Research Article

Detection, Molecular Characterization and Antibiotic Susceptibility of Riemerella anatipestifer in Muscovy Ducks in Dong Thap Province

Nguyen Thu Tam*, Nguyen Tran Phuoc Chien, Nguyen Khanh Thuan

Faculty of Veterinary Medicine, College of Agriculture, Can Tho University, Can Tho, Vietnam.

Received | December 18, 2024; Accepted | March 10, 2025; Published | May 20, 2025

*Correspondence | Nguyen Thu Tam, Faculty of Veterinary Medicine, College of Agriculture, Can Tho University, Can Tho, Vietnam; Email: [email protected]

Citation | Tam NT, Chien NTP, Thuan NK (2025). Detection, molecular characterization and antibiotic susceptibility of Riemerella anatipestifer in muscovy ducks in Dong Thap province. Adv. Anim. Vet. Sci. 13(6): 1283-1291.

DOI | https://dx.doi.org/10.17582/journal.aavs/2025/13.6.1283.1291

ISSN (Online) | 2307-8316; ISSN (Print) | 2309-3331

Copyright: 2025 by the authors. Licensee ResearchersLinks Ltd, England, UK.

This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https://creativecommons.org/licenses/by/4.0/).

INTRODUCTION

Riemerella anatipestifer is a bacterial pathogen that affects ducklings, gosling, turkeys, and other fowl. Clinical signs include ocular and nasal discharge, mild coughing and sneezing, watery green feces, and neurologic signs progressing to obtundation and death (Pathanasophon et al., 1995). Symptoms typically include nasal discharge, lethargy, neurological signs, and severe systemic infections, with Muscovy ducks (Cairina moschata) being particularly vulnerable. In Dong Thap province, Viet Nam, where Muscovy ducks are widely raised in intensive farming systems, outbreaks of R. anatipestifer pose a considerable threat to local livelihoods and food security.

The region’s intensive farming practices often facilitate R. anatipestifer, co-infections including Escherichia coli, Pasteurella multocida, and avian influenza virus, together with environmental stressors, exacerbate the illness (Hu et al., 2011; Megahed et al., 2023). Molecular techniques are essential for diagnosing and researching Riemerella anatipestifer, facilitating precise and rapid diagnosis for epidemic control and prevention. PCR techniques aimed at conserved genes such as 16S rRNA and the ompA gene have validated R. anatipestifer infections in ducks and other avian species (Tsai et al., 2005; Wang et al., 2014). Recent findings indicate that molecular diagnostics can differentiate Riemerella anatipestifer from other bacterial illnesses exhibiting similar clinical symptoms, facilitating timely intervention. The phylogenetic study elucidates genetic connections across R. anatipestifer strains, augmenting molecular detection. Diverse serotypes of R. anatipestifer influence disease severity, host susceptibility, and vaccine efficacy (Wang et al., 2014; Yang et al., 2024a). Globally, strains from different regions have distinct molecular signatures, reflecting environmental adaptations and agricultural practices (Zheng et al., 2023). The genetic characteristics of R. anatipestifer isolates in Dong Thap province remain unidentified since host specificity, environmental factors, and intensive agricultural practices may influence local strains.

This research may improve techniques for addressing the R. anatipestifer epidemic in Dong Thap and other areas. Comprehending the genetic diversity and antibiotic susceptibility of local R. anatipestifer strains may facilitate the development of effective regional vaccines (Yang et al., 2024b). The study underscores the need to use molecular and phylogenetic techniques in disease surveillance, management initiatives, and antibiotic susceptibility to promote sustainable poultry production in Mekong Delta, Viet Nam. These results will provide the foundation for further research on R. anatipestifer strain genetic evolution, serotype diversity, antibiotic susceptibility, and adaptation (Xihui et al., 2023), ensuring sustainable disease control in Muscovy duck populations.

