Duck Bacterial Diseases and Zoonotic Risks: A Comprehensive Guide

1. Introduction

Duck production systems, ranging from smallholder backyard flocks to large-scale commercial operations, are susceptible to a diverse array of bacterial pathogens that cause significant economic losses and pose potential zoonotic hazards. The aquatic environment and gregarious behavior of ducks facilitate the transmission of enteric and respiratory bacteria, while the increasing density of waterfowl farming amplifies antimicrobial selection pressure [1, 2, 24]. This article provides a systematic review of the major bacterial diseases affecting ducks, with emphasis on pathogenesis, antimicrobial resistance (AMR) profiles, molecular diagnostics, and zoonotic risk assessment. The discussion integrates recent genomic and proteomic findings to inform veterinary diagnostic and control strategies.

2. Riemerella anatipestifer Infection (Duck Serositis)

Riemerella anatipestifer is a Gram-negative, non-motile, rod-shaped bacterium belonging to the family Flavobacteriaceae. It is the etiological agent of epizootic infectious serositis in ducks, a disease characterized by fibrinous pericarditis, perihepatitis, airsacculitis, and meningitis [3, 4, 5]. The pathogen is responsible for high mortality rates in ducklings aged 2 to 7 weeks, with morbidity often exceeding 50% in naive flocks [6, 33].

2.1 Virulence Factors and Pathogenesis

R. anatipestifer employs a multifactorial virulence strategy. The type IX secretion system (T9SS) and its associated PorV protein function as critical adhesins, facilitating bacterial attachment to host epithelial cells [3, 39]. The outer membrane protein OMP85, a member of the BamA family, enhances virulence by recruiting host complement regulator vitronectin, thereby mediating complement evasion [4]. The chaperone protein DnaK contributes to antibiotic resistance and pathogenicity by maintaining protein homeostasis under stress conditions [40]. The ATPase MoxR is involved in anti-stress responses and is required for full virulence in duck models [56]. The ZntR regulator controls zinc homeostasis, and its disruption attenuates pathogenicity, indicating that metal ion acquisition is essential for in vivo survival [53]. Mutations in the clpS gene, which encodes a component of the Clp protease system, affect stress response and bacterial virulence [7]. The NuoB and SdhC subunits of respiratory complexes have been functionally characterized as contributors to both antibiotic resistance and pathogenicity [8].

2.2 Serotypes and Genetic Diversity

R. anatipestifer exhibits extensive serological diversity, with at least 21 serotypes described globally. A study in Thailand identified multiple serotypes circulating in duck and chicken populations, with serotypes 1, 2, and 6 being predominant [5]. Genomic analysis of chicken-derived isolates in China revealed considerable genetic heterogeneity and the presence of serotype-specific virulence gene clusters [29]. The emergence of R. anatipestifer infection in laying hens has been documented, leading to decreased egg production and hatchability, indicating host range expansion [9, 32]. A large-scale epidemiological investigation across 29 provinces in China from 2021 to 2024 confirmed the widespread distribution of this pathogen in chicken flocks, underscoring its cross-species transmission potential [36].

2.3 Antimicrobial Resistance

R. anatipestifer has acquired resistance to multiple antimicrobial classes. A novel aminoglycoside phosphotransferase, APH(3')-IVb, was identified in R. anatipestifer, conferring resistance to kanamycin and neomycin [10]. Isolates from clinical cases in Hungary between 2022 and 2023 demonstrated high levels of resistance to sulfonamides and tetracyclines, with variable susceptibility to florfenicol and doxycycline [41]. Pharmacokinetic/pharmacodynamic (PK/PD) studies have established optimal dosing regimens for florfenicol alone and in combination with doxycycline against R. anatipestifer in ducks, with the combination therapy showing reduced resistance development [55, 65]. Outer membrane vesicles (OMVs) produced by R. anatipestifer mediate horizontal transfer of antibiotic resistance genes through natural transformation, a mechanism that accelerates the dissemination of resistance determinants within bacterial populations [63].

