Salmonella Dublin in Cattle: Emerging Pathogen, Diagnostic Challenges, and Public Health Impact

Introduction

Salmonella enterica subspecies enterica serovar Dublin (S. Dublin) is a host-adapted bacterial pathogen that primarily affects cattle, with a particular tropism for dairy operations [1, 2]. Unlike broad-host-range serovars such as S. Typhimurium, S. Dublin has evolved a specialized relationship with bovine hosts, enabling it to cause systemic, typhoid-like disease rather than self-limiting enteritis [26, 46]. The pathogen has emerged as a significant threat to the global cattle industry, with increasing prevalence documented across North America and Europe [3, 2, 4]. This emergence is characterized by high morbidity and mortality in young calves, reduced production efficiency in adult animals, and the establishment of asymptomatic carrier states that perpetuate within-herd transmission [1, 2, 38].

The clinical and economic burden of S. Dublin is compounded by its zoonotic potential and the increasing prevalence of multidrug-resistant (MDR) strains [5, 18, 54]. Infected cattle can shed the organism intermittently in feces, contaminating the environment and serving as a source of infection for both other animals and humans [2, 39]. The pathogen is frequently associated with invasive human infections, including bacteremia and focal abscesses, with a reported fatality rate of approximately 3% in human cases [6, 24, 42]. This review provides a comprehensive examination of S. Dublin as an emerging bovine pathogen, focusing on its pathogenesis, diagnostic challenges, antimicrobial resistance profiles, and public health impact.

Taxonomy and Host Adaptation

S. Dublin belongs to the species Salmonella enterica subspecies enterica, which encompasses over 2,500 serovars differentiated by somatic (O) and flagellar (H) antigens. The serovar is classified under serogroup D1, sharing the O9 antigen with S. Enteritidis and S. Gallinarum. Genomically, S. Dublin is closely related to S. Enteritidis but has undergone host restriction, losing or modifying genetic elements that facilitate broad-host-range infection while retaining those necessary for systemic disease in cattle [5, 32].

The host-adapted nature of S. Dublin is reflected in its genomic content. The pathogen harbors multiple Salmonella pathogenicity islands (SPIs), including SPI-1 and SPI-2, which encode type III secretion systems (T3SS) essential for invasion and intracellular survival [7, 29]. Additionally, S. Dublin possesses two type VI secretion systems (T6SS) encoded within SPI-6 and SPI-19 [28, 49]. The T6SSSPI-6 contributes to interbacterial competition and colonization of the avian and murine gastrointestinal tract, while T6SSSPI-19 also mediates antibacterial activity [28, 49]. The presence of these secretion systems underscores the pathogen's capacity to compete within the gut microbiota and establish infection in the bovine host [7, 28].

Pathogenesis and Host-Pathogen Interactions

The pathogenesis of S. Dublin in cattle involves a complex cascade of events beginning with oral ingestion of the bacterium, followed by colonization of the distal ileum and invasion of intestinal epithelial cells [7, 26]. Invasion is mediated by genes encoded within SPI-1, which assemble a needle-like T3SS-1 apparatus that injects effector proteins into host cells, triggering cytoskeletal rearrangements and bacterial internalization [7, 29]. Following invasion, S. Dublin can survive and replicate within macrophages, a process dependent on SPI-2-encoded T3SS-2 [29, 43].

Studies using bovine ileal monolayer models derived from adult organoids have demonstrated that S. Dublin invades epithelial cells, replicates intracellularly, and disrupts epithelial barrier integrity [8]. Infected monolayers exhibit a specific inflammatory response, including upregulation of cytokines and chemokines, providing a physiologically relevant platform for studying host-pathogen interactions [8]. In vivo studies using GFP-expressing S. Dublin in orally challenged calves revealed that bacteria are predominantly extracellular in the distal ileal mucosa and gut-associated lymph nodes at 48 hours post-infection. Intracellular bacteria were found primarily within MHCII+ macrophage-like cells, and infected cells showed elevated levels of MHCII and CD40 compared to uninfected cells [26, 46].

Systemic translocation of S. Dublin occurs predominantly via efferent lymphatics in a cell-free niche, a process that requires T3SS-1 but not T3SS-2 [29, 33]. This finding contrasts with the intracellular route commonly described for S. Typhimurium and highlights the unique pathogenic strategy of S. Dublin. The ability to disseminate systemically while remaining extracellular in the lymphatic system may contribute to the establishment of chronic carrier states, a hallmark of S. Dublin infection [38, 39].