MATERIALS AND METHODS

Sampling

Muscovy ducks suspected of septicemia and those undergoing necropsy exhibited characteristic lesions associated with Riemerella anatipestifer infection, including fibrinous airsacculitis, fibrinous pericarditis, fibrinous perihepatitis, and splenomegaly with a marbled appearance.

Samples were collected from raised-in free-range systems in Dong Thap province between April 2024 and November 2024. Clinical specimens, including blood, heart, liver, spleen, and lungs displaying characteristic lesions of Riemerellosis, were collected under sterile conditions, placed in sterile bags, and stored at low temperatures in compliance with QCVN 01-83:2011/BNNPTNT regulations. Random sampling was conducted in duck flocks suspected of R. anatipestifer infection based on reports from Muscovy duck farms and local veterinary authorities. A total of 68 suspected R. anatipestifer-infected flocks were sampled, with five symptomatic ducks selected per flock, forming one pooled representative sample per flock for further laboratory analysis.

Cultivation and Isolation of Riemerella anatipestifer

The clinical samples were homogenized under sterile conditions in the laboratory. The samples were finely ground and placed in test tubes containing 3 ml of phosphate-buffered saline (PBS), followed by homogenization using a vortex mixer. The homogenized suspensions were cultured on trypticase soy agar (TSA) supplemented with 5% sheep blood and incubated at 37°C for 24–48 hours under 5% CO₂ anaerobic conditions.

Bacterial identification was performed based on colony morphology observed on blood agar, following the descriptions of Sandhu (2008) and Soman et al. (2014). Colonies of R. anatipestifer were identified as 1–2 mm in diameter, raised, round, transparent, and shiny. Suspected R. anatipestifer colonies were selected and subcultured onto blood agar for further purification. Purified colonies were propagated on nutrient agar (NA) under anaerobic conditions at 37°C for 24–48 hours to ensure bacterial growth and isolate pure strains.

A series of biochemical tests were performed to confirm the identity of R. anatipestifer, including A series of biochemical tests were performed, including carbohydrate fermentation, citrate utilization, motility and acetoin production, multiple sugar utilization, indole production, urease activity, mannitol utilization, and oxidase and catalase tests using paper disks.

Gram staining was performed on suspected bacterial broth or colonies of R. anatipestifer to prepare bacterial smears for microscopic examination. Additionally, PCR targeting the 16S rRNA gene (Tsai et al., 2005) was conducted to confirm the presence of R. anatipestifer strains molecularly. This molecular approach ensured high specificity and accuracy in bacterial identification.

Extract DNA, PCR, Sequence Alignment and Phylogenetics

The genomic DNA of R. anatipestifer was isolated from six representative samples utilizing the TopPURE® RNA/DNA Viral Extraction Kit (ATB, Viet Nam) and quantified at 100 ng/µL using the NanoDrop™ 2000. PCR targeted the 16S rRNA gene with primers F-5’-CAGCTTAACGTAGAACTGC-3’ and R-5’-TCGAGATTTGCATCACTTCG-3’ (662 bp). The PCR comprised a 25 μL reaction mixture containing 12 μL GoTaq® DNA Polymerase (Promega, USA), 01 μL of each 10 μM primer, 09 μL deionized water, and 02 μL DNA template and utilizing a thermal cycling regimen of t 95°C for 4 min, 35 cycles of 95°C for 1 min, 55°C for 1 min, and 72°C for 1 min, followed by a final extension of 72°C for 7 min. The PCR amplicons were stained with 6X GelRed Loading Buffer with Tricolour (ABT, Viet Nam) and visualized using 1.5% agarose gel electrophoresis under LED illumination, followed by photography. Nuclease-free water served as a negative control. A 100 bp DNA Ladder (PhuSa, Viet Nam) was utilized as a reference for determining the molecular mass of PCR results.