2.4 Vaccination and Control

Live attenuated vaccines delivered via the intranasal route have demonstrated efficacy against R. anatipestifer serotype 1 in ducklings, inducing robust mucosal and systemic immune responses [6]. The PorV protein has been identified as a cross-protective antigen, offering protection against multiple serotypes [3]. Subunit vaccines based on recombinant OMP85 and other conserved surface proteins are under development [4, 33]. Compound Chinese herbal medicine formulations administered in drinking water have shown therapeutic efficacy against multidrug-resistant R. anatipestifer infections, providing an alternative to conventional antibiotics [11]. Sophoraflavanone G, a flavonoid compound, ameliorates R. anatipestifer infection both in vivo and in vitro by reducing bacterial load and modulating inflammatory responses [21].

3. Pasteurella multocida and Fowl Cholera

Pasteurella multocida is a Gram-negative coccobacillus that causes fowl cholera in ducks, a septicemic disease characterized by sudden death, fever, mucoid discharge, and cyanosis [12, 13]. The disease is particularly devastating in waterfowl, with acute outbreaks resulting in mortality rates exceeding 50% [31, 57]. For a detailed discussion of serotypes and outbreak dynamics, see the article on Avian Cholera in Waterfowl: Pasteurella multocida Serotypes, Outbreak Dynamics, and Vaccination Approaches in Wild and Domestic Birds.

3.1 Virulence and Genomic Characteristics

P. multocida strains isolated from ducks harbor a range of virulence-associated genes, including those encoding dermonecrotoxin (toxA), fimbriae (ptfA), and outer membrane proteins (ompH) [13, 57]. The kmt1 gene, which encodes a species-specific outer membrane protein, is a conserved target for molecular identification [57]. Attenuation of virulence through serial passage has been exploited to develop live vaccine strains, such as PMZ8, which shows reduced pathogenicity while retaining immunogenicity in ducks [12].

3.2 Antimicrobial Resistance

Genomic analysis of avian P. multocida isolates has revealed the presence of resistance genes against tetracyclines (tetR, tetH), beta-lactams (blaROB-1), and sulfonamides (sul2) [13]. Multidrug-resistant strains have been recovered from poultry and rabbits, with resistance profiles including aminoglycosides and fluoroquinolones [57]. Metformin has been shown to enhance the efficacy of doxycycline against P. multocida in vitro and in vivo, suggesting a potential role for metabolic modulators in combination therapy [31].

3.3 Zoonotic Considerations

P. multocida is a zoonotic pathogen capable of causing wound infections and bacteremia following scratches or bites from infected ducks. A case report documented P. multocida bacteremia in a human following a scratch from an adopted Pekin duck, highlighting the risk of direct contact transmission [49]. Immunocompromised individuals are at increased risk for severe systemic infections.

4. Avian Pathogenic Escherichia coli (APEC) and Colibacillosis

Avian pathogenic Escherichia coli (APEC) is a major cause of colibacillosis in ducks, manifesting as airsacculitis, pericarditis, perihepatitis, and septicemia [14, 38, 66]. APEC strains belong to a diverse group of extraintestinal pathogenic E. coli (ExPEC) that share virulence genes with human uropathogenic E. coli (UPEC) and neonatal meningitis E. coli (NMEC) [62, 66].

4.1 Pathogenesis and Host Adaptation

APEC O145 strains isolated from ducks exhibit host adaptation genes that facilitate colonization of the avian respiratory and gastrointestinal tracts [66]. The pathogenicity of APEC is mediated by adhesins (e.g., type 1 fimbriae, P fimbriae), iron acquisition systems (e.g., aerobactin, yersiniabactin), and toxins (e.g., hemolysin, cytotoxic necrotizing factor) [62]. Probiotic interventions using Lactobacillus plantarum ZG-7 have been shown to improve intestinal barrier function and modulate gut microbiota in Muscovy ducks infected with APEC [14]. Sheep bile acids exert a protective effect against APEC infection in ducklings by reducing intestinal inflammation and bacterial translocation [38].

