 base pair fragment from serotype 1 strains, a 1125 base pair fragment from serotype 2 strains, and an 843 base pair fragment of the 16S rRNA gene from all R. anatipestifer strains [3]. The sensitivity test showed a detection limit of 10^2 CFU [3]. When tested against 60 clinical isolates, the multiplex PCR results showed 100% consistency with the slide agglutination test [3].

Serotyping

Serotyping is performed using slide agglutination or agar gel precipitation tests with specific antisera [19, 28]. Rabbit antisera prepared against reference strains are used to classify isolates into serotypes [19]. The development of comprehensive serotyping panels is complicated by the large number of serotypes and the emergence of untypable strains [19, 25].

Genomic Typing

Whole-genome sequencing and pan-genome analysis have been applied to characterize R. anatipestifer strains and investigate their phylogenetic relationships [5, 16, 19, 25, 31, 35]. Multilocus sequence typing (MLST) provides a standardized method for molecular typing [19]. Phylogenetic analysis of whole-genome sequences showed that strains from different hosts, including chicken, duck, goose, and tadorna, did not form distinct independent branches but were intermixed throughout the evolutionary tree [5].

Antimicrobial Resistance

Antimicrobial resistance in R. anatipestifer is a critical concern that severely limits therapeutic options [1, 7, 13, 29, 35]. The bacterium exhibits intrinsic and acquired resistance to multiple antibiotic classes, and resistance rates are alarmingly high in many regions [2, 7, 25, 29, 35].

Resistance Profiles

A large-scale study of more than 400 nonredundant R. anatipestifer isolates collected from 22 provinces in China between 1994 and 2021 revealed that over 90% of isolates were resistant to sulfamethoxazole, kanamycin, gentamicin, ofloxacin, norfloxacin, and trimethoprim [35]. Alarmingly, 88.48% of isolates were resistant to tigecycline, a last-resort antibiotic [35]. Resistance to oxacillin, norfloxacin, ofloxacin, and tetracycline increased over time [35].

In chicken isolates from China, severe resistance was observed to enrofloxacin (91.54%), polymyxin (88.22%), and amikacin (86.10%) [2]. In Hungarian waterfowl isolates, high resistance rates were observed for spectinomycin, lincomycin, and tiamulin, while beta-lactam antibiotics (amoxicillin, ceftriaxone, and imipenem) demonstrated strong efficacy [7]. In Anhui Province, China, all 74 isolates from waterfowl were resistant to multiple drugs, ranging from 13 to 26 kinds of antimicrobials, with significant resistance to aminoglycosides and macrolides [29].

Molecular Mechanisms of Resistance

The molecular mechanisms underlying antimicrobial resistance in R. anatipestifer are diverse and include target gene mutations, acquisition of resistance genes, and efflux pump activity [13, 24, 25, 29, 30].

Quinolone Resistance: Quinolone resistance in R. anatipestifer from Thai ducks was primarily associated with a single point mutation at codon 83 of gyrA, either C248T (Ser83Ile) or C248G (Ser83Arg) [13]. No mutations were observed in parC, and no plasmid-mediated quinolone resistance (PMQR) genes (qnrA, qnrB, qnrS) were detected [13].

Beta-Lactam Resistance: Beta-lactam resistance is mediated by beta-lactamase enzymes [29]. A novel blaRASA-1 variant (16.2%), the class A extended-spectrum beta-lactamase blaRAA-1 (12.2%), and a blaOXA-209 variant (98.6%) were identified among isolates from Anhui Province [29]. The blaRAA-1 gene could undergo horizontal transmission among different bacteria via the insertion sequence IS982 [29]. Genome-wide analysis revealed the alarmingly high prevalence of blaOXA-like (93.05%) and tet(X) (90.64%) genes [35].

Polymyxin Resistance: The d-alanine pathway plays a critical role in polymyxin resistance in R. anatipestifer [24]. Alanine racemase (Alr), a pyridoxal 5'-phosphate-dependent enzyme critical for d-alanine synthesis in bacterial peptidoglycan, was identified as a target for polymyxin antibiotics [24]. Alr deletion was lethal to R. anatipestifer, and exogenous addition of d-alanine was unable to restore polymyxin resistance phenotype and virulence, suggesting that the Alr gene exhibits activity beyond alanine racemase functions [24].

