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Equine paratyphoid, caused by the host-adapted pathogen Salmonella enterica subspecies enterica serovar Abortusequi, remains a critical threat to equine husbandry worldwide. While historical eradication efforts significantly reduced prevalence in the West, recent epidemiological data highlights a concerning re-emergence across Asia and parts of Europe over the last decade. This comprehensive review synthesizes current knowledge on the pathogen, integrating recent literature on its pathogenesis, epidemiology, and the growing challenge of antimicrobial resistance. Furthermore, it explores modern diagnostic frameworks essential for mitigating abortion storms in horses and donkeys. As part of an expanding body of veterinary immunology and diagnostic resources, this synthesis is designed to provide actionable insights for researchers, laboratory managers, and clinicians managing equine reproductive health.
Salmonella enterica serovar Abortusequi (S. Abortusequi) is a highly specialized, Gram-negative bacterium primarily recognized as the etiological agent of equine paratyphoid. Historically described in the late 19th century, the pathogen is notorious for causing sudden, late-term abortions in pregnant mares, often without any prior clinical warning signs. In addition to catastrophic reproductive failure, S. Abortusequi is responsible for neonatal septicemia, polyarthritis (often referred to as joint ill), and orchitis in equid populations. For decades, rigorous surveillance, strict biosecurity, and vaccination programs in regions such as the United States and Western Europe led to a dramatic decline in the incidence of equine abortus salmonellosis, effectively pushing the disease to the periphery of veterinary research.
However, the epidemiological landscape has shifted dramatically. In recent years, S. Abortusequi has re-emerged as a formidable veterinary pathogen, precipitating devastating economic losses within the global equine and asinine (donkey) industries. The sudden surge in abortion rates—sometimes reaching between 30% and 100% in affected breeding herds—has catalyzed renewed scientific interest. As the agricultural sector scales up the commercial breeding of donkeys for various byproducts, the high density of susceptible animals has created ideal conditions for bacterial transmission. Consequently, understanding the contemporary dynamics of S. Abortusequi infection is no longer just an academic exercise but an urgent necessity for farm management and animal welfare.
The geographic distribution of S. Abortusequi has evolved considerably, transitioning from a global scourge to a highly localized, yet potent, threat. Throughout the mid-to-late 20th century, the pathogen was considered virtually eradicated in many developed nations. Today, recent epidemiological surveillance reveals widespread circulation in Central and East Asia, the Indian subcontinent, and isolated pockets of Europe, such as Italy and Croatia. Over the last decade, unexpected and severe abortion storms have been frequently documented in China and India, where the expanding donkey farming industry has been hit particularly hard.
Transmission occurs primarily through the fecal-oral route, facilitated by the ingestion of feed or water contaminated by aborted fetuses, placental fluids, or the feces of shedding animals. One of the most insidious aspects of S. Abortusequi epidemiology is the role of asymptomatic carriers. Mares that have previously aborted, as well as ostensibly healthy stallions, can harbor the bacteria in their reproductive and gastrointestinal tracts, intermittently shedding the pathogen into the environment. This persistent shedding creates silent reservoirs of infection that can rapidly ignite outbreaks when herd immunity wanes or when naïve pregnant animals are introduced to a farm.
Figure 1. A Minimum-spanning tree based on cgMLST and HierCC analysis of 11 Salmonella Abortusequi serovar.
(Source: Manikandan R, et al. 2024)
The pathogenesis of S. Abortusequi is deeply intertwined with its ability to invade host cells, evade the immune system, and selectively target the reproductive tract. Upon ingestion, the bacteria traverse the intestinal epithelium, utilizing specialized virulence factors, such as the Type III secretion system, to survive and multiply within macrophages. This intracellular lifestyle shields the pathogen from neutralizing antibodies and facilitates systemic dissemination via the lymphatic and circulatory systems.
In pregnant equids, the bacteria exhibit a profound tropism for the gravid uterus. S. Abortusequi colonizes the placenta, inducing severe purulent placentitis. The resulting inflammatory cascade compromises the maternofetal barrier, leading to fetal bacteremia, hypoxia, and ultimately, the expulsion of the fetus. These abortions typically occur between the seventh and tenth months of gestation. Notably, mares generally do not exhibit systemic signs of illness prior to the abortion, making predictive clinical monitoring exceptionally difficult. In neonatal foals that survive the initial infection, the pathogen frequently localizes in the synovial joints, resulting in debilitating polyarthritis that requires intensive veterinary intervention.
Recent immunological studies have identified specific antigens that play a crucial role in the host-pathogen interaction. The flagellin protein FljB, for instance, has been identified as a major immunogenic target. Understanding these molecular interactions is vital for developing targeted therapies and advanced diagnostic assays that can detect the pathogen before an abortion storm decimates a susceptible herd.
Rapid and accurate diagnosis is the cornerstone of effective outbreak management. Historically, the gold standard for detecting S. Abortusequi has been bacterial isolation and culture from fetal tissues, vaginal swabs, or feces. While definitive, culture-based methods are time-consuming and prone to false negatives, particularly in chronic carriers with low shedding rates.