Clinical Manifestations in Cattle

Clinical disease caused by S. Dublin varies markedly with age. In neonatal and young calves, infection typically manifests as acute septicemia, pneumonia, and enteritis, with mortality rates ranging from 20% to 100% [2, 23, 39]. A retrospective analysis of postmortem cases in British Columbia found that over half of S. Dublin-positive cases presented with respiratory symptoms, and calves were 38 times more likely to have S. Dublin compared to adults [3]. The respiratory presentation is often mistaken for primary bacterial pneumonia, leading to treatment with first-line antibiotics such as florfenicol, which may be ineffective against MDR strains [3].

In adult cattle, S. Dublin infection is often subclinical, with animals serving as asymptomatic carriers that intermittently shed the organism in feces [2, 38]. Clinical disease in adults can include abortion, decreased milk production, and occasional enteritis [1, 40]. The carrier state is a critical feature of S. Dublin epidemiology, as carriers can shed the pathogen during periods of stress, such as calving or transport, perpetuating the infection cycle within the herd [2, 38]. Risk factors for becoming a carrier include age at first infection, parity, and management practices.

Epidemiology and Transmission

The prevalence of S. Dublin has increased substantially in dairy cattle populations across North America and Europe [1, 2, 4]. In British Columbia, Canada, bulk tank milk (BTM) surveillance from 2021 to 2023 revealed that 30% of 461 dairy herds were positive for S. Dublin based on ELISA serology [4]. Herd-level risk factors included larger herd size (greater than 500 lactating cattle, odds ratio = 21.78) and higher farm density (greater than 5 farms within a 4 km radius, odds ratio = 1.73), while satisfactory pest control was protective (odds ratio = 0.28) [4]. In Ontario, Canada, 25% of 100 dairy farms were classified as S. Dublin positive, with risk factors including introduction of purchased animals and returning animals from shows or embryo collection centers.

In Austria, seroprevalence in dairy herds was 14.8% in Tyrol and 18.2% in Salzburg, based on bulk milk screening of 6,973 farms [9]. Individual animal testing revealed that 11.3% of animals were shedding S. Dublin in feces by culture, and 17.0% were positive by qPCR [9]. Alpine pasturing, where cattle from multiple farms are herded together, was identified as a significant risk factor for S. Dublin transmission, with surface water on pastures testing positive for the pathogen in 6.4% of samples [10, 30].

Transmission of S. Dublin occurs primarily through the fecal-oral route, with contaminated feed, water, and environment serving as sources of infection [2, 17]. The pathogen can survive for extended periods in the environment, particularly in moist, cool conditions. Movement of live cattle is a well-documented risk factor for between-herd transmission, and social network analysis has demonstrated that cattle movement activities in the previous month are the strongest predictive factor for farms becoming infected [11, 41]. Despite stringent movement restrictions imposed on infected farms in some national control programs, network analysis has revealed similar trends for non-infected and infected farms, indicating the failure of movement restrictions alone to reduce transmission.

Wildlife vectors have been investigated as potential reservoirs. Studies on common starlings (Sturnus vulgaris) captured on S. Dublin-infected Danish dairy farms found no trace of the pathogen, suggesting that starlings are unlikely to be significant vectors [12]. Wild mice, however, have been identified as potential reservoirs in closed dairy herds. Herding dogs have also been found to shed S. Dublin asymptomatically, potentially contributing to environmental contamination.

Diagnostic Challenges

Diagnosis of S. Dublin infection presents significant challenges due to the intermittent shedding pattern of the pathogen, the high proportion of asymptomatic carriers, and the limitations of conventional culture-based methods [2, 13, 9]. Traditional bacterial culture of fecal samples is considered the gold standard for detection, but its sensitivity is limited, particularly in samples with low bacterial loads or from subclinical carriers [13, 9]. The pathogen can be outcompeted by other enteric bacteria on selective media, and the use of enrichment steps is often necessary to improve recovery [13].

Serological methods, particularly ELISA for detection of antibodies against S. Dublin lipopolysaccharide (LPS) in bulk tank milk or serum, are widely used for herd-level screening [10, 4, 9]. BTM ELISA testing has demonstrated high sensitivity for identifying infected herds, with a percent positivity (PP) threshold of 35% commonly used to define positivity [4]. Monitoring variability in BTM PP over time can function as an early warning tool, with positive farms exhibiting high variability (median PP variability = 35.84%) compared to negative farms (median = 5.79%) [4]. However, serology cannot distinguish between current infection and past exposure, and antibodies may persist for months after clearance of the organism [9].

Molecular methods, including conventional and real-time PCR, offer improved sensitivity and specificity for detection of S. Dublin in clinical and environmental samples [13, 9, 21]. A novel dual-gene target TaqMan real-time PCR assay targeting a chromosomal locus (SeD_A1104) and a plasmid-encoded gene (vagD) demonstrated detection limits as low as 1.9 x 10^1 genome copies per reaction, with amplification efficiencies of 91% to 102%. Field testing of this assay on samples from subclinical cattle and farm environments detected S. Dublin in 50% of fecal samples, 40% of vaginal swabs, 30% of nasal swabs, and 75% of environmental boot swabs. The dual-target design reduces false positives caused by genetically related Salmonella serovars and improves confidence in detection.