The 16S rRNA inserts were sequenced by Sanger sequencing, analyzed using BLAST (NCBI, https://blast.ncbi.nlm.nih.gov/), and submitted to GenBank. The 16S rRNA gene sequences were aligned using the MUSCLE technique, and genetic analysis was conducted using a phylogenetic tree incorporating select gap-free sites. Phylogenetic trees were rebuilt utilizing the maximum likelihood (ML) approach used in MEGA software version 11.0.26. Bootstrap analysis with 1000 iterations was employed to evaluate the reliability of the branching pattern of the maximum likelihood trees. The evolutionary distances were assessed using the Kimura-2 parameter model. The trees were generated and annotated utilizing the Interactive Tree of Life (iTOL) tool (https://itol.embl.de/).

Bacterial Antibiotic Susceptibility Test

Based on the Clinical and Laboratory Standards Institute (CLSI) antibiotic susceptibility test standards (CLSI: 2019) and local waterfowl antibiotic use, cefotaxime, tetracycline, gentamycin, florfenicol, ampicillin, and enrofloxacin were chosen for bacterial antibiotic susceptibility testing. CLSI recommends the Kirby-Bauer technique to assess the isolated isolates’ resistance profile. TSA plate colonies were inoculated into TSB medium (5% newborn sheep serum) and incubated at 37℃ on a shaker for 12 hours. After centrifuging 1 mL of the bacterial sample to remove the culture medium, the purified sample was adjusted to 0.5 McFarland units using normal saline. A sterile cotton swab was dipped into the diluted bacterial solution and placed uniformly on Mueller-Hinton (MH) agar medium (5% newborn sheep serum). The culture dish lid was opened and blow-dried for 3 min. An antibiotic-sensitive tablet (Oxoid Co., Ltd.) was connected to MH medium and cultured at 37℃ for 24 hours in a 5% CO2 incubator. The inhibition zone width of each strain was measured, and antibiotic resistance was evaluated using the CLSI antibiotic sensitivity test standard breakpoint. MDR bacteria are resistant to three or more antibiotic classes (Magiorakos et al., 2012).

Statistical Analysis

A map of sampled Muscovy duck flocks was created utilizing QGIS version 3.28.0. Raw data were statistically processed using Microsoft Excel 2019. Prevalence trends and pathogen detection across tissue samples were analyzed using the Chi-square test in R version 4.2.1, with a 95% confidence level (p<0.05). If the expected value was more significant than 5, statistical analysis was performed using the Chi-square test in R version 4.2.1.

 

 

RESULTS AND DISCUSSION

Detection and Sample Information

A total of 26/68 (38.24%) isolates were confirmed as R. anatipestifer based on colony morphology, biochemical characteristics, and molecular identification. The colonies exhibited typical R. anatipestifer characteristics, including 1–2 mm diameter, raised, round, transparent, and shiny appearance (Figure 1), consistent with descriptions by Sandhu (2008) and Soman et al. (2014). Biochemical characterization revealed (Figure 2) weak fermentation of select sugars without gas production, negative results for citrate utilization, Voges-Proskauer (VP) test, indole production, urease activity, and mannitol fermentation. Additionally, R. anatipestifer was oxidase- and catalase-positive (Figure 3), differentiating it from Pasteurella multocida, which vigorously ferments sugars and is both catalase- and oxidase-positive, and Escherichia coli, which is citrate-positive, indole-positive, and exhibits vigorous sugar fermentation. Gram staining of bacterial isolates confirmed Gram-negative pleomorphic rods, consistent with R. anatipestifer morphology (Figure 4). Molecular confirmation through PCR targeting the 16S rRNA gene identified all 26 isolates as R. anatipestifer with a 100% positivity rate (Figure 5). These findings reinforce the reliability of combining traditional bacterial isolation methods with molecular diagnostics for accurately identifying and surveilling R. anatipestifer infections in backyard Muscovy ducks.

