Bridging Continents and Pathogens: The Role of Migratory Birds in Antimicrobial Resistance Dissemination in Tropical Ecosystems

1. Introduction

Antimicrobial resistance [AMR] has emerged as one of the most critical global health challenges of the 21st century, undermining the efficacy of antibiotics that revolutionized modern medicine [1]. The World Health Organization has characterized AMR as a ‘silent pandemic’ with the potential to reverse decades of therapeutic progress and drive global mortality rates beyond those of tuberculosis, HIV/AIDS, and malaria by 2050 [2,3]. Although AMR is a global phenomenon, its effects are particularly severe in tropical and subtropical regions, where high population density, unregulated antimicrobial use, and weak sanitation infrastructure create ideal conditions for the emergence and dissemination of resistant pathogens [4,5]. In addition, tropical ecosystems sustain warm temperatures, high microbial diversity, and year-round aquatic productivity—environmental characteristics known to accelerate bacterial growth, horizontal gene transfer [HGT], and the persistence of antimicrobial resistance genes [ARGs] [6,7].

The persistence and global spread of ARGs are driven by complex ecological and evolutionary processes that extend beyond clinical environments [8]. Mobile genetic elements [MGEs], including plasmids, transposons, integrons, and bacteriophages, facilitate HGT and enable bacteria to acquire resistance traits across taxonomic and ecological boundaries[8,9]. Through this genetic mobility, hospitals, farms, wastewater systems, and natural habitats become linked within a single, interconnected global resistome [10]. Importantly, tropical regions often exhibit higher ARG and MGE abundance than temperate regions, reflecting climatic and ecological conditions that favour rapid microbial turnover and gene exchange [11].

Within this interconnected resistome, wildlife represents an often overlooked yet critical component of AMR ecology [12,13,14]. Among wildlife taxa, migratory birds warrant particular attention because they routinely traverse multiple ecological zones, including many of the world’s most contaminated tropical wetlands, rice paddies, deltas, and coastal lagoons—environments where exposure to ARGs is consistently high [12,15,16]. Through the ingestion of contaminated food or water, the gastrointestinal tracts of migratory birds may become colonized by antimicrobial-resistant bacteria [ARB], allowing these species to function as mobile reservoirs and dispersal agents of ARGs [17].

Each year, billions of migratory birds travel along major flyways spanning Africa, Asia, Europe, and the Americas, frequenting wetlands, agricultural landscapes, and urban waste sites often contaminated with antibiotic residues and resistant microorganisms [18,19,20]. Studies conducted along tropical flyways report widespread detection of clinically relevant ARGs through fecal shedding, environmental contamination, or direct interface with human-dominated ecosystems [18]. Through their transcontinental movements, migratory birds link distant ecosystems and facilitate the long-range redistribution of resistant bacteria and genetic material across ecological and geopolitical boundaries [17,19,20].

Despite this growing body of evidence, the role of migratory birds in the dissemination of AMR remains inadequately represented in global surveillance systems. While the One Health paradigm emphasizes the interconnectedness of human, animal, and environmental health [21], wildlife, particularly migratory birds traversing heavily impacted tropical ecosystems, is largely excluded from national and international AMR reporting frameworks such as WHO-GLASS [14]. At key tropical interfaces, including wetlands, rice paddies, and coastal lagoons, ecological and anthropogenic pressure converge, promoting bidirectional transfer of resistant bacteria and genes among wildlife, livestock, and human populations [22]. Surveillance efforts addressing these interfaces remain fragmented, geographically uneven, and disproportionately focused on high-income regions, despite the fact that many major migratory flyways intersect tropical areas where ARG contamination is most intense [14,23]

Against this backdrop, the present review consolidates current evidence on the role of migratory birds in the global dissemination of antimicrobial-resistant bacteria and ARGs, with a specific focus on tropical and subtropical flyways [19]. It addresses three guiding questions: [i] what evidence demonstrates that migratory birds in tropical ecosystems carry clinically relevant ARGs; [ii] which ecological interfaces and anthropogenic drivers facilitate ARG acquisition and dispersal; and [iii] how migratory birds can be effectively integrated into One Health–oriented AMR surveillance frameworks [24,25]. By synthesizing findings from surveillance studies and molecular epidemiology, this review highlights critical knowledge gaps and underscores the urgent need for harmonized, wildlife-inclusive AMR monitoring strategies that capture the transboundary flow of resistance across tropical ecosystems.