Multiplex PCR assays have also been developed for serovar-level discrimination. A multiplex PCR targeting the tcpS, lygD, and flhB genes can simultaneously identify S. Enteritidis, S. Pullorum/Gallinarum, and S. Dublin, with a detection limit of 58.5 pg/μL of genomic DNA and 100 CFU. Endpoint multiplex PCR combined with culture has been used to detect S. Dublin in environmental samples from calf production systems, including calf marketing operations and veal farms [13].

The following table summarizes the diagnostic methods available for S. Dublin detection:

Antimicrobial Resistance

Antimicrobial resistance (AMR) in S. Dublin is a growing concern, with MDR strains increasingly isolated from cattle and humans [5, 18, 54]. Analysis of 247 clinical S. Dublin isolates from California cattle over a 27-year period (1993 to 2019) revealed a predominant core MDR pattern, with reduced susceptibility to third-generation cephalosporins and quinolones observed in more recent year groups (2011 to 2019) compared to older isolates (1993 to 2005) [18, 53]. Discriminant analysis of MIC log2-transformed data showed a time-sequence pattern of increasing resistance, highlighting the need for continued monitoring and judicious antimicrobial use.

Genomic analysis of 2,150 S. Dublin strains collected from bovine, human, and environmental sources in the United States between 2002 and 2023 identified the highest prevalence of drug-specific AMR genes and the MDR plasmid IncA/C2 in bovine clinical strains [5]. Despite source-dependent differences in AMR gene frequency, 72% of strains differed by 20 or fewer SNPs, indicating a high degree of genomic similarity and potential for cross-transmission between reservoirs [5]. The MDR plasmid pVPS18S0911-1-like was strongly associated with human-associated isolates, suggesting that plasmid-mediated resistance may contribute to prolonged infections and increased clinical severity in humans [6].

In Canada, genomic investigation of invasive MDR S. Dublin strains from humans and animals revealed the emergence of a distinct clade associated with increased virulence and resistance. The presence of AMR genes encoding resistance to ampicillin, chloramphenicol, streptomycin, sulfonamides, and tetracycline (ACSSuT phenotype) is common, and additional resistance to extended-spectrum cephalosporins and fluoroquinolones has been documented [5, 18, 54]. The isolation of MDR S. Dublin from cryopreserved Holstein semen underscores the potential for sexual transmission and the importance of screening breeding stock.

Genomic Evolution and Virulence Factors

Whole-genome sequencing has provided insights into the evolution and virulence of S. Dublin. Comparative genomic analysis of S. Dublin isolates from cattle and humans has identified genetic features associated with human-associated (HA) versus non-human-associated (NHA) strains [6]. Premature stop codons (PMSCs) in genes putatively involved in virulence, such as pgtC, sadA, yahO, ratB, gsiB, and zirT, were differentially distributed between HA and NHA groups [6]. Disruption of sadA may contribute to attenuated virulence in NHA isolates, while disruption of zirT may enhance virulence in HA isolates [6]. An IS1 transposon insertion within panE, located between two genes putatively involved in oxidative stress, was observed exclusively among HA isolates, although these isolates did not show significantly different oxidative stress survival compared to NHA isolates [6].

The Type VI Secretion System (T6SS) is a key virulence factor in S. Dublin. Both T6SSSPI-6 and T6SSSPI-19 contribute to interbacterial competition, with effector/immunity modules encoded within each cluster. The T6SSSPI-6 is required for efficient colonization of the avian and murine gastrointestinal tract, while T6SSSPI-19 is not necessary for colonization of these hosts. The identification of T6SS effectors that target bacterial cells highlights the role of these systems in niche competition within the gut microbiota [7, 28].

The homolog of the gene bstA from the BTP1 phage of S. Typhimurium ST313 has been identified as an antivirulence gene in S. Dublin. This finding suggests that phage-mediated gene transfer can modulate virulence in S. Dublin, potentially affecting host adaptation and disease outcome. Complete closed genome sequences of S. Dublin strains isolated from cattle at harvest have facilitated genomic comparisons to characterize virulence determinants and AMR genes.