 

This study investigated the prevalence of R. anatipestifer infection across six districts in Dong Thap province, surveying 68 backyard Muscovy duck farms. The overall infection rate was 38.24% (26/68 farms), with significant district-specific variations in morbidity rates (Table 1). Bacterial isolation and cultivation methods confirmed 26 R. anatipestifer strains from 1–2-month-old ducks, with regional distribution patterns of positive and negative cases depicted in Figure 6. The highest morbidity rate was recorded in Lai Vung (55.56%), followed by Lap Vo (50.00%) and Thap Muoi (40.00%) (p>0.05). These districts may have higher environmental risk factors, including poor biosecurity, increased duck densities, and favorable bacterial survival and transmission conditions. Conversely, Cao Lanh (29.41%) and Tam Nong (30.77%) had the lowest morbidity rates, suggesting better farm management practices, reduced bacterial load, or environmental factors that limit pathogen spread. The overall morbidity rate of 38.24% was higher than the 16.91% prevalence reported in commercial ducks by Vo et al. (2022) but lower than the 90.0 to 96.8% infection rates recorded in wild ducks in South Korea (Cha et al., 2015). The difference between commercial and backyard poultry systems may be attributed to variations in biosecurity measures, vaccination protocols, and overall farm management practices.

 

 

Table 1: Prevalence and epidemiological trends of Riemerella anatipestifer infection in backyard Muscovy ducks.

Districts	Farms
	No.	positive	Morbidity rate (%)
Hong Ngu	9	3	33.33
Tam Nong	13	4	30.77
Thap Muoi	10	4	40.00
Cao Lanh	17	5	29.41
Lap Vo	10	5	50.00
Lai Vung	9	5	55.56
Total	68	26	38.24

 

 

Table 2: Pathogen detection across different tissue samples in Riemerella anatipestifer-infected Muscovy ducks.

Specimen	Samples	Sample positive	Pathogen detection rate (%)
Blood	68	24	35.29
Heart	68	17	25.00
Liver	68	19	27.94
Spleen	68	22	32.35
Lungs	68	12	17.65

 

The detection rate of R. anatipestifer varied among tissue samples collected from 68 backyard Muscovy ducks suspected of infection (Table 2). The highest detection rate was observed in blood samples (35.29%), indicating bacteremia and systemic dissemination, a hallmark of R. anatipestifer septicemia. This finding aligns with the systemic nature of R. anatipestifer infections, where the bacterium spreads via the bloodstream to multiple organs. The spleen (32.35%) and liver (27.94%) exhibited high positivity rates, suggesting that R. anatipestifer exhibits a strong affinity for reticuloendothelial organs, which play a key role in the immune response. This supports previous findings that R. anatipestifer can persist and replicate in immune-related tissues, as Soman et al. (2014) noted. The heart (25.00%) showed a moderate detection rate, consistent with fibrinous pericarditis observed during necropsy, indicating cardiac involvement in systemic infection. The lungs had the lowest detection rate (17.65%), possibly due to lower bacterial colonization in pulmonary tissues compared to systemic circulation and immune-associated organs. This suggests that respiratory transmission may not be the primary route of infection, and systemic spread plays a more significant role in disease progression. However, the differences in R. anatipestifer detection rates across tissue samples were not statistically significant (p>0.05), indicating that the pathogen disseminates widely rather than exhibiting strong organ-specific localization. Soman et al. (2014) described R. anatipestifer infection in ducks as a septicemia disease that affects multiple internal organs, including the heart, liver, spleen, and lungs. Following initial infection, the bacteria enter the bloodstream and spread to major organs, leading to typical pathological lesions such as fibrinous pericarditis, perihepatitis, and splenomegaly. The current study’s findings support this mechanism, as R. anatipestifer was detected in all major organs examined, though at varying rates.

 