Role of Chaperone DnaK: The chaperone DnaK promotes the emergence and accumulation of antibiotic-resistant clones through its ATPase activity [30]. The DnaK-deficient strain (delta-dnaK) was more sensitive than the wild type to various antibacterial agents [6]. DnaK is important for alleviating oxidative stress damage and maintaining normal cell morphology and membrane permeability [6]. The broad DnaK molecular chaperone inhibitor telaprevir effectively decreases the frequency of antibiotic resistance in R. anatipestifer by inhibiting the ATPase activation of DnaK [30].

Horizontal Gene Transfer

Horizontal gene transfer plays a significant role in the dissemination of antimicrobial resistance genes in R. anatipestifer [34]. Outer membrane vesicles (OMVs) from antibiotic-resistant strains were shown to transfer antibiotic resistance gene fragments and plasmids to sensitive strains, relying on the natural transformation system [34]. The OMVs had approximately 2.04 Mb genomic size, representing 88.3% of the genomic length, and contained 577 proteins representing 27.2% of the bacterial proteins [34].

Treatment and Control

Antimicrobial Therapy

Antimicrobial therapy remains a primary approach for controlling R. anatipestifer infections, although the high prevalence of multidrug resistance complicates treatment decisions [1, 7, 15]. Beta-lactam antibiotics, including amoxicillin and ceftriaxone, have demonstrated strong efficacy against many isolates [7]. The combination of florfenicol and doxycycline has been investigated as a therapeutic strategy [15]. Pharmacokinetic/pharmacodynamic studies in ducks showed that a single dose of doxycycline (greater than or equal to 10 mg/kg) combined with florfenicol (20 mg/kg) could exert a bactericidal effect on some tissues within 24 hours [15]. Continuous passage under antibiotic pressure for 30 days suggested that resistance to florfenicol was delayed in the presence of doxycycline [15].

Alternative Therapeutic Agents

The escalating prevalence of antibiotic resistance has prompted the search for alternative therapeutic agents [9, 23]. Sophoraflavanone G (SFG), a lavandulylated flavanone derived from Sophora flavescens, exhibits rapid bactericidal activity against R. anatipestifer in vitro and shows a low propensity for inducing drug resistance [23]. Mechanistic studies indicate that SFG binds to phosphatidylglycerol, compromising the structural and functional integrity of the cytoplasmic membrane, disrupting bacterial proton motive force, promoting intracellular accumulation of ATP and reactive oxygen species, and inhibiting DNA and RNA biosynthesis [23].

Marine-derived compounds have also shown promise [9]. Five compounds isolated from marine Streptomyces sp. C2-13 demonstrated potent activity against R. anatipestifer, with an IC50 value of 200 micromolar and bactericidal effect at 300 micromolar [9]. Transcriptome analysis showed that the active compound inhibited 30S and 50S ribosomal subunits, resistance mechanisms, and gliding motility proteins of the T9SS [9].

Vaccination

Vaccination is considered an essential component of disease control strategies for R. anatipestifer [11, 19, 28]. Various vaccine types have been developed, including live attenuated vaccines, inactivated (killed) vaccines, subunit vaccines, and vector vaccines [11].

A novel bivalent inactivated vaccine (WZX-XT5) containing propolis adjuvant was prepared based on serotype distribution in southern China [28]. The vaccine strains XT5 (serotype 1) and WZX (serotype 2) were selected based on virulence and immunogenicity [28]. Immunization induced high levels of RA-specific IgY, IFN-gamma, IL-2, and IL-4 in serum and offered over 90% protection against RA with ultra-high lethal dose in ducks [28].

Pan-genome analysis has been employed to identify potential cross-protective vaccine proteins [19]. Analysis of 1116 core genome genes revealed 5 genes of interest that could serve as better cross-protective vaccine antigens [19]. The oprM-1 protein, a highly reactive protein, was expressed and purified, and its immunoreactivity with five antisera (anti-serotypes 1, 2, 5, 11, and 18) was demonstrated by Western blotting [19].

Phage Therapy

Bacteriophages represent a potential alternative for controlling R. anatipestifer infections [14]. Temperate phages (PJA1, PJO17, PJR4, PJL1, and PJX6) were isolated from clinical strains and showed various lytic abilities, high similarity between different Riemerella phages, and stable properties [14]. Lysogenic conversion reduced the minimum inhibitory concentration of rifampin and florfenicol for certain lysogens [14]. Genomic analysis revealed high similarity to the first characterized Riemerella phage, RAP44 [14].