4.2 Antimicrobial Resistance in Duck-Derived E. coli

E. coli isolates from ducks exhibit high rates of resistance to critically important antimicrobials. Extended-spectrum beta-lactamase (ESBL) production, mediated by CTX-M and TEM genes, is widespread in duck populations [20, 25, 50]. A study in Indonesia detected the CTX-M gene in E. coli from ducks sold in traditional markets, indicating a reservoir for community dissemination [20]. The blaTEM gene has been identified in multidrug-resistant E. coli from cloacal swabs of ducks in Indonesian farms [25, 50]. In Poland, E. coli isolated from duck farm environments showed high levels of cephalosporin resistance, with genomic characterization revealing the presence of blaCTX-M-1 and blaCMY-2 genes [44]. Multidrug-resistant E. coli coharboring fosA3 and ESBL genes have been detected along the duck slaughter line, posing a food safety risk [46]. Molecular typing of duck-derived E. coli in China identified a predominance of sequence types ST10, ST48, and ST117, with many isolates carrying multiple resistance determinants [58]. A comprehensive analysis of ESBL-producing E. coli from apparently healthy birds in Nigerian live bird markets revealed high genetic diversity and the presence of pandemic clones [47]. In Taiwan, ESBL-producing E. coli from diseased livestock and poultry, including ducks, harbored blaCTX-M-15 and blaCTX-M-55 genes [67].

4.3 Zoonotic Risk

APEC strains share virulence gene profiles with human ExPEC, raising concerns about zoonotic transmission through the food chain or direct contact [62, 66]. The presence of ESBL and fosA3 genes in duck-derived E. coli represents a One Health concern, as these resistance determinants can be transferred to human pathogens via mobile genetic elements [44, 46]. Bacteriophage therapy using broad-spectrum phages has been explored as an alternative to antibiotics for controlling multidrug-resistant E. coli in waterfowl [61]. Polyvalent inactivated vaccines based on prevalent APEC serogroups have been developed for use in ducks [62].

5. Salmonella Infections in Ducks

Salmonella enterica subsp. enterica is a significant pathogen in ducks, causing enteritis, septicemia, and reduced egg production [1, 2, 34]. Ducks are frequently asymptomatic carriers, shedding Salmonella in feces and contaminating the environment and carcasses [42, 54]. Non-typhoidal Salmonella (NTS) serovars, including Typhimurium, Enteritidis, and Potsdam, are commonly isolated from waterfowl [1, 64].

5.1 Antimicrobial Resistance Dynamics

A prospective study in West Bengal, India, documented the dynamics of AMR in NTS from chickens and ducks, revealing increasing resistance to ciprofloxacin and ceftriaxone over the study period [1]. In Sichuan, China, waterfowl-derived Salmonella isolates from 2021 to 2023 showed high resistance to tetracyclines, sulfonamides, and ampicillin, with a significant proportion being multidrug-resistant [2]. A study in Shandong, China, identified Salmonella from food animals, including ducks, with resistance to ciprofloxacin and third-generation cephalosporins [34]. In South Korea, ciprofloxacin-resistant Salmonella enterica isolated from food animals between 2010 and 2023 included duck-derived strains, with resistance mediated by mutations in the gyrA and parC genes [52].

5.2 Virulence and Pathogenesis

Duck beta-defensin 10 (AvBD10) inhibits Salmonella enterica by disrupting bacterial membrane integrity, and its expression is associated with gut microbiota dynamics [15]. The synergistic pathogenicity of novel duck Orthoreovirus and Salmonella Typhimurium has been demonstrated, with co-infection resulting in more severe clinical signs and higher mortality than either pathogen alone [54]. An indirect ELISA based on tandem expression of PrgH-PagN proteins has been developed for serological detection of Salmonella infection in ducks, providing a tool for flock-level surveillance [42].

5.3 Zoonotic Transmission

Salmonella is a major foodborne zoonotic pathogen, and duck meat and eggs are recognized sources of human salmonellosis [1, 16, 34]. A restaurant-associated campylobacteriosis outbreak was likely linked to duck liver pate, although Salmonella was also considered a potential co-contributor [16]. The emergence of multidrug-resistant Salmonella serovars in duck production systems poses a direct threat to public health, necessitating enhanced biosecurity and surveillance [2, 52, 64].