Phylogenetic Analysis

The phylogenetic relationships of R. anatipestifer isolates from six Muscovy duck domestic in Dong Thap province, Viet Nam, were established using comparative analysis of 16S rRNA gene sequences, as depicted in Figure 7. Alongside the field isolates (n=6), the study used reference strains of R. anatipestifer (n=54), R. columbina (n=4), R. columbipharyngis (n=5), and Bergeyella zoohelcum (n=6), all of which are closely associated with R. anatipestifer within the Weeksellaceae family. The strains were categorized into four primary branches, each representing a unique bacterial species—the field isolates of Muscovy ducks grouped with the 54 R. anatipestifer strains. Phylogenetic clustering revealed three distinct groups (Figure 7). The first cluster contained 23 reference sequences retrieved from GenBank, derived from ducks, geese, and chickens across Poland, China, South Korea, Central Vietnam, India, and Mexico. The second cluster, which included 31 sequences, predominantly comprised strains from ducks and turkeys in China, South Korea, Taiwan, and Poland. Notably, all six Vietnamese isolates (GenBank accession numbers PQ770683–PQ770688) were positioned within this second cluster, reinforcing their close genetic proximity to East Asian R. anatipestifer strains. The third cluster contained diverse strains from multiple avian hosts, showing more significant genetic variability, potentially reflecting strain adaptations to different poultry production systems.

 

The pairwise identity analysis of the 16S rRNA gene sequences revealed a high genetic similarity (99.01–100%) between the R. anatipestifer strains isolated from Muscovy ducks in Dong Thap province, Vietnam, and those from various global locations (Figure 8). The color-coded matrix of pairwise identity scores highlights four distinct clusters corresponding to different geographic regions and countries, reflecting the phylogenetic grouping of R. anatipestifer strains. Notably, six R. anatipestifer strains from Muscovy ducks in Dong Thap exhibited 100% genetic identity with strains isolated from ducks in China and central Vietnam. This finding suggests a close evolutionary relationship and potential transmission links between these regions, likely facilitated by cross-border poultry trade, migratory birds, or similar ecological and environmental conditions supporting bacterial persistence.

The study revealed a substantial prevalence of R. anatipestifer in Muscovy ducks, and phylogenetic analysis of incomplete 16S rRNA gene sequences demonstrated a strong genetic relationship with R. anatipestifer strains from commercial ducks reared on breeding farms in Vietnam and China. Additionally, the R. anatipestifer isolates obtained in this investigation exhibited substantial genetic similarity to those derived from migratory ducks, geese, poultry, and turkeys. These results suggest that R. anatipestifer strains from commercial poultry may persist in wild avian populations; they are not always reintroduced into poultry farms. The data refutes the hypothesis that domesticated Muscovy ducks serve as intermediate hosts for the transmission of R. anatipestifer to commercial duck populations; however, additional research is required with a larger sample of wild ducks.

The phylogenetic tree and pairwise identity analysis of the 16S rRNA gene confirmed a high genetic similarity between R. anatipestifer isolates from Muscovy ducks in Dong Thap, Vietnam, and strains from China, South Korea, Taiwan, and Poland (Figure 7 and Figure 8). The sequences showed minimal genetic diversity, indicating strong genetic conservation across different regions. However, the lack of host-specific clustering suggests that R. anatipestifer can infect multiple avian hosts, including ducks, geese, chickens, and turkeys. This finding highlights its potential for cross-species transmission, emphasizing the need for enhanced biosecurity and surveillance in poultry farming to mitigate its spread.

 

Table 3: Antibiotic susceptibility profile of Riemerella anatipestifer isolates (n=26).

Antibiotic	Concentration (µg)	Susceptible		Resistant
		Isolates	rate (%)	Isolates	rate (%)
Enrofloxacin	5	5	19.23	21	80.77
Ampicillin	10	7	26.92	19	73.08
Florfenicol	30	13	50.00	13	50.00
Gentamycin	10	15	57.69	11	42.31
Tetracycline	30	22	84.62	4	15.38
Cefotaxime	30	26	100	0	0.00

 