2. Antimicrobial Resistance [AMR] and Antimicrobial Resistance Genes [ARGs]

2.1. Mechanisms of Resistance

Antimicrobial resistance in bacteria associated with migratory birds results from a limited set of well-characterized mechanisms, including enzymatic inactivation of antibiotics, modification of antimicrobial targets, decreased membrane permeability, and active efflux [22,23,24,25,26,27]. In tropical aquatic environments, where resistant bacteria are prevalent, these mechanisms support colonization of avian gastrointestinal tracts and promote the persistence and dissemination of antimicrobial resistance genes [ARGs] across ecological boundaries ([28]; Table 1).

2.1.1. Enzymatic Degradation and β-Lactamases

Enzymatic degradation mediated by β-lactamases represents one of the most frequently documented resistance mechanisms in bacteria isolated from migratory birds [29,30]. These enzymes confer resistance to widely used β-lactam antibiotics and have been commonly reported in Enterobacteriaceae recovered from avian fecal samples, particularly in birds inhabiting or frequenting contaminated aquatic environments [31]. From an ecological perspective, the relevance of β-lactamases in migratory birds lies primarily in their environmental mobility rather than in their biochemical diversity [32]. β-lactamase–producing bacteria, including those carrying extended-spectrum β-lactamases [ESBLs] and carbapenemases, are consistently detected in tropical wetlands, wastewater-impacted waters, and agricultural runoff—key foraging and roosting habitats for migratory birds [33]. These environments facilitate the acquisition, persistence, and subsequent long-distance dispersal of clinically relevant resistance genes along migratory flyways.

2.1.2. Target Modification and Efflux Systems

Resistance in bacteria associated with migratory birds can also arise through modification of antimicrobial target sites and activation of efflux systems, mechanisms that reduce antibiotic susceptibility by limiting effective drug binding or intracellular accumulation [34]. Such resistance traits have been reported in bacteria recovered from avian fecal samples and from contaminated aquatic environments frequented by migratory birds, where persistent antibiotic pressure favors their maintenance [35,36]. In polluted tropical habitats, including wetlands, wastewater-impacted waters, and agricultural runoff, these mechanisms enhance bacterial persistence and contribute to the acquisition and retention of multidrug-resistant strains within avian hosts. Through repeated exposure at key foraging and roosting sites, migratory birds may subsequently facilitate the dispersal of these resistance traits along established flyways.

2.1.3. Mobile Genetic Elements: Plasmids, Integrons, and Transposons

Mobile genetic elements [MGEs] play a central role in shaping antimicrobial resistance profiles in bacteria associated with migratory birds by facilitating the transfer of antimicrobial resistance genes [ARGs] across environmental and host-associated microbial communities [37,38]. In tropical aquatic and terrestrial habitats frequently used by migratory birds, plasmid- and transposon-mediated gene exchange supports the persistence of resistance traits within bacterial populations exposed to sustained antimicrobial inputs [39]. Warm temperatures, high microbial diversity, and the presence of antibiotic residues in soil and water further enhance ARG retention and recombination, increasing the likelihood that migratory birds acquire and subsequently disseminate ARG-bearing bacteria along their migratory flyways [40,41,42].

2.2. Diminished Permeability and Limited Uptake

Reduced membrane permeability contributes to intrinsic antimicrobial resistance by limiting antibiotic entry into bacterial cells [43]. Such traits have been reported in Enterobacteriaceae isolated from tropical aquatic environments impacted by human and livestock wastewater, habitats commonly used by migratory birds for foraging and roosting [44]. Exposure to these contaminated systems may therefore facilitate the acquisition and persistence of intrinsically resistant bacteria within avian hosts, supporting their continued circulation along migratory flyways.

2.3. Horizontal Gene Transfer [HGT]

HGT is a key driver of antimicrobial resistance gene [ARG] dissemination within environmental bacterial communities and underlies the emergence of multidrug-resistant bacteria encountered by migratory birds [10,56,57,58,59]. In tropical aquatic and sedimentary environments frequented by migratory birds, conditions such as high nutrient loads, warm temperatures, and dense microbial assemblages promote frequent gene exchange and long-term ARG persistence [6,60,61,62,63]. These settings increase the likelihood that migratory birds acquire bacteria carrying transferable resistance determinants, thereby facilitating the redistribution of ARGs across geographic regions through migratory movement.

Table 1. Central mechanisms of antimicrobial resistance [AMR], molecular basis, mechanism of action, and representative antibiotics affected.