Control and Prevention Strategies

Control of S. Dublin in cattle herds requires a multifaceted approach encompassing biosecurity, management practices, vaccination, and surveillance [1, 2, 17]. Biosecurity best management practices (BBMP) include tracking herd health status, proactive response to disease risks, limiting animal purchases, routine facility disinfection, and controlling visitor access [14]. However, focus group studies with dairy farmers have identified significant barriers to implementation, including lack of perceived risk, financial constraints, labor limitations, and the belief that biosecurity does not register as a daily priority [14]. Farmers who had experienced a S. Dublin outbreak expressed a shift toward proactive biosecurity, while those without outbreak experience did not view the pathogen as a substantial risk [14].

A semi-quantitative biosecurity assessment framework has been developed to evaluate the risk of introduction and establishment of S. Dublin in dairy herds. The framework includes a weighted assessment method for 12 biosecurity sections, evaluated through on-farm observations and farmer interviews. The tool can be used for systematic data collection in epidemiological studies and to identify high-risk practices.

Mathematical modeling has been employed to evaluate control strategies. A modified Susceptible-Infected-Recovered-Susceptible (SIRS) model of S. Dublin transmission in a heifer-raising operation predicted a median of 37 carriers and 92 asymptomatic infections among 2,330 heifers over a 2-year simulation [15]. Increasing barn floor cleaning frequency to 12 times per day meaningfully reduced epidemiological outcomes and improved operating income by 1.2% to 10.6% in the first year, depending on cleaning costs [15]. In most cost scenarios, vaccination was not supported, even when paired with stringent cleaning measures [15].

Vaccination against S. Dublin is available in some regions, but its efficacy is variable. Live attenuated vaccines, such as those derived from gene deletion mutants (e.g., Sdu189ΔspiC and Sdu189ΔspiCΔaroA), have shown promise in mouse models, providing 100% protection against challenge with the wild-type strain. However, field efficacy in cattle remains to be fully established, and vaccination does not eliminate the carrier state [1, 2]. The use of recombinant S. Dublin expressing heterologous antigens, such as intimin from E. coli O157:H7, has been explored as a vector for vaccination against other pathogens, demonstrating transient reduction in fecal shedding of the target organism.

The following Mermaid diagram illustrates a decision tree for S. Dublin control on dairy farms:

flowchart TD
 A[Initial Herd Assessment] --> B{BTM ELISA Screening}
 B -->|Negative| C[Maintain Biosecurity]
 B -->|Positive| D[Individual Animal Testing]
 D --> E{Identify Carriers}
 E -->|Yes| F[Isolate and Manage Carriers]
 E -->|No| G[Implement Enhanced Biosecurity]
 F --> H[Increase Cleaning Frequency]
 G --> H
 H --> I[Restrict Animal Movements]
 I --> J[Vaccination Consideration]
 J --> K{Re-evaluate Herd Status}
 K -->|Still Positive| L[Review Management Practices]
 K -->|Negative| C
 L --> H

Public Health Impact

S. Dublin is a zoonotic pathogen capable of causing severe invasive disease in humans [5, 6, 24]. Unlike non-typhoidal Salmonella serovars that typically cause self-limiting gastroenteritis, S. Dublin is associated with a high rate of extra-intestinal infections, including bacteremia, meningitis, osteomyelitis, and abscess formation [6, 24, 42]. A case report of a 78-year-old man with a paravertebral abscess caused by S. Dublin highlights the pathogen's ability to cause focal infections, particularly in individuals with underlying conditions such as diabetes or immunosuppression. The patient had consumed raw beef and unpasteurized milk, suggesting a foodborne source of infection.

Genomic analysis of S. Dublin strains from human clinical cases has identified genetic features associated with enhanced virulence, including the presence of MDR plasmids and specific PMSCs in virulence genes [6]. The high degree of genomic similarity between bovine, human, and environmental strains (72% of strains differing by 20 or fewer SNPs) underscores the potential for cross-transmission and the importance of a One Health approach to surveillance and control [5]. The emergence of MDR S. Dublin in the food supply chain, particularly in ground beef and unpasteurized dairy products, poses a significant public health risk [5, 39].

Future Directions

Critical knowledge gaps remain in the understanding of S. Dublin pathogenesis, transmission, and control. The role of the gut microbiota in modulating susceptibility to S. Dublin infection is an area of active investigation, with studies suggesting that propionate, a microbial fermentation product, inhibits SPI-1 transcription and may be leveraged for prevention [7]. Lactobacilli endogenous to the small intestine of calves may be harnessed to inhibit S. Dublin invasion through propionate synthesis and nutrient blocking [7].

The development of improved diagnostic tools, particularly those capable of detecting subclinical carriers and low-level environmental contamination, is essential for effective surveillance and control [13, 21]. The integration of mathematical modeling with economic analysis can inform cost-effective control strategies and support decision-making at the farm level [15]. Continued genomic surveillance is needed to monitor the emergence of new AMR profiles and virulence determinants [5, 6, 54].

References

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