Antibiotic Sensitivity and Resistance

This visual (Table 3 and Figure 9) highlights the high resistance rates of R. anatipestifer to enrofloxacin (21/26 isolates, 80.77%), ampicillin (19/26 isolates, 73.08%), florfenicol (13/26 isolates, 50.00%), gentamicin (11/26 isolates, 42.31%), and tetracycline (4/26 isolates, 15.38%) while demonstrating complete sensitivity to cefotaxime (100%). Field evaluations of Muscovy ducks raised in backyards in Dong Thap province suggest that cefotaxime and tetracycline are seldom employed, which may account for the high susceptibility of R. anatipestifer to these antibiotics. Conversely, the prevalent treatment for duck septicemia caused by R. anatipestifer and other bacterial infections is the administration of antibiotics, including gentamicin, enrofloxacin, florfenicol, and ampicillin. Antimicrobial resistance has been exacerbated by the frequent and frequent indiscriminate use of these antibiotics, making treatment more difficult. The injudicious use of antibiotics, particularly for treating septicemic disease caused by R. anatipestifer, has led to the selective survival of antibiotic-resistant strains carrying resistance plasmids. These plasmids can be horizontally transferred to other bacterial strains, further propagating antimicrobial resistance (Chang et al., 2003). A study by Zhong et al. (2009) analyzed the antibiotic resistance patterns of 224 R. anatipestifer isolates collected between 1998 and 2005 in China, revealing high resistance levels to trimethoprim (75.9%) and sulfamethoxazole (79.2%). Similarly, Gyuris et al. (2017) investigated the antibiotic susceptibility of 185 R. anatipestifer isolates obtained from geese and ducks in Hungary between 2000 and 2014, showing that most isolates were susceptible to florfenicol (97.9%), ampicillin (95.1%), penicillin (93%), sulfamethoxazole/trimethoprim (92.4%), and spectinomycin (86.5%). In Vietnam, a study by Khai and Hau (2018) demonstrated that R. anatipestifer exhibited high susceptibility to ampicillin (80%), amoxicillin (70%), and trimethoprim (70%), while showing moderate sensitivity to tetracycline (60%) and doxycycline (60%). However, the study also reported high resistance to gentamicin (95%), sulfamethoxazole/trimethoprim (95%), and streptomycin (90%), highlighting a concerning trend of antimicrobial resistance in R. anatipestifer strains in Vietnam.

 

Antimicrobial overuse and improper application, particularly in the treatment of septicemia, are the primary causes of the emergence of antibiotic-resistant R. anatipestifer strains. This selective pressure enhances the persistence of antibiotic-resistant strains that contain resistance plasmids, which can be horizontally transmitted to other bacterial populations (Chang et al., 2003). This information is consistent with prior investigations. Khai and Hau (2018) conducted a study in Ben Tre province that revealed R. anatipestifer isolates exhibited complete sensitivity to cefotaxime, while they also exhibited variable degrees of resistance to other antibiotics, such as ampicillin and enrofloxacin. Additionally, research in China revealed that over 90% of R. anatipestifer isolates were resistant to various antibiotics, such as enrofloxacin and ampicillin (Yang et al., 2024b).

The adverse effects of excessive antibiotic use in veterinary medicine are highlighted by the elevated resistance rates of commonly used antibiotics. This highlights the pressing necessity of implementing rational antibiotic use policies and routine antimicrobial susceptibility testing to inform effective treatment tactics. Cefotaxime may be a viable treatment option for R. anatipestifer-induced infections, as evidenced by the complete sensitivity to this antibiotic observed in this study. In conclusion, the ongoing monitoring of antimicrobial resistance patterns in R. anatipestifer is essential to ensure the efficacy of treatment strategies and to reduce the risk of further resistance development, as evidenced by the complete susceptibility to cefotaxime and the increasing resistance to enrofloxacin and ampicillin.

 

Table 4: Results of multidrug-resistant testing of Riemerella anatipestifer isolates (n=26).

No. of Antibiotics	Resistance patterns	No. of isolates	(%)	Cumulative (%)
3	ENR+FL+AMP	6	23.08	38.46
	ENR+AMP+CN	4	15.38
4	TE+ENR+AMP+CN	1	3.85	3.85

 

ENR: enrofloxacin; FL: florfenicol; AMP: ampicillin; TE: tetracycline; CN: gentamicin.