To address these limitations, modern veterinary diagnostics have pivoted toward molecular and serological techniques. Real-time polymerase chain reaction (PCR) assays targeting specific genomic markers have revolutionized the speed and sensitivity of direct pathogen detection. Concurrently, competitive enzyme-linked immunosorbent assays (cELISA) utilizing monoclonal antibodies against the FljB flagellin have emerged as powerful tools for large-scale seroepidemiological monitoring. These assays allow clinicians to identify animals with elevated antibody titers, thereby pinpointing potential asymptomatic carriers within a herd rapidly.
Implementing these advanced diagnostic methodologies requires a robust clinical infrastructure. For laboratories transitioning to these modern molecular and serological platforms, comprehensive QMS setup is absolutely critical. Proper Quality Management System integration ensures that sample handling, assay calibration, and data interpretation adhere to strict standardization protocols, thereby minimizing diagnostic errors and providing reliable, actionable data for farm management decisions. Ensuring high-quality, standardized testing environments will remain essential for effectively combating infectious disease outbreaks and protecting herd health.
Figure 2. Schematic diagram of solid phase competition ELISA.
(Source: Guo K, et al. 2023)
Compounding the challenge of S. Abortusequi is the alarming rise of antimicrobial resistance (AMR). In farm environments, the persistent use of broad-spectrum antibiotics for both prophylactic and therapeutic purposes has exerted immense selective pressure on bacterial populations. Recent whole-genome sequencing (WGS) and core genome multilocus sequence typing (cgMLST) studies have characterized numerous S. Abortusequi isolates, frequently identifying the ST251 sequence type as a dominant lineage in recent outbreaks.
Genomic analyses have revealed that many contemporary strains harbor multiple resistance genes, conferring resistance to critical antibiotic classes, including beta-lactams, aminoglycosides, and tetracyclines. This multidrug-resistant profile severely limits treatment options for septicemic foals and complicates efforts to clear the carrier state in adult animals. The integration of genomic epidemiology into routine surveillance is therefore vital for tracking the evolution of resistance plasmids and guiding prudent antimicrobial stewardship in veterinary medicine.
The resurgence of S. Abortusequi presents a multifaceted challenge to the global equine and asinine industries. Driven by intensive breeding practices, international animal movement, and the insidious nature of asymptomatic carriers, this pathogen continues to cause profound economic and animal welfare impacts. Mitigating this threat requires a holistic approach that combines rigorous biosecurity, ongoing serological surveillance, and the rapid deployment of molecular diagnostics. By leveraging modern genomic tools and committing to stringent diagnostic quality standards, the veterinary community can better anticipate outbreaks, manage antimicrobial resistance, and ultimately protect the reproductive health of equid populations worldwide.
References
| Target | Cat. No. | Product Name | Size | Species | Application | Detection Sample | |
| Salmonella | DEIABL37 | Chicken Salmonella Enteritidis IgY ELISA Kit | 96T | Chicken | Quantitative | Chicken serum, blood plasma, and other biological fluids. | Inquiry |
| DEIA4172 | Salmonella IgG ELISA Kit | 96T | Human | Quantitative | Serum, plasma | Inquiry | |
| DEIA2563 | Salmonella Antigen In Food ELISA Kit | 96T | N/A | Quantitative | Food, feed and environmental samples | Inquiry | |
| DEIA-WZ1001S | Anti-Salmonella abortusovis ELISA Kit | 5 96T | Sheep | Qualitative | Sheep Serum and Plasma | Inquiry |
| Target | Cat. No. | Product Name | Expression System | Tag/Conjugate | Application | |
| Salmonella | DAGF-121 | Salmonella pullorum and gallinarum positive serum | Chicken | Unconjugated | Inquiry | |
| DAGF-122 | Salmonella pullorum and gallinarum negative serum | Chicken | Unconjugated | Inquiry | ||
| DAG-WT7198 | Inactivated Natural Salmonella spp. Quality Control | N/A | N/A | Immunoassays | Inquiry | |
| DAG-WT7183 | Inactivated Natural Salmonella choleraesuis Quality Control | N/A | N/A | Immunoassays | Inquiry | |
| DAG3590 | Recombinant Salmonella beta Lactamase | E. coli | Unconjugated | N/A | Inquiry | |
| DAGA-3049 | Native Salmonella paratyphi A antigen | S. paratyphi A culture | Unconjugated | ELISA, LF | Inquiry | |
| DAGA-3050 | Native Salmonella paratyphi B antigen | S. paratyphi B culture | Unconjugated | ELISA, LF | Inquiry | |
| DAGA-3051 | Native Salmonella typhimurium antigen | S. typhimurium | Unconjugated | ELISA, LF | Inquiry | |
| DAG-WT525 | Inactivated Native Salmonella enteritidis Antigen | S. enteritidis | N/A | ELISA, LF | Inquiry | |
| DAG-WT526 | Inactivated Native Salmonella typhi Antigen | S. typhi | N/A | ELISA, LF | Inquiry |
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