6. Clostridium perfringens and Necrotic Enteritis

Clostridium perfringens type A is a Gram-positive, spore-forming anaerobe that causes necrotic enteritis in ducks, characterized by intestinal necrosis, diarrhea, and sudden death [43]. The disease is often precipitated by dietary factors or concurrent coccidial infections that disrupt the intestinal mucosa. Untargeted metabolomics analysis of spleens from ducks infected with C. perfringens type A revealed alterations in amino acid, lipid, and energy metabolism, providing insights into systemic host responses [43]. For a broader discussion of this pathogen in poultry, see the article on Necrotic Enteritis in Broiler Chickens: Clostridium perfringens Virulence Factors, Gut Microbiome, and Probiotic Control Strategies.

7. Botulism (Clostridium botulinum Type C)

Avian botulism, caused by Clostridium botulinum type C, is a paralytic disease affecting ducks and other waterfowl [17, 37]. The bacterium produces a potent neurotoxin that blocks acetylcholine release at neuromuscular junctions, leading to flaccid paralysis of the neck, wings, and legs. Outbreaks are often associated with stagnant water bodies containing decaying organic matter and high temperatures, which promote bacterial growth and toxin production [17]. A botulism type C outbreak in free-ranging wild birds in a public urban park in Brazil resulted in high mortality, with ducks being among the affected species [37]. The presence of duckweed blooms and Euglena sanguinea in coastal ponds has been linked to recurrent botulism events, suggesting that algal blooms may create anaerobic conditions favorable for C. botulinum proliferation [17].

8. Mycoplasma Infections in Ducks

Mycoplasma gallisepticum and Mycoplasma anserisalpingitidis are important pathogens of ducks, causing respiratory disease, airsacculitis, and reproductive disorders [22, 23, 45]. M. gallisepticum has been detected in large-scale duck farms using molecular methods, with droplet digital PCR (ddPCR) offering improved sensitivity and quantification over conventional PCR [22]. Epidemiological investigations have identified M. gallisepticum in duck flocks across multiple provinces, indicating widespread circulation [45]. M. anserisalpingitidis shows temperature-dependent survival in water, with implications for biosecurity and transmission in waterfowl farming systems [23].

9. Other Bacterial Pathogens

9.1 Erysipelothrix rhusiopathiae

Erysipelothrix rhusiopathiae is a Gram-positive rod that causes erysipelas in waterfowl, characterized by septicemia, skin lesions, and sudden death. Antimicrobial susceptibility profiles of E. rhusiopathiae isolates from clinical cases in Hungary between 2022 and 2023 showed susceptibility to penicillins and cephalosporins, with resistance to aminoglycosides [41].

9.2 Enterococcus cecorum

Enterococcus cecorum is an emerging pathogen in poultry, including ducks, causing osteomyelitis, spondylitis, and septicemia. Phenotypic and genomic analysis of UK isolates revealed the presence of virulence genes and resistance to tetracyclines and macrolides [26].

9.3 Klebsiella pneumoniae

Klebsiella pneumoniae has been isolated from duck cloacal swabs in Indonesia, with multidrug-resistant strains carrying ESBL genes [51]. The presence of K. pneumoniae in duck populations is a concern due to its potential for zoonotic transmission and its role as a nosocomial pathogen in humans.

9.4 Nocardia cyriacigeorgica

Nocardia cyriacigeorgica was identified in a Mallard (Anas platyrhynchos) from Arizona, USA, representing a rare case of nocardiosis in waterfowl [35]. The bacterium caused granulomatous lesions in the respiratory tract, highlighting the diversity of opportunistic pathogens affecting ducks.

9.5 Bergeyella anatis

A novel species, Bergeyella anatis sp. nov., was isolated from the upper respiratory tract of ducks, expanding the known microbiota of waterfowl [30]. The clinical significance of this organism remains to be determined.

9.6 Chlamydia sp.

Chlamydia sp. has been detected at the interface between wild birds and free-range duck farms, with limited transmission observed [59]. Chlamydia psittaci is a zoonotic pathogen that causes psittacosis in humans, and its presence in duck flocks warrants surveillance.

