Salmonella in Poultry: Prevalence, Transmission Dynamics, and Epidemiological Trends
By Dr. Zubair Khalid, DVM, MS, PhD ·
Key Takeaways
- Salmonella prevalence in poultry is highly variable globally, exceeding 20% in some African regions and 8% in Chinese eggs, with serovars like S. Enteritidis, S. Typhimurium, S. Infantis, and S. Kentucky being predominant.
- Transmission occurs via horizontal (fecal-oral, contaminated feed/water/equipment) and vertical (internal egg contamination) routes, with environmental persistence facilitated by biofilm formation and adaptation to litter.
- Epidemiological trends indicate a significant shift towards S. Infantis and S. Kentucky as major poultry-associated serovars, often linked to multidrug resistance.
- Antimicrobial resistance, particularly to cephalosporins and fluoroquinolones, is a growing concern, driven by plasmid-borne genes like *bla*CTX-M and emerging colistin resistance (mcr genes).
- Biosecurity measures, vaccination of breeder flocks, and the use of alternatives to antimicrobials (probiotics, bacteriophages) are critical for control, while genomic epidemiology and advanced surveillance tools like WGS are revolutionizing outbreak investigations.
Introduction
Salmonella enterica is a Gram-negative, facultative anaerobic bacillus belonging to the family Enterobacteriaceae. In poultry, Salmonella can cause a spectrum of conditions ranging from asymptomatic intestinal carriage to severe systemic disease, depending on the serovar and host immune status [<a href="#ref-1">1</a>, <a href="#ref-2">2</a>]. The genus is divided into two species: S. enterica and S. bongori, with S. enterica further subdivided into six subspecies [<a href="#ref-3">3</a>]. More than 2,600 serovars have been identified; those associated with poultry include host-adapted serovars such as S. Gallinarum and S. Pullorum (causing fowl typhoid and pullorum disease, respectively) and broad-host-range serovars such as S. Enteritidis, S. Typhimurium, S. Infantis, and S. Kentucky [<a href="#ref-2">2</a>, <a href="#ref-4">4</a>, <a href="#ref-5">5</a>]. The economic burden of salmonellosis in poultry production arises from mortality, reduced performance, and trade restrictions. Understanding the prevalence patterns, transmission mechanisms, and evolving epidemiology is essential for designing effective control programs. This review synthesizes recent data on Salmonella in poultry with emphasis on prevalence, transmission dynamics, and epidemiological trends across global production systems.
Prevalence of Salmonella in Poultry
Prevalence estimates vary widely by geographic region, production type (broiler, layer, breeder, backyard), and sampling point (farm, slaughter, retail). A continent-wide systematic review of African poultry reported pooled prevalence estimates exceeding 20% in some countries, with S. Enteritidis and S. Typhimurium being the most commonly isolated serovars [<a href="#ref-3">3</a>]. In Plateau State, Nigeria, Salmonella species were detected in 38% of samples from live bird markets and processing environments, indicating substantial environmental contamination [<a href="#ref-1">1</a>]. In Northern Algeria, broiler chicken samples yielded a prevalence of 12.6%, with a high proportion of multidrug-resistant isolates [<a href="#ref-6">6</a>]. In Burkina Faso, household-level infection prevalence in chickens reached 41% and was associated with free-range management and lack of biosecurity.
In Asia, prevalence dynamics are equally heterogeneous. In Bangladesh, S. Gallinarum-Pullorum isolates from poultry showed high antimicrobial resistance, with 89% of isolates resistant to at least three antibiotic classes [<a href="#ref-2">2</a>]. In Northeast Thailand, seasonal surveillance of chickens from slaughterhouses revealed prevalence peaks during the rainy season, with S. Enteritidis being the dominant serovar [<a href="#ref-4">4</a>]. In South Korea, temporal trends between 2019 and 2024 showed a shift in dominant serovars from S. Enteritidis to S. Infantis in livestock [<a href="#ref-7">7</a>]. In China, a meta-analysis of egg-derived Salmonella found an overall prevalence of approximately 8% in eggs, with S. Enteritidis predominating. Fifty years of chicken-source Salmonella in China demonstrated a progressive increase in S. Infantis and a decline in S. Pullorum.