Genetic Manipulation Tools

The development of genetic manipulation tools has advanced research on R. anatipestifer pathogenesis and virulence mechanisms [21]. The CRISPR/Cas9 system was adapted for genome editing in R. anatipestifer through the construction of the pCasRA-SacB shuttle vector [21]. This system enables rapid, efficient, and scarless genome editing, including gene deletion (54.2% efficiency), insertion (100.0% efficiency), and point mutation (50.0% efficiency) [21]. The vector contains replication origins from R. anatipestifer plasmid pRA0726 and p15A, chloramphenicol and cefoxitin resistance genes, BsaI and SalI cloning sites, and the sucrose-sensitive gene sacB for plasmid curing [21].

Frequently Asked Questions

What is Riemerella anatipestifer?

Riemerella anatipestifer is a Gram-negative, non-spore-forming bacterium belonging to the family Weeksellaceae that causes epizootic infectious polyserositis, a septicemic disease primarily affecting domestic waterfowl and increasingly chickens [1].

Which animals are susceptible to Riemerella anatipestifer infection?

Domestic ducks, geese, turkeys, wild waterfowl, and chickens are susceptible to R. anatipestifer infection, with ducks being the most commonly affected host species [1, 2, 4, 5, 8, 11, 31].

How is Riemerella anatipestifer transmitted?

Transmission occurs through direct contact with infected birds or contaminated environments, and potentially through vertical transmission from parent to offspring as suggested by detection in embryos [2, 31].

What are the clinical signs of Riemerella anatipestifer infection in ducks?

Clinical signs in ducks include depression, anorexia, ocular and nasal discharge, diarrhea, ataxia, tremors, opisthotonos, and high mortality, particularly in ducklings aged 2 to 7 weeks [1].

What are the clinical signs of Riemerella anatipestifer infection in chickens?

In chickens, clinical signs include leg lesions in broilers, decreased egg production and reduced hatching rates in layers, and neurological symptoms, with lower mortality compared to ducks [2, 4, 5, 8, 31].

How is Riemerella anatipestifer diagnosed?

Diagnosis is based on bacterial isolation from clinical specimens, biochemical characterization, PCR targeting species-specific genes (e.g., 16S rRNA, Z gene), and serotyping using slide agglutination [1, 3, 26].

What serotypes of Riemerella anatipestifer are most common?

Serotypes 1 and 2 are the most prevalent globally, although serotype distribution varies by geographic region and host species [2, 19, 25, 28].

Why is antimicrobial resistance a concern for Riemerella anatipestifer?

Riemerella anatipestifer exhibits high resistance rates to multiple antibiotic classes, including fluoroquinolones, aminoglycosides, polymyxins, and tetracyclines, severely limiting therapeutic options [2, 7, 13, 29, 35].

What are the main virulence factors of Riemerella anatipestifer?

Key virulence factors include the type IX secretion system (T9SS), adhesins (PorV, PosF, OMP71), the haemophore RhuH for iron acquisition, and various other determinants such as hemolysin, lipooligosaccharide, and capsule biosynthesis components [1, 12, 16, 20, 32].

Are there effective vaccines against Riemerella anatipestifer?

Yes, various vaccine types have been developed including inactivated, live attenuated, subunit, and vector vaccines, with bivalent inactivated vaccines based on prevalent serotypes showing over 90% protection in ducks [11, 28].

Conclusion

Riemerella anatipestifer remains a significant threat to the global poultry industry, with its impact expanding due to cross-species adaptation to chickens and the emergence of multidrug-resistant strains [1, 2, 4, 5]. The bacterium's extensive serotypic diversity, complex virulence mechanisms involving the T9SS and unique iron acquisition systems, and high capacity for horizontal gene transfer of resistance determinants present ongoing challenges for disease control [1, 12, 20, 25, 32, 34]. Continued research into pathogenesis, the development of cross-protective vaccines, and the exploration of alternative therapeutic agents including natural compounds and bacteriophages are essential for sustainable management of this pathogen [9, 11, 14, 23].

References

[1] Hao, J., Zhang, J., He, X., et al. Unveiling the silent threat: A comprehensive review of Riemerella anatipestifer – From pathogenesis to drug resistance. Poultry Science. 2025. https://www.semanticscholar.org/paper/5e33f49d1c9715b4f01ad77fcee32fda990cf6c1

[2] Zhang, C., Liu, D., Sui, Z., et al. Epidemiological investigation of Riemerella anatipestifer in large-scale chicken farms in 29 provinces of China from 2021 to 2024. Poultry Science. 2025. https://www.semanticscholar.org/paper/f0a6d390b1a4e9e7c59feaea89acbb9adc2e2e71

[3] Wu, M., Guo, R., Chen, M., et al. Development of a multiplex PCR assay for detection of Riemerella anatipest