Mechanism of Resistance	Representative Genes or Enzymes	Mechanism of Action	Representative Antibiotics	References
1. Enzymatic inactivation	blaTEM, blaSHV, blaCTX-M[ESBLs]; blaNDM, blaKPC, blaOXA-48	Hydrolysis of β-lactam ring or other structural modification that inactivates the antibiotic	Penicillins, Cephalosporins, Carbapenems	[31,32,33,45,46]
2. Target modification	ermB [23S rRNA methylation]; qnrA/B/S [DNA gyrase protection]; tetM [ribosomal protection protein]	Alteration or protection of antibiotic binding sites reduces affinity for the drug target	Macrolides, Quinolones, Tetracyclines	[34]
3. Efflux pump activation	Multidrug efflux systems — ABC, MFS, RND families [acrAB-tolC, norA, mexAB-oprM]	Active transport of antibiotics out of the cell, lowering intracellular concentration	Fluoroquinolones, Tetracyclines, Chloramphenicol	[47,48,49,50]
4. Mobile genetic elements	Broad-host-range and conjugative plasmids [IncF, IncI, IncA/C types]	Horizontal transfer of ARGs via conjugation between bacteria	Multidrug resistance [across classes]	[51,52,53,54,55]
5. Integrons	intI1 and associated gene cassettes	Site-specific recombination	Multiple antibiotic classes	[39,40]
6. Transposons	Tn3, Tn21, and related insertion sequences	Movement of ARGs between plasmids and chromosomes	Multiple antibiotic classes.	[41,42]

Table 1. Central mechanisms of antimicrobial resistance [AMR], molecular basis, mechanism of action, and representative antibiotics affected.

Mechanism of Resistance	Representative Genes or Enzymes	Mechanism of Action	Representative Antibiotics	References
1. Enzymatic inactivation	blaTEM, blaSHV, blaCTX-M[ESBLs]; blaNDM, blaKPC, blaOXA-48	Hydrolysis of β-lactam ring or other structural modification that inactivates the antibiotic	Penicillins, Cephalosporins, Carbapenems	[31,32,33,45,46]
2. Target modification	ermB [23S rRNA methylation]; qnrA/B/S [DNA gyrase protection]; tetM [ribosomal protection protein]	Alteration or protection of antibiotic binding sites reduces affinity for the drug target	Macrolides, Quinolones, Tetracyclines	[34]
3. Efflux pump activation	Multidrug efflux systems — ABC, MFS, RND families [acrAB-tolC, norA, mexAB-oprM]	Active transport of antibiotics out of the cell, lowering intracellular concentration	Fluoroquinolones, Tetracyclines, Chloramphenicol	[47,48,49,50]
4. Mobile genetic elements	Broad-host-range and conjugative plasmids [IncF, IncI, IncA/C types]	Horizontal transfer of ARGs via conjugation between bacteria	Multidrug resistance [across classes]	[51,52,53,54,55]
5. Integrons	intI1 and associated gene cassettes	Site-specific recombination	Multiple antibiotic classes	[39,40]
6. Transposons	Tn3, Tn21, and related insertion sequences	Movement of ARGs between plasmids and chromosomes	Multiple antibiotic classes.	[41,42]

3. Environmental Interface and Migratory Birds in AMR Dissemination

Antimicrobial resistance [AMR] extends beyond clinical settings and has become a pervasive ecological challenge shaped by interactions among humans, animals, and the environment within the One Health continuum [71,72]. The natural environment functions as both a reservoir and conduit for ARGs, continually enriched by antibiotic residues, resistant bacteria, and mobile genetic elements released from hospitals, livestock operations, aquaculture systems, and municipal wastewater [6,35,64]. These anthropogenic inputs generate intense selective pressures that accelerate bacterial adaptation and horizontal gene transfer within environmental microbial communities [65,66,67,68]. Tropical wetlands, estuaries, and wastewater-impacted habitats are particularly vulnerable to AMR enrichment. Warm temperatures, high organic loads, and continuous water availability support dense microbial populations and enhance ARG persistence, allowing these ecosystems to function as ecological mixing zones where resistant strains emerge and circulate [70,73,74,75,76,77]. Such conditions amplify ARG mobility and increase exposure risk for wildlife species that rely on these habitats.

At the wildlife–environment interface, migratory birds serve as biological connectors, linking human-impacted environments across continents [78,79,80]; specifically, species that feed, roost, or nest near landfills, sewage lagoons, aquaculture ponds, and agricultural fields risk acquiring multidrug-resistant bacteria that harbor clinically relevant resistance determinants, such as β-lactamase and colistin resistance genes, reflecting exposure to anthropogenically contaminated environments [12,14,17,18,19,79,81,82,83,84,85,86,87,88]. Following acquisition, these birds may disseminate resistant microorganisms across ecological and geopolitical boundaries through fecal shedding during migration, thereby linking highly contaminated tropical sites with distant ecosystems [17,18,23,89].