In the Americas, prevalence in retail meats from Indiana, USA, was 4.3% in chicken samples, with S. Kentucky being the most common serovar [<a href="#ref-8">8</a>]. Backyard poultry in the United States presents a growing concern, with outbreaks linked to contact with live poultry increasing over the past decade. In Ecuador, high-Andean wild waterbirds were found to carry multidrug-resistant Salmonella, suggesting wildlife reservoirs contribute to the epidemiology [<a href="#ref-9">9</a>]. In Brazil, S. Minnesota emerged as a significant multidrug-resistant serovar in poultry.
In Europe, monitoring of the poultry chain in Sardinia, Italy, revealed S. Enteritidis as the most frequent serovar in broilers, with moderate antimicrobial resistance rates [<a href="#ref-10">10</a>]. In Germany, mobile slaughter units showed comparable Salmonella contamination rates to conventional slaughterhouses. The Israeli layer flock survey from 2018 to 2023 found S. Enteritidis prevalence of 6.8% and identified risk factors including flock size and housing type.
These data underscore that Salmonella prevalence remains high in many regions, heavily influenced by production system biosecurity, climate, and antimicrobial use practices.
Transmission Dynamics
Transmission of Salmonella in poultry occurs through horizontal and vertical routes. Horizontal transmission involves the fecal-oral pathway, contaminated feed, water, litter, equipment, and vectors including insects, rodents, and humans [<a href="#ref-11">11</a>]. In broiler production, within-flock transmission is rapid; models indicate that infection can spread to most birds within days after introduction [<a href="#ref-12">12</a>]. A study using CRISPR-SeroSeq in Ecuador detected up to five serovars simultaneously in a single broiler flock, revealing complex within-host and within-flock dynamics. Contaminated eggshells used in poultry feed were identified as the vehicle for a diffuse nationwide S. Enteritidis outbreak in the Netherlands.
Vertical transmission is particularly important for S. Enteritidis and S. Pullorum, which can colonize the reproductive tract of hens and contaminate eggs internally [<a href="#ref-13">13</a>]. In breeder companies, whole-genome sequencing can trace clonal spread of S. Enteritidis from grandparent to parent and broiler flocks [<a href="#ref-13">13</a>]. The role of primary breeders in disseminating Salmonella has been highlighted by genomic epidemiology studies that quantified the contribution of breeding stock to downstream contamination [<a href="#ref-14">14</a>].
Environmental persistence is a key factor. S. Infantis can survive osmotic stress and adapt to poultry litter, with transcriptomic profiling showing upregulation of stress response genes in litter environments [<a href="#ref-15">15</a>]. The pESI megaplasmid, common in S. Infantis, carries multiple antimicrobial resistance and virulence genes and enhances persistence in poultry [<a href="#ref-5">5</a>]. Similarly, S. Kentucky ST198 strains in Bangladesh showed clonal spread and high environmental survival [<a href="#ref-16">16</a>]. Biofilm formation on surfaces such as plastic, rubber, and metal facilitates long-term contamination of poultry houses [<a href="#ref-17">17</a>].
The following Mermaid diagram summarizes the major transmission pathways of Salmonella in poultry production systems:
graph TD
A["Contaminated Feed/Water"] -->|"Horizontal"| B["Broiler Flock"]
C["Infected Breeders"] -->|"Vertical"| D["Eggs"]
D -->|"Hatch"| B
B --> F["Slaughter/Processing"]
F --> G["Contaminated Meat"]
E["Environmental Reservoir"] -->|"Litter/Equipment"| B
H["Wildlife/Vectors"] --> E
B --> I["Manure/Litter"]
I -->|"Land Application"| J["Crop Contamination"]
J --> A
B -->|"Within-flock spread"| BTransmission is also influenced by host factors. Co-infection with other pathogens (e.g., Campylobacter jejuni) can alter cecal microbiota and serum metabolome, potentially affecting Salmonella colonization dynamics. The use of competitive exclusion products containing cecal microbiota from healthy chickens reduces Salmonella colonization by outcompeting the pathogen for nutrients and binding sites.
