Bacterial Carriage in Ducks: Pathogens of Public Health Concern

Introduction

Ducks (Anas platyrhynchos domesticus and other species) are increasingly recognized as reservoirs for a diverse array of bacterial pathogens with zoonotic potential [1, 2, 3]. Commercial duck production, backyard flocks, and free-range systems create interfaces between ducks, humans, and the environment that facilitate bacterial transmission [4, 5]. The question "what bacteria do ducks carry" is central to veterinary public health, as many avian bacterial species can colonize ducks asymptomatically while posing risks to immunocompromised individuals, food handlers, and downstream consumers [6, 7]. This article provides an exhaustive review of the bacterial pathogens of public health concern carried by ducks, addressing etiology, epidemiology, clinical signs, pathology, diagnostic approaches, treatment, and control strategies within a strict veterinary and molecular diagnostic framework.

Etiology: Major Bacterial Pathogens Carried by Ducks

Ducks carry a broad spectrum of Gram-negative and Gram-positive bacteria. The most frequently reported pathogens of public health relevance include Salmonella enterica, Campylobacter spp., Escherichia coli (including extended-spectrum beta-lactamase (ESBL)-producing strains), Listeria monocytogenes, Arcobacter spp., Riemerella anatipestifer, and Acinetobacter indicus [1, 2, 3, 4, 5, 6, 7, 8, 9]. Table 1 summarizes key characteristics of these pathogens.

Table 1. Major bacterial pathogens carried by ducks and their public health significance.

Salmonella enterica is frequently isolated from duck cloacal swabs and feces, with serovars such as Salmonella Enteritidis and Salmonella Typhimurium being predominant [7]. Campylobacter jejuni and Campylobacter coli are thermophilic species commonly shed in duck feces [8]. E. coli strains from ducks often harbor ESBL genes (e.g., blaCTX-M) and the colistin resistance gene mcr-1, as documented in studies from Zimbabwe and Nigeria [3, 4, 5]. Listeria monocytogenes carriage was reported in healthy free-range ducks in Kenya, underscoring its presence in low-biosecurity settings [2]. Arcobacter butzleri and Arcobacter cryaerophilus have been detected in duck cecal contents in Denmark [9]. Acinetobacter indicus carrying tet(X5) and blaNDM-3 has been identified in duck isolates in China, illustrating the role of ducks in the dissemination of resistance determinants [6].

Epidemiology

The carriage of bacterial pathogens in ducks is influenced by production system, age, diet, and environmental exposure. In open-house systems, ducks have high contact with soil, water, and wild birds, leading to sustained fecal shedding [5]. Longitudinal studies in meat-type ducks show that Salmonella shedding can persist over weeks, with intermittent peaks [5]. Similarly, Campylobacter carriage in waterfowl from Southern Ontario was found to be widespread, with prevalence rates varying by sampling location and season [8]. In live bird markets in Nigeria, ESBL-producing E. coli was detected in 34.7% of apparently healthy ducks, indicating that these birds serve as reservoirs for resistant clones [3]. In Zimbabwe, fecal carriage of ESBL and colistin-resistant E. coli in ducks was documented over a two-year period, demonstrating temporal stability [4].

Ducks also carry Riemerella anatipestifer, which causes epornitic outbreaks of septicemia and serositis in ducklings, though this pathogen is not a major human health threat [1]. However, diagnostic confusion with other bacteria (e.g., Pasteurella multocida) underscores the need for specific detection [1].

Clinical Signs and Pathology

Many bacterial pathogens carried by ducks do not cause overt disease in adult birds. Salmonella infection in ducks is often subclinical, with birds functioning as asymptomatic shedders [7]. In ducklings, however, Salmonella can cause diarrhea, depression, and increased mortality [1]. Campylobacter colonization in ducks is typically asymptomatic, with high bacterial loads in the ceca [8]. Pathology associated with E. coli in ducks ranges from colibacillosis (airsacculitis, pericarditis, peritonitis) in young birds to asymptomatic fecal carriage in adults [1, 3, 4]. Riemerella anatipestifer infection manifests as fibrinous polyserositis, meningitis, and arthritis in ducklings, with case fatality rates up to 75% [1].

Listeria monocytogenes does not typically cause clinical disease in ducks, but its excretion poses a foodborne risk [2]. Arcobacter spp. are considered commensals in the duck intestinal tract, with no specific pathology described [9]. Acinetobacter indicus is an emerging pathogen that can cause septicemia in immunosuppressed ducks and may serve as a reservoir for carbapenemase genes [6].