Recent genomic studies conducted across Africa, Europe, and Asia have identified antimicrobial resistance gene [ARG] signatures in migratory birds that closely resemble those observed in local wastewater isolates, indicating active exchange between anthropogenic sources and wildlife along tropical flyways [12,79,90,91,92]. This genomic overlap reflects sustained exposure of migratory birds to contaminated tropical wetlands, wastewater-impacted waters, and other human-influenced aquatic environments, where resistant bacteria and ARGs are abundant. Species such as gulls, waterfowl, and shorebirds that forage in these habitats frequently harbor extended-spectrum β-lactamase–producing Escherichia coli and carbapenemase-producing Enterobacterales, mirroring resistance profiles reported in human and livestock populations [24,74,75,91,93,94,95,96,97].

Within avian gastrointestinal tracts, microbiome analyses indicate that ARGs can persist and undergo horizontal transfer, supporting long-distance carriage during migration and subsequent deposition into new environments [12,20,23,98]. The ecological plasticity of migratory birds, combined with broad habitat use, diverse foraging strategies, and long-distance movement, therefore enables the coupling of environmental reservoirs with transcontinental dispersal processes. In this way, tropical ecosystems function not only as hotspots for the emergence of antimicrobial resistance but also as critical nodes in a feedback loop linking environmental reservoirs, migratory birds, and anthropogenic AMR sources, particularly in regions where porous ecological boundaries, high human–wildlife contact, and limited biosecurity amplify opportunities for ARG transmission [99,100,101].

4. Evidence of AMR in Migratory Birds and Associated Pathogens

4.1. Extended-Spectrum β-Lactamase [ESBL] Producers

ESBL–producing Escherichia coli and other Enterobacteriaceae are among the most frequently reported antimicrobial-resistant organisms detected in migratory birds [14,20]. Since the first identification of ESBL-producing E. coli in pigeons in 1975, wild birds have increasingly been recognized as reservoirs of clinically relevant ESBL genes [14]. Exposure to agricultural fields treated with antimicrobials, wastewater-impacted wetlands, and urban refuse sites represents a significant pathway for migratory birds to acquire resistant strains [20,79]. As a widely used indicator organism for environmental AMR surveillance, E. coli has been extensively investigated in avian populations across multiple regions [80]. In Egypt, Ahmed et al. [2019] reported plasmid-mediated colistin resistance genes in migratory birds, with mcr-1 detected in 20% and mcr-2 in 3.6% of isolates [82]. In Pakistan, Mohsin et al. [2017] found that 17.3% of wild birds carried faecal ESBL-producing E. coli, of which 88.4% exhibited multidrug resistance [83]. Similarly, in Europe, Vergara et al. [2017] reported E. coli in 54.5% of faecal samples collected from gulls in Barcelona, with more than half of these isolates identified as ESBL producers [84].

4.2. Other Clinically Relevant Pathogens

4.2.1. Carbapenem-Resistant Enterobacteriaceae [CRE]

Although less frequently reported than ESBL producers, carbapenem-resistant Enterobacteriaceae [CRE] have been identified in migratory birds, raising concern regarding the dissemination of high-risk resistance determinants [102,103]. A notable example involved Salmonella enterica serovar Corvallis isolated from a black kite [Milvus migrans] in Germany carrying blaNDM-1, conferring resistance to cefotaxime and reduced susceptibility to carbapenems [104]. Phylogenetic analyses suggested dissemination from South or Southeast Asia to Europe via migratory routes, potentially involving stopover regions in North Africa and West Africa [104]. Supporting this hypothesis, Villa et al. [2015] demonstrated that plasmid pRH-1738 recovered from avian isolates closely resembled a human-associated plasmid from Afghanistan, indicating possible wildlife–anthropogenic exchange [103]. These findings underscore the ecological significance of migratory birds as potential long-distance carriers of carbapenemase-producing bacteria.

4.2.2. Colistin Resistance [mcr Genes]

Colistin represents a last-resort antimicrobial for the treatment of infections caused by carbapenem-resistant Gram-negative bacteria. The emergence of plasmid-mediated mcr genes therefore poses a substantial public-health concern. Multiple studies have reported mcr-positive Enterobacteriaceae in migratory birds, including detection of mcr-1 in European herring gulls [Larus argentatus] and in kelp gulls sampled in Ushuaia, Argentina [105,106]. In these cases, mcr-1 was frequently carried on IncI2 plasmids lacking ESBL genes, suggesting the circulation of wildlife-associated resistance lineages distinct from those commonly reported in livestock or clinical settings. The detection of mcr genes in migratory birds highlights their potential role in maintaining the environment and disseminating last-resort resistance determinants across geographic regions.