Epidemiological Trends
Serovar Distribution
Globally, the serovar landscape has undergone significant shifts. While S. Enteritidis and S. Typhimurium have historically dominated, S. Infantis has emerged as a major serovar in poultry worldwide [<a href="#ref-5">5</a>, <a href="#ref-18">18</a>]. In the United States, a persistent multidrug-resistant clade of S. Infantis (REPJFX01) was identified in humans and chickens from 2010 to 2023, indicating stable reservoir perpetuation [<a href="#ref-18">18</a>]. In Mexico, S. Infantis clones carrying blaCTX-M-65 and gyrA_D87Y mutations have become predominant. In Canada, IncI1 plasmids in S. Heidelberg and S. Kentucky from poultry are associated with production type and geographic region, demonstrating that plasmid-mediated resistance determinants spread across serovars [<a href="#ref-19">19</a>].
S. Kentucky has become increasingly common in many regions, often associated with multidrug resistance and the ST198 clone [<a href="#ref-16">16</a>]. Machine learning approaches applied to S. Kentucky genomes have identified genetic determinants of host specificity and geographic origin, aiding source attribution. S. Hadar, another serovar with emerging importance, shows high pangenome diversity and population structure linked to poultry reservoirs. In Thailand, clonal dissemination of multidrug-resistant S. Enteritidis has been documented in chicken production systems. In Iran, non-typhoidal Salmonella serovars from poultry production chains exhibited high virulence gene prevalence and multidrug resistance.
Antimicrobial Resistance (AMR)
Antimicrobial resistance in poultry-associated Salmonella is a rapidly evolving threat. The prevalence of extended-spectrum beta-lactamase (ESBL) producers, particularly those carrying blaCTX-M genes, has increased globally [<a href="#ref-20">20</a>, <a href="#ref-21">21</a>]. In China, blaCTX-M genes were found in 34% of Salmonella isolates from retail chicken, with both plasmid-mediated and chromosomal integration mechanisms identified [<a href="#ref-20">20</a>]. In South Korea, a hybrid transposon Tn1721/Tn21 was responsible for the dissemination of blaCTX-M-15 in S. Enteritidis from poultry. In Bangladesh, ESBL-producing and carbapenem-resistant Salmonella were detected in retail chicken meat and live bird market sewage, reflecting environmental contamination [<a href="#ref-22">22</a>].
Plasmid-mediated colistin resistance (mcr genes) has emerged in livestock-derived Salmonella in South Korea [<a href="#ref-7">7</a>]. Colistin resistance is particularly concerning as it is a last-resort antibiotic. In West Bengal, India, non-typhoidal Salmonella from chickens and ducks showed increasing resistance to fluoroquinolones and third-generation cephalosporins [<a href="#ref-23">23</a>]. In Lagos, Nigeria, preharvest antibiotic use on poultry farms was directly correlated with AMR levels in Salmonella isolates [<a href="#ref-24">24</a>]. In Burkina Faso, asymptomatic carriage of resistant Salmonella was common in both poultry and humans, with shared resistance profiles suggesting transmission between hosts.
The convergence of multidrug resistance, virulence, and biofilm formation capacity has been documented in isolates from Xinjiang, China [<a href="#ref-17">17</a>]. Genomics has been essential for characterizing these phenotypes. For example, S. Dublin from poultry exhibited ciprofloxacin resistance linked to mutations in gyrA and efflux pump overexpression, identified through whole-genome sequencing and gene expression analysis. In retail meats in Hong Kong, whole-genome sequencing revealed diverse AMR genes and plasmid types, underscoring the complexity of resistance dissemination.
Biosecurity and Risk Factors
Farm management practices heavily influence Salmonella prevalence. In commercial poultry premises in Saint Kitts, seroprevalence of Salmonella was 14%, with higher risk associated with poor biosecurity scores [<a href="#ref-25">25</a>]. A large study in Israel identified flock size, multi-age housing, and prior Salmonella history as significant risk factors for layer flock infection. In Thailand, factors such as open-house housing and lack of footbaths increased Salmonella risk [<a href="#ref-4">4</a>]. Machine learning models applied to farm management data have improved prediction of on-farm Salmonella occurrence, allowing targeted interventions.