 

Multidrug-Resistant Testing

The emergence of multidrug-resistant (MDR) strains of R. anatipestifer poses a significant challenge to poultry health management. The most prevalent resistance patterns were ENR+FL+AMP (23.08%) and ENR+AMP+CN (15.38%), with 42.31% (11/26) of the isolates demonstrating resistance to at least three antibiotics (Table 4). Despite this, all isolates maintained a 100% susceptibility to cefotaxime, while enrofloxacin (80.77%) and ampicillin (73.08%) exhibited elevated resistance rates (Table 4 and Figure 10). These findings are consistent with previous studies; according to Zhong et al. (2009), 87.8% of 224 R. anatipestifer isolates from 1998 to 2005 were trimethoprim-resistant, and over 50% were multi-resistant to 10 antimicrobials. Similarly, studies by Xihui et al. (2023) and Sun et al. (2012) found that 51 R. anatipestifer isolates from waterfowl were resistant to aminoglycosides, trimethoprim, lincosamides, polypeptides, and macrolides. Given the high prevalence of antibiotic resistance, the effective management of R. anatipestifer infections requires prudent antimicrobial use and comprehensive immunization strategies. The potential of cefotaxime as a viable treatment option is highlighted by the sustained susceptibility of R. anatipestifer to this antibiotic. Nevertheless, ensuring the long-term efficacy of cefotaxime in controlling R. anatipestifer infections by monitoring the emergence of resistance and constant surveillance is essential.

 

The emergence of MDR R. anatipestifer, with 42.31% of isolates resistant to at least three antibiotics, poses a significant challenge to poultry health management and disease control. Several strategies have been proposed to mitigate the spread of MDR strains. Enhanced biosecurity measures, such as implementing strict farm hygiene protocols, controlling poultry movement, and minimizing interactions with wild birds that may act as reservoirs for resistant strains, are crucial in preventing bacterial transmission. Additionally, judicious antibiotic use through antimicrobial susceptibility testing (AST) before treatment is essential to ensure targeted therapy and reduce antibiotic overuse, contributing to resistance selection. Continuous surveillance and monitoring programs are necessary to track emerging resistance patterns and guide policy decisions regarding antimicrobial use in poultry farming.

CONCLUSIONS AND RECOMMENDATIONS

This study highlights the genetic relatedness, phylogenetic clustering, and antimicrobial resistance (AMR) patterns of Riemerella anatipestifer in backyard Muscovy ducks from Dong Thap province. The isolates exhibited high genetic similarity with strains from commercial poultry in China, South Korea and Vietnam, suggesting potential transmission links. Phylogenetic analysis confirmed their close relationship with strains from ducks, geese, chickens, and turkeys. The findings also reveal a high prevalence of multidrug resistance, with increasing resistance to enrofloxacin and ampicillin, though susceptibility to cefotaxime remains intact. These results underscore the need for enhanced biosecurity, vaccination strategies, and continuous AMR surveillance to control R. anatipestifer infections and preserve antibiotic efficacy.

ACKNOWLEDGEMENTS

The authors thank the technical staff at the Dong Thap Provincial Sub-Department of Animal Husbandry and Veterinary Medicine for their assistance with sample collection. We also thank the Faculty of Veterinary Medicine, College of Agriculture, Can Tho University, for supporting sample sequencing and analysis. Finally, we sincerely appreciate the cooperation of poultry owners in the area.

NOVELTY STATEMENTS

This study is the first to investigate the presence, prevalence, and antibiotic susceptibility of Riemerella anatipestifer associated with septicemia in Muscovy ducks in Dong Thap province, providing essential data for effective diagnosis, treatment, and control strategies in the region.

AUTHOR’S CONTRIBUTIONS

All authors contributed to the research by participating in study design, data analysis, interpretation of findings, and manuscript preparation. All authors have reviewed and approved the final version of the manuscript.

Ethics Statement

This research was conducted on poultry with naturally occurring infections. Diagnostic samples were collected without performing any experimental procedures on the animals. Informed consent was obtained from poultry owners before sample collection, ensuring their voluntary participation in the study.

Conflict of Interest

We certify that there is no conflict of interest.

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