4.2.3. Zoonotic Pathogens

Salmonella enterica remains a zoonotic pathogen of major relevance to both human and livestock health and frequently exhibits antimicrobial-resistant phenotypes [107]. Migratory birds have been implicated in the dissemination of ESBL-producing and multidrug-resistant Salmonella during migration [85]. Prevalence studies have reported Salmonella isolation rates of 28.26% in migratory birds sampled in Egypt and 21.21% in Bangladesh [86]. Evidence of overlapping Salmonella serotypes between migratory birds and poultry farms in the Middle East further suggests potential cross-transmission at the wildlife–livestock interface [87].

Table 2. Evidence of antimicrobial resistance [AMR] in migratory birds and associated pathogens.

Pathogen	Host group	Resistance category	Antibiotics affected	Tropical region / flyway interface	References
Escherichia coli	Gulls, waterfowl, pigeons	ESBLs; MDR common; sporadic mcr	β-lactams, third-generation cephalosporins; colistin	Sub-Saharan Africa; South Asia [primary]; Europe [comparative]	[82,83,105,106,108]
Salmonella entericaserovar Corvallis	Raptors [black kite]	Carbapenemase [blaNDM-1]	Carbapenems	Africa–Eurasia flyway interface	[103,104]
Salmonellaspp.	Waterfowl, mixed wild birds	MDR; ESBL-associated	Multiple antibiotic classes	South Asia; Middle East	[85,86,87,158]
Campylobacterspp.	Shorebirds, waterfowl	Fluoroquinolone and macrolide resistance	Fluoroquinolones, macrolides	East Asia [tropical–subtropical interface]	[125,160]

Table 2. Evidence of antimicrobial resistance [AMR] in migratory birds and associated pathogens.

Pathogen	Host group	Resistance category	Antibiotics affected	Tropical region / flyway interface	References
Escherichia coli	Gulls, waterfowl, pigeons	ESBLs; MDR common; sporadic mcr	β-lactams, third-generation cephalosporins; colistin	Sub-Saharan Africa; South Asia [primary]; Europe [comparative]	[82,83,105,106,108]
Salmonella entericaserovar Corvallis	Raptors [black kite]	Carbapenemase [blaNDM-1]	Carbapenems	Africa–Eurasia flyway interface	[103,104]
Salmonellaspp.	Waterfowl, mixed wild birds	MDR; ESBL-associated	Multiple antibiotic classes	South Asia; Middle East	[85,86,87,158]
Campylobacterspp.	Shorebirds, waterfowl	Fluoroquinolone and macrolide resistance	Fluoroquinolones, macrolides	East Asia [tropical–subtropical interface]	[125,160]

5. Risk Factors for Antimicrobial Resistance Genes [ARGs] Dissemination

The dissemination of ARGs by migratory birds is driven by an interconnected system of anthropogenic, agricultural, and environmental factors that create ecological hotspots for exposure, acquisition, and spread of ARB [111]. These interfaces are particularly prevalent in tropical and subtropical ecosystems, where inadequate sanitation, high antimicrobial use, and intense human–animal–environment interactions converge. Within such settings, migratory birds frequently encounter ARG-enriched habitats, acquire resistant microorganisms, and redistribute them across continents along major flyways [Figure 1].

5.1. Human Activity

5.1.1. Antibiotic Misuse in Clinical Settings

Extensive and often inappropriate antibiotic use in clinical settings significantly contributes to the persistence and amplification of resistant bacteria in the environment [112]. Between 30% and 90% of administered antibiotics are excreted in biologically active forms and enter hospital wastewater systems [113]. Consequently, hospital effluents contain concentrated mixtures of antimicrobial residues, ARGs, and pathogenic bacteria [114,115]. Studies from China, the Netherlands, and Nepal have documented tetracycline, quinolone, carbapenemase, and last-resort resistance genes, including blaNDM and mcr, in untreated or partially treated hospital wastewater [114]. When discharged into municipal sewage systems or natural water bodies, these effluents create heavily contaminated aquatic zones that migratory birds frequently use for feeding and resting [116,117].

5.1.2. Urban Wastewater and Sewage as Hotspots

Industrial, domestic, and hospital effluents commonly converge in wastewater treatment plants [WWTPs] or are discharged directly into natural waterways, often with incomplete removal of ARB and ARGs despite multiple treatment stages [118]. ARGs and resistant bacteria have been detected throughout wastewater processing chains [113], and resistance determinants may persist for up to 20 km downstream of treated effluent discharge points [119]. Migratory birds frequently exploit these anthropogenic water bodies, where ARG concentrations remain elevated [18]. Studies from Europe and Asia report that gulls and other migratory species using wastewater-impacted habitats carry ESBL genes, carbapenemases, aminoglycoside resistance genes, and quinolone resistance determinants [18,70,120].