Diagnostics

Accurate detection of bacterial carriage in ducks requires culture-based isolation, molecular assays, and antimicrobial susceptibility testing. Conventional culture methods using selective media (e.g., MacConkey agar for E. coli, XLD agar for Salmonella, Campy-Cefex for Campylobacter, PALCAM for Listeria) remain the gold standard [2, 7, 8]. However, these methods are time-consuming and may fail to detect low-level carriage or mixed infections.

A significant advance is the development of a multiplex PCR (mPCR) assay targeting four major duck pathogens: Pasteurella multocida (KMT1 gene), Salmonella enterica (invasion protein gene), Riemerella anatipestifer (16S rDNA), and Escherichia coli (alkaline phosphatase gene) [1]. This mPCR has a detection limit of 10 pg of genomic DNA, and shows no cross-reactivity with other avian bacteria such as Staphylococcus aureus or Mycoplasma spp. [1]. The mPCR can be applied directly to field samples (cloacal swabs, tissues) and yields results within 4 hours, making it highly suitable for surveillance [1].

For antimicrobial resistance profiling, disk diffusion and broth microdilution are standard [3, 4]. Molecular detection of resistance genes (e.g., blaCTX-M, mcr-1, tet(X5), blaNDM-3) is performed using PCR or whole-genome sequencing [3, 4, 6]. High-throughput sequencing is increasingly used to characterize the resistome and virulence gene repertoire of duck-origin bacteria [3, 6].

The following decision tree illustrates a diagnostic workflow for investigating bacterial carriage in ducks.

flowchart TD
    A[Clinical specimen: cloacal swab, fecal sample, or tissue], > B{Selective culture}
    B, > C[MacConkey / XLD / Campy-Cefex / PALCAM]
    C, > D[Isolate colonies]
    D, > E{Initial identification}
    E, > F[Biochemical tests / MALDI-TOF]
    E, > G[Multiplex PCR (Wei et al. 2013)]
    G, > H{Positive for target?}
    H, >|Salmonella enterica| I[Serotyping & AMR testing]
    H, >|Escherichia coli| J[ESBL/mcr PCR]
    H, >|Riemerella anatipestifer| K[Confirm 16S rDNA]
    H, >|Pasteurella multocida| L[Confirm KMT1]
    I, > M[Report & isolate banking]
    J, > M
    K, > M
    L, > M
    H, >|Negative| N[Consider other pathogens: Arcobacter, Listeria, Acinetobacter]
    N, > O[Species-specific PCR or WGS]
    O, > M

Treatment and Antimicrobial Resistance

Antimicrobial therapy in ducks should be guided by culture and susceptibility results. For colibacillosis, amoxicillin-clavulanate or fluoroquinolones may be used, but emerging resistance is a major concern [1, 3, 4]. Salmonella infections in ducks are generally not treated with antibiotics to avoid selection of resistance; instead, depopulation and hygiene measures are preferred [5]. Campylobacter carriage is not routinely treated in live ducks due to the difficulty of eradication [8].

Resistance profiles in duck-origin bacteria are alarming. ESBL-producing E. coli from ducks in Nigeria showed high resistance to ceftriaxone, cefotaxime, and ciprofloxacin [3]. Colistin resistance mediated by mcr-1 was detected in E. coli from ducks in Zimbabwe and Thailand [4, 5]. The co-occurrence of tet(X5) (tigecycline resistance) and blaNDM-3 (carbapenem resistance) in Acinetobacter indicus of duck origin highlights the role of ducks as reservoirs of last-resort antibiotic resistance [6]. Salmonella isolates from ducks in Trinidad harbored multiple virulence genes and resistance to tetracycline and sulfonamides [7]. Campylobacter from waterfowl in Canada showed resistance to ciprofloxacin and erythromycin [8].

Control and Prevention

Strategies to reduce bacterial carriage in ducks include biosecurity, vaccination, feed additives, and flock management. All-in/all-out production, cleaning and disinfection of facilities, and control of wild bird access are critical to reduce environmental contamination [5]. Probiotics and competitive exclusion products can decrease Salmonella colonization in young ducklings. Vaccination against Riemerella anatipestifer and Pasteurella multocida is available in some regions but does not target zoonotic pathogens [1].

At the live bird market level, improved sanitation and reduced holding times can lower fecal shedding of ESBL E. coli [3]. In free-range systems, separation of ducks from other poultry and livestock reduces cross-contamination [2]. Surveillance programs using PCR-based methods enable rapid detection of carriers and implementation of quarantine measures [1].