Biosecurity measures including all-in-all-out management, rodent control, feed treatment, and water sanitation are critical for reducing prevalence [<a href="#ref-11">11</a>]. Vaccination of breeder flocks with killed or live attenuated vaccines can reduce vertical transmission [<a href="#ref-26">26</a>]. Outer membrane vesicle vaccines derived from S. Typhimurium have shown cross-protective efficacy against multiple serovars in SPF chickens. Subunit vaccines targeting OmpX fused with chicken IL-9 have enhanced immunogenicity and protection [<a href="#ref-27">27</a>]. However, differentiation between vaccine and field strains is necessary for surveillance; disc diffusion methods validated by multiple suppliers enable such differentiation [<a href="#ref-28">28</a>].
Alternatives to antimicrobials include probiotics, prebiotics, synbiotics, bacteriophages, and essential oils. Synbiotic supplementation reduced S. Enteritidis colonization in broilers by modulating gut microbiota [<a href="#ref-29">29</a>]. A direct-fed microbial combined with xylanase enzyme improved performance and reduced S. Typhimurium cecal load. Oregano essential oil administered preharvest reduced S. Enteritidis in market-age broilers [<a href="#ref-30">30</a>]. Bacteriophage cocktails have shown efficacy in reducing Salmonella in turkey farms when applied in feed or water. Nanoencapsulation of gentamicin, ciprofloxacin, and lysozyme in chitosan-metal-organic frameworks enhanced antimicrobial activity against drug-resistant Salmonella [<a href="#ref-31">31</a>].
Detection and Surveillance
Detection of Salmonella in poultry relies on traditional culture methods, serotyping, and increasingly on molecular and genomic tools. Culture-based isolation remains the gold standard but is time-consuming [<a href="#ref-32">32</a>]. Molecular methods such as PCR and quantitative PCR enable rapid detection from samples. An indirect ELISA based on the Sptp protein has been developed for serological detection of Salmonella infection in poultry [<a href="#ref-33">33</a>]. CRISPR-SeroSeq is a high-resolution amplicon sequencing method that can detect serovar diversity directly from cecal samples, revealing mixed infections missed by culture.
Whole-genome sequencing (WGS) has revolutionized epidemiological investigations. Core-genome multilocus sequence typing (cgMLST) of IncI1 plasmids from Canadian poultry revealed associations with production factors and geographic region [<a href="#ref-19">19</a>]. WGS was used to track clonal spread of S. Infantis across poultry and human sources in the United States [<a href="#ref-18">18</a>] and to characterize pESI megaplasmid distribution [<a href="#ref-5">5</a>]. In Thailand, WGS demonstrated clonal dissemination of multidrug-resistant S. Enteritidis. In China, WGS enabled phylodynamic analysis of meat-derived Salmonella, revealing population structure and AMR gene flow. Genomic epidemiology also quantified the contribution of primary breeders to downstream contamination [<a href="#ref-14">14</a>].
Surveillance programs vary by country. In the United States, chick paper sampling during hatchery outbreaks linked to backyard poultry represents a One Health approach to inform public health actions. In Europe, the poultry chain is monitored under national control programs [<a href="#ref-10">10</a>]. Temporal sampling of turkey flocks highlighted challenges in preharvest monitoring due to intermittent shedding [<a href="#ref-32">32</a>]. Artificial intelligence models integrating multi-source food safety data provide early warnings of Salmonella foodborne risk [<a href="#ref-34">34</a>]. Spatial risk modeling in Nigeria identified geographic predictors of non-typhoidal salmonellosis perpetuation in poultry farms and human communities.
Conclusion
Salmonella remains a persistent challenge in global poultry production. Prevalence is influenced by region, production system, and biosecurity. Transmission is driven by both horizontal and vertical routes, with environmental persistence and biofilm formation enhancing spread. Epidemiological trends show a shifting serovar landscape, with S. Infantis and S. Kentucky emerging as major problems. Antimicrobial resistance, especially to cephalosporins and fluoroquinolones, is escalating and driven by plasmid-borne genes. Genomic tools have improved our understanding of transmission dynamics, resistance spread, and host adaptation. Integrated control strategies incorporating biosecurity, vaccination, alternatives to antimicrobials, and robust surveillance are essential to reduce the burden of Salmonella in poultry.