5.1.3. Ecotourism and Bird–Human Contact Zones

The expansion of ecotourism activities, including birdwatching, hiking, and nature-based recreation, has intensified direct and indirect interactions between humans and wild birds [121,122]. Human presence in previously undisturbed habitats can introduce resistant bacteria into wildlife environments and promote cross-species bacterial exchange [123]. In China and Egypt, Escherichia coli, Campylobacter, and Salmonella strains harboring ARGs have been isolated from migratory birds nesting near heavily visited tourist sites [124,125]. These contact zones increase opportunities for bidirectional ARG spillover between humans and wildlife, highlighting ecotourism as a subtle but increasingly relevant driver of AMR dissemination.

5.2. Livestock and Agricultural Practices

5.2.1. Antibiotics in Animal Feed and Aquaculture

Antimicrobial use in livestock production and aquaculture remains a major contributor to environmental contamination with AMR [99,126,127]. Up to 90% of administered antibiotics are excreted unmetabolized in manure and urine [128], introducing AMR residues and ARB into soils and aquatic systems. Manure application as fertilizer adds ARGs to agricultural fields, where they may spread to soil microbiota or enter surface waters through runoff [129,130,131]. Slaughterhouse effluents also contain clinically significant ARGs and persistent pathogens [132,133]. Downstream river systems have also been shown to harbor multidrug resistance determinants that accumulate in fish and other aquatic organisms [131,134,135].

5.2.2. Interaction Between Migratory Birds and Farm Environments

Farmlands frequently serve as stopover sites for migratory birds because of the availability of food and water [101]. Shared access to irrigation channels, harvested crop fields, and livestock watering points creates opportunities for ARG exchange among birds, domestic animals, and agricultural landscapes [101]. These interactions reinforce One Health linkages and facilitate the circulation of resistance determinants between wildlife and livestock populations.

5.2.3. Spillover of Resistance Genes into the Food Chain

ARGs carried by migratory birds pose food safety risks by contaminating crop and livestock environments. Bird fecal deposition can introduce resistant bacteria into soil, irrigation water, and animal feed [101,135]. Livestock exposed to ARG-contaminated water or feed may subsequently transmit resistant bacteria to humans through meat, milk, feces, or direct contact [126,137]. Fresh produce may also be affected; resistant E. coli and multidrug-resistant Acinetobacter baumannii have been isolated from fruits and vegetables irrigated with contaminated water [138,139]. These pathways highlight the role of migratory birds in linking environmental AMR reservoirs with food production systems.

6. One Health Implications

The dissemination of antimicrobial resistance by migratory birds exemplifies the interconnectedness of human, animal, and environmental health central to the One Health paradigm [4,5,12,21,23]. By moving between natural wetlands, tropical agricultural landscapes, coastal estuaries, and densely populated urban areas, migratory birds facilitate ecological mixing of antimicrobial-resistant bacteria [ARB] and antimicrobial resistance genes [ARGs] across geographic and political boundaries [18,70,96,120]. This transboundary connectivity highlights that AMR is not sustained within isolated sectors but through continuous biological exchange among wildlife, humans, livestock, and the environment [5,21,140,141,142]. Understanding these linkages is essential for developing One Health strategies that address the emerging role of migratory birds in the dissemination of resistance [8,14,143,144].

6.1. Human Health

6.1.1. Spillover of Multidrug-Resistant Pathogens

Migratory birds function as biological bridges connecting distant ecosystems and human populations [12]. Long-distance migrants using the American, African–Eurasian, and East Asian–Australasian flyways have been shown to harbor human-associated resistant pathogens, including Escherichia coli, Salmonella enterica, and Campylobacter jejuni [145,146,147,148,149]. Urban expansion intensifies bird–human interactions, particularly at peri-urban wetlands, garbage dumps, and wastewater-adjacent communities, increasing opportunities for pathogen spillover and indirect human exposure [150,151].

6.1.2. Public Health Surveillance Gaps

Current AMR surveillance systems remain heavily biased toward clinical and livestock settings, with limited integration of wildlife reservoirs. As climate change and land-use modification alter migratory routes and stopover sites, bird–human contact zones are shifting in ways that existing surveillance frameworks fail to capture [142]. Addressing these gaps requires wildlife-inclusive One Health surveillance, including AMR screening at significant migratory stopovers, coordinated public health alerts along flyways, genomic monitoring of avian isolates, and the integration of GPS tracking and remote-sensing data to identify high-risk ecological nodes [141].

6.2. Animal Health

6.2.1. Exposure of Domestic and Wild Animals

Migratory birds interact with resident birds, livestock, and wildlife at shared water sources, crop fields, and coastal wetlands, creating opportunities for interspecies exchange of ARB and ARGs [91,96,97]. Overlapping use of aquatic habitats and foraging grounds facilitates microbial exchange through direct contact and environmental contamination, promoting the local establishment of resistance within resident animal populations [91,152]. Over time, these processes blur traditional boundaries between wildlife and domestic animal reservoirs of AMR [152].

