Salmonella typhimurium in Horses: Enteric Fever and Septicemia in Foals and Adults

Introduction

Salmonella enterica subsp. enterica serovar Typhimurium (S. Typhimurium) is a host-generalist, Gram-negative enteric bacterium capable of causing a spectrum of clinical diseases in equids, ranging from subclinical carriage to severe, life-threatening enteric fever and septicemia [1, 2, 3]. In horses, S. Typhimurium is one of the most frequently isolated serovars from clinical cases of salmonellosis [4, 5]. The clinical presentation differs markedly between foals and adults, with foals typically developing septicemia and adults presenting with acute colitis and enteric fever [6, 7]. Equine salmonellosis represents a diagnostic and therapeutic challenge due to intermittent shedding, the emergence of multidrug-resistant (MDR) strains, and its significance as a zoonotic pathogen within the One Health framework [8, 4, 5]. This article provides an exhaustive review of the virological (bacteriological), pathophysiological, diagnostic, and epidemiological aspects of S. Typhimurium infection in horses, drawing exclusively on peer-reviewed evidence from the veterinary literature.

Pathogenesis and Virulence Mechanisms

S. Typhimurium invades the intestinal mucosa via M cells and enterocytes, using a type III secretion system (T3SS) encoded on Salmonella pathogenicity island 1 (SPI-1) to inject effector proteins that induce membrane ruffling and bacterial internalization [1, 3]. Following invasion, the bacteria reside within Salmonella-containing vacuoles (SCVs) and employ a second T3SS (SPI-2) to survive intracellularly and evade host defenses [3]. In horses, the cecum and large colon are the primary sites of attachment and invasion [7, 9, 10]. The bacterium then triggers a robust inflammatory response characterized by neutrophil infiltration, mucosal damage, and fluid secretion, leading to profuse diarrhea [7, 10]. In foals, the immature immune system and underdeveloped gut-associated lymphoid tissue predispose them to rapid systemic dissemination, resulting in septicemia [6, 11]. The ability of S. Typhimurium to resist serum bactericidal activity is a key determinant of invasive disease; serum resistance is mediated by O-antigen chain length and the presence of specific virulence plasmids [12]. The pangenome of S. Typhimurium is notably larger than that of host-restricted serovars such as S. Typhi, contributing to its metabolic versatility and capacity to infect multiple host species including horses [3, 5].

Clinical Features in Foals and Adults

Foals: Septicemia and Enteric Disease

Neonatal foals (birth to 3 months) are highly susceptible to S. Typhimurium infection, which most often manifests as septicemia within the first one to two weeks of life [6, 11]. The primary route of infection is fecal-oral, with sources including contaminated environment, dam feces, or fomites [11]. Clinical signs include pyrexia (temperature > 39.5 degrees Celsius), lethargy, hypoglycemia, and rapid progression to septic shock [6]. Diarrhea may be absent in the early stages of septicemia, making diagnosis challenging [6]. In a retrospective study of blood culture isolates from 243 neonatal foals in Normandy, Gram-negative enteric bacteria including Salmonella spp. were frequently identified, and survival correlated strongly with pathogen-specific antimicrobial susceptibility patterns [6]. Foals infected with S. Typhimurium often develop severe endotoxemia secondary to lipopolysaccharide (LPS) release, resulting in cardiovascular collapse, disseminated intravascular coagulation (DIC), and high mortality if untreated [6, 11]. The presence of MDR strains further worsens prognosis [8]. Gross pathological findings include extensive hemorrhagic enteritis, enlarged mesenteric lymph nodes, and petechial hemorrhages on serosal surfaces [10].

Adult Horses: Enteric Fever and Colitis

In adult horses, S. Typhimurium infection typically presents as acute colitis, often referred to as enteric fever in the equine context due to the presence of systemic signs including fever, toxemia, and colic [7, 4]. The clinical features of S. Typhimurium enteritis are similar to those of equine coronavirus (ECoV) infection; horses with either disease commonly present with fever, depression, decreased appetite, and abdominal pain [7]. Diarrhea may range from soft feces to profuse, watery, and hemorrhagic diarrhea [7, 10]. Leukopenia due to neutropenia is a consistent hematological finding, reflecting the consumption of neutrophils at the site of intestinal inflammation [7]. In a comparative study, horses with Salmonella infection had significantly lower mean leukocyte counts than horses without a diagnosis [7]. Adult horses may also become chronic asymptomatic carriers, intermittently shedding S. Typhimurium in feces; such horses serve as reservoirs for environmental contamination and transmission to other animals and humans [4, 5]. Stressors including transportation, surgery, hospitalization, or antimicrobial therapy predispose adult horses to clinical disease [7, 4].

Diagnostic Approaches

Detection of Salmonella Typhimurium in Horses

Accurate diagnosis of equine salmonellosis requires a combination of microbiological, serological, and molecular methods. Fecal culture remains the gold standard, but sensitivity is limited by intermittent shedding; therefore, multiple sequential fecal samples (at least three to five) are recommended [2, 4]. Enrichment broths (e.g., tetrathionate or Rappaport-Vassiliadis) followed by selective plating on XLD or MacConkey agar increase isolation rates [2]. Biochemical confirmation and serotyping are essential for definitive identification of serovar Typhimurium [4].

Molecular techniques offer greater sensitivity and rapidity. PCR assays targeting the invA gene or other Salmonella-specific sequences are widely used for direct detection in feces [2, 13]. Whole-genome sequencing (WGS) provides the highest resolution for genotyping, antimicrobial resistance (AMR) gene profiling, and phylogenetic analysis [2, 3, 5]. WGS of S. Typhimurium isolates from foals has revealed diverse sequence types and many virulence genes [2]. For epidemiological investigations, pulsed-field gel electrophoresis (PFGE) has been extensively applied, with standardized protocols enabling comparison of equine isolates with those from other hosts via national databases such as PulseNet [1, 4, 5].

Matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS) can rapidly identify Salmonella at the genus level; serovar-specific biomarkers have been described for S. Typhimurium (notably a peak at approximately 7095 