One Health and Public Health Implications

Ducks are a nexus for the transmission of a wide range of zoonotic bacterial pathogens. Direct contact with duck feces, handling of raw duck meat, and consumption of undercooked duck products are primary exposure routes [7, 8]. The presence of multidrug-resistant organisms (MDROs) in apparently healthy ducks poses a risk of environmental dissemination through manure used as fertilizer [4, 6]. Food safety interventions must target the entire farm-to-fork continuum, with special attention to the asymptomatic nature of carriage.

For further reading on related topics, see the articles on Duck Diseases: Comprehensive Review of Viral and Bacterial Pathogens, Salmonella in Poultry: Food Safety and Public Health Concerns, and Bacterial Pathogens in Poultry: Prevalence, Public Health Risks, and Control Strategies. The reader is also directed to the article on Duck Disease: Comprehensive Veterinary Reference on Common Pathogens and Clinical Management for additional clinical context.

Conclusion

Ducks carry a diverse array of bacterial pathogens of public health concern, including Salmonella, Campylobacter, ESBL- and colistin-resistant E. coli, Listeria monocytogenes, Arcobacter, and multidrug-resistant Acinetobacter. Asymptomatic carriage is common, complicating surveillance and control. Multiplex PCR assays provide rapid, specific detection of multiple pathogens simultaneously, aiding in epidemiological studies and outbreak investigations [1]. Antimicrobial resistance in duck-origin bacteria is a growing global threat requiring coordinated One Health action. Effective control depends on stringent biosecurity, prudent antimicrobial use, and routine monitoring of both clinical and subclinical infections.

References

[1] Wei B, Cha S, Kang M, et al. Development and application of a multiplex PCR assay for rapid detection of 4 major bacterial pathogens in ducks. Poult Sci. 2013. URL: https://www.semanticscholar.org/paper/546ce9389a94c127f94a1ce872964fc9d7e6627e

[2] Njagi L, Mbuthia P, Bebora L, et al. Carrier status for Listeria monocytogenes and other Listeria species in free range farm and market healthy indigenous chickens and ducks. East Afr Med J. 2004. URL: https://www.semanticscholar.org/paper/10b466cef201ddf0fe0200627e968af32459066d

[3] Bakre AA, Adekanmbi AO, Ajani I, et al. Genetic diversity and antibiogram of ESBL-producing Escherichia coli isolated from apparently healthy birds sold at two selected live bird markets in Nigeria. Mol Biol Rep. 2025. URL: https://pubmed.ncbi.nlm.nih.gov/40172679/

[4] Takawira FT, Pitout JDD, Thilliez G, et al. Faecal carriage of ESBL producing and colistin resistant Escherichia coli in avian species over a 2-year period (2017-2019) in Zimbabwe. Front Cell Infect Microbiol. 2022. URL: https://pubmed.ncbi.nlm.nih.gov/36619741/

[5] Assawatheptawee K, Punyadi P, Luangtongkum T, et al. Research Note: Longitudinal fecal shedding patterns and characterization of Salmonella enterica and mcr-positive Escherichia coli in meat-type ducks raised in an open-house system. Poult Sci. 2022. URL: https://pubmed.ncbi.nlm.nih.gov/36055024/

[6] Tang B, Yang H, Jia X, et al. Coexistence and characterization of Tet(X5) and NDM-3 in the MDR-Acinetobacter indicus of duck origin. Microb Pathog. 2021. URL: https://pubmed.ncbi.nlm.nih.gov/33347964/

[7] Kumar N, Mohan K, Georges K, et al. Occurrence of Virulence and Resistance Genes in Salmonella in Cloacae of Slaughtered Chickens and Ducks at Pluck Shops in Trinidad. J Food Prot. 2021. URL: https://pubmed.ncbi.nlm.nih.gov/32818228/

[8] Vogt NA, Pearl DL, Taboada EN, et al. Carriage of Campylobacter, Salmonella, and Antimicrobial-Resistant, Nonspecific Escherichia coli by Waterfowl Species Collected from Three Sources in Southern Ontario, Canada. J Wildl Dis. 2019. URL: https://pubmed.ncbi.nlm.nih.gov/31021686/

[9] Atabay HI, Wainø M, Madsen M. Detection and diversity of various Arcobacter species in Danish poultry. Int J Food Microbiol. 2006. URL: https://pubmed.ncbi.nlm.nih.gov/16516995/ *** 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.