6.2.2. AMR Threat to Biodiversity and Veterinary Medicine

The introduction of ARGs into naïve wildlife populations can disrupt ecological equilibria and facilitate the emergence of novel pathogen lineages [143,153,154]. Widespread antimicrobial use in livestock, aquaculture, and companion-animal medicine contributes to environmental contamination with resistant bacteria [18,155], which are subsequently encountered by migratory birds foraging at waste sites, ponds, and agricultural interfaces [79]. Migratory birds may also introduce resistant parasites and associated microbiota into new wildlife populations, while limited therapeutic options for wildlife infections amplify AMR-related risks to biodiversity and veterinary medicine [150,154].

6.3. Environmental Health

6.3.1. Ecosystems as Persistent ARG Reservoirs

Each year, billions of migratory birds traverse diverse ecological zones while carrying bacteria that may harbor clinically relevant ARGs [19,140,156]. Owing to their ecological flexibility, many species, particularly waterbirds, act as effective environmental vectors and amplifiers of AMR [19,25,157]. Fecal shedding during migration and stopovers results in continuous deposition of ARB and ARGs into wetlands, estuaries, and agricultural landscapes [92,159]. Escherichia coli, a key indicator organism for AMR surveillance, is frequently isolated in these contexts due to its high capacity for gene exchange and environmental persistence [157,160].

6.3.2. Long-Term Cycling and Evolution of Resistance

Migratory birds contribute to the persistence and global cycling of ARGs, reinforcing a dynamic “global resistome” shaped by repeated introduction, amplification, and redistribution across continents [14,20,93,161]. Resident avian species further amplify these processes by sustaining ARGs within local environments [14]. Within these ecological networks, avian-associated microbial communities may undergo local adaptation, co-selection, and recombination, generating novel resistance profiles that can ultimately spill back into human and livestock microbiomes.

7. Perspective Piece: Recommendations and Future Directions

AMR has evolved from a primarily clinical challenge into an ecological and transboundary crisis, particularly across tropical regions where environmental contamination, dense human–animal interfaces, and shifting migratory pathways converge. Migratory birds, due to their intercontinental mobility and regular use of human-modified tropical habitats, function as both biological vectors and sentinels of ARGs within the One Health framework. Addressing this complexity requires integrated surveillance, genomic-scale monitoring, and coordinated policies linking environmental microbiology, veterinary medicine, conservation biology, and public health

7.1. Strengthening AMR Surveillance in Migratory Bird Populations

Current AMR surveillance systems remain heavily focused on human and livestock sectors, leaving major wildlife reservoirs, particularly migratory birds, poorly represented [14,94]. To address this gap, avian surveillance should be integrated into existing platforms such as the WHO Global Antimicrobial Resistance and Use Surveillance System [GLASS] and the OIE World Animal Health Information System [WAHIS]. Routine sampling at key ecological interfaces, including wetlands, rice paddies, landfills, sewage lagoons, and agricultural landscapes, would enable spatiotemporal mapping of ARG dynamics along tropical flyways [18,95]. Standardized protocols and harmonized molecular workflows are essential to ensure data comparability and effective early-warning systems.

7.2. Genomic and Metagenomic Approaches to Track ARGs

Advances in next-generation sequencing enable high-resolution characterization of resistomes in migratory birds and their environments. Whole-genome sequencing and metagenomic profiling provide insights into ARG diversity, plasmid mobility, and microbial community structure across tropical ecosystems [8,11,23]. When integrated with ecological metadata, migratory corridors, habitat characteristics, and climatic parameters, these data can support predictive models to identify emerging ARG dissemination routes [89,98].

7.3. Integrated One Health Policies for AMR Control

Effective AMR mitigation requires coordinated governance that explicitly incorporates wildlife into national and regional action plans [5,21]. Cross-sectoral collaboration among ministries of health, agriculture, and environment, supported by WHO, FAO, OIE, and UNEP, should prioritize integrated wildlife–environment–human AMR databases, shared early-warning tools, transboundary data exchange, and joint outbreak investigations in tropical regions [4]. Strengthening collaboration among microbiologists, veterinarians, ornithologists, ecologists, and public health authorities will ensure that AMR surveillance is ecologically grounded and globally harmonized [4].

7.4. Mitigation Strategies and Sustainable Interventions

Reducing selective pressure in tropical ecosystems requires coordinated interventions across agriculture, wastewater management, and habitat stewardship. Limiting non-therapeutic antibiotic use in livestock and aquaculture remains a central priority [126,127]. Upgrading wastewater treatment technologies, including membrane bioreactors, advanced oxidation processes, and biofiltration, can substantially reduce the release of ARGs into the environment [112,115,162]. Ecological restoration of wetlands and the establishment of buffer zones around migratory bird habitats can further reduce contamination and interspecies contact [79,144].