10. Diagnostic Approaches

10.1 Molecular Diagnostics

Polymerase chain reaction (PCR) and real-time PCR are the primary methods for detecting bacterial pathogens in ducks. Multiplex PCR panels targeting species-specific genes (e.g., ompA for R. anatipestifer, kmt1 for P. multocida, invA for Salmonella) enable rapid identification [42, 57]. Droplet digital PCR (ddPCR) provides absolute quantification of target DNA without the need for standard curves, offering advantages for detecting low-abundance pathogens such as Mycoplasma gallisepticum [22]. High-throughput sequencing technologies facilitate genomic characterization of bacterial isolates, enabling AMR gene profiling and phylogenetic analysis [13, 26, 44].

10.2 Serological Assays

Enzyme-linked immunosorbent assays (ELISAs) are used for serological surveillance of Salmonella and R. anatipestifer infections in duck flocks [42]. Indirect ELISAs based on recombinant proteins (e.g., PrgH-PagN for Salmonella) offer high sensitivity and specificity [42]. For a general discussion of ELISA principles, see the article on Enzyme-Linked Immunosorbent Assay (ELISA) for Feline Leukemia Virus: p27 Antigen Detection and Diagnostic Interpretation.

10.3 Antimicrobial Susceptibility Testing

Broth microdilution and disk diffusion methods are used to determine minimum inhibitory concentrations (MICs) for duck bacterial isolates [41, 55]. PK/PD modeling integrates pharmacokinetic data with MIC values to establish clinical breakpoints and optimize dosing regimens [55, 65].

11. Zoonotic Risk Assessment

Ducks serve as reservoirs for several zoonotic bacterial pathogens, including Salmonella, Campylobacter, P. multocida, and ESBL-producing E. coli [1, 16, 34, 49]. Direct contact with infected ducks or contaminated environments poses a risk to farm workers, veterinarians, and consumers. The presence of multidrug-resistant bacteria in duck production systems amplifies the public health threat, as infections with resistant strains are more difficult to treat [2, 44, 46]. For a broader overview of zoonotic pathogens from farm animals, see the article on Livestock Zoonoses: A Comprehensive Overview of Bacterial and Viral Diseases Transmitted from Farm Animals to Humans.

12. Integrated Control Strategies

Control of bacterial diseases in ducks requires a multifaceted approach combining biosecurity, vaccination, antimicrobial stewardship, and alternative therapies. Biosecurity measures include all-in/all-out production, disinfection of facilities, and control of wild bird and rodent access [59]. Vaccination against R. anatipestifer and P. multocida is recommended in endemic areas [6, 12]. Antimicrobial use should be guided by susceptibility testing and PK/PD principles to minimize resistance development [55, 65]. Probiotics, bacteriophages, and herbal medicines offer alternatives to antibiotics for disease prevention and treatment [14, 11, 21, 61].

flowchart TD
    A["Clinical Signs in Ducks: Mortality, Serositis, Enteritis, Respiratory Distress"] --> B["Sample Collection: Cloacal Swabs, Tissues, Blood"]
    B --> C{Initial Diagnostic Triage}
    C --> D[Gram Stain and Culture on Selective Media]
    D --> E[Biochemical Identification and MALDI-TOF MS]
    E --> F{Pathogen Confirmed?}
    F -->|Yes| G[Antimicrobial Susceptibility Testing]
    F -->|No| H["Molecular Detection: PCR/ddPCR/Sequencing"]
    H --> I[Species-Specific Gene Targets]
    I --> J[AMR Gene Profiling and Genotyping]
    G --> K[PK/PD-Guided Therapy Selection]
    J --> K
    K --> L[Treatment and Biosecurity Implementation]
    L --> M[Post-Treatment Surveillance and Flock Monitoring]

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Disclaimer: This article is for educational and informational purposes only. It is not intended to substitute for professional veterinary advice, diagnosis, treatment, or regulatory guidance. Always consult a licensed veterinarian or qualified specialist regarding animal health, disease diagnosis, and therapeutic decisions.