7.5. Practical, Low-Cost Veterinary and Farm Biosecurity Measures

Simple, affordable biosecurity interventions can significantly reduce opportunities for wildlife–livestock exchange of ARB and ARGs [22,79]. These include covering livestock feed and water troughs, restricting wild bird access to poultry areas, limiting shared water points, improving manure management through composting, and mitigating runoff from agricultural sites [152]. Such measures are particularly critical in tropical regions characterized by high wildlife–livestock overlap and have been shown to reduce environmental contamination and cross-species transmission [163].

7.6. Integrating Climate-Sensitive Monitoring into AMR Surveillance

Climate change is reshaping migratory bird phenology, habitat use, and flyway connectivity across tropical ecosystems, altering opportunities for ARG exchange [78,140]. To address these emerging risks, AMR surveillance should incorporate climate-based habitat suitability models, anomaly-driven alerts for drying or newly formed wetlands, and targeted sampling at climate-altered peri-urban and agricultural water bodies [22,152]. Early-warning systems combining climate anomaly detection with targeted microbiological sampling could help identify evolving ARG hotspots [112,115].

7.7. Ethical Considerations for Habitat-Level Interventions

Interventions aimed at mitigating AMR risks in wildlife habitats must be carefully designed to avoid unintended ecological harm. Habitat modification, avian removal, or environmental disinfection may disrupt migratory connectivity, affect non-target species, or damage sensitive ecosystems [78,143,153]. In line with One Health principles, habitat-level interventions should undergo rigorous environmental and conservation impact assessments, involve interdisciplinary expertise, and demonstrate clear, evidence-based benefits that outweigh potential ecological risks [21,22,142].

8. Limitations

This review highlights several limitations that reflect broader deficiencies in the global evidence base on antimicrobial resistance [AMR] in migratory birds [12,13,14]. First, most studies rely on detection-based approaches that document antimicrobial-resistant bacteria or resistance genes [ARGs] without demonstrating direct transmission, host-to-host spread, or long-distance dispersal mediated by birds. The predominance of cross-sectional and opportunistic study designs means that most findings remain correlational rather than causal [19,69,79]. In addition, DNA-based methods often fail to distinguish viable bacteria from residual genetic material, limiting their epidemiological and veterinary relevance and constraining inference about active transmission [79].

Second, avian AMR research is geographically uneven. Surveillance efforts are concentrated in Europe, East Asia, and parts of North America, where laboratory capacity and wildlife monitoring systems are well established [70,79,88,91]. In contrast, tropical low- and middle-income regions, including Southeast Asia, sub-Saharan Africa, and Latin America, remain markedly underrepresented due to limited microbiological infrastructure, weaker veterinary surveillance, and fewer coordinated monitoring programs [14,18,101]. This imbalance restricts the generalizability of existing findings and likely underestimates ARG diversity and circulation along tropical flyways. Finally, substantial taxonomic and flyway biases persist. Research focuses disproportionately on gulls and waterfowl, while passerines, raptors, and long-distance migrants, many of which rely on tropical stopover sites, are comparatively understudied [25]. Methodological heterogeneity across studies, including differences in sampling design, host selection, analytical workflows, genomic resolution, and metadata reporting, further complicates cross-study comparisons and synthesis [25,164].

9. Conclusion

AMR is a global ecological crisis that extends beyond clinical boundaries, encompassing environmental and geographic dimensions. Within the One Health framework, migratory birds act as both sentinels and potential vectors of ARGs, linking human settlements, agricultural systems, and natural ecosystems along continental flyways [19,90,140]. Their repeated use of human-modified habitats exposes them to resistant bacteria derived from wastewater effluents, agricultural runoff, and landfill waste, reinforcing connectivity between clinical and environmental resistomes. Evidence across regions shows that avian-associated bacteria frequently harbor extended-spectrum β-lactamases, carbapenemases, and mcr genes—resistance determinants of significant clinical concern [82,158]. These findings underscore that AMR is not solely a biomedical problem, but an ecological and evolutionary process shaped by interactions among species, ecosystems, and human activity. Effective responses require integrated molecular surveillance, environmental context, and coordinated policy action. Genomic and metagenomic tools should underpin global AMR monitoring, supported by strengthened antimicrobial stewardship and cross-sectoral collaboration. Recognizing migratory birds not merely as passive carriers but as ecological indicators of emerging resistance pathways is essential. Protecting avian and environmental health is therefore integral to sustaining microbial balance and safeguarding long-term human health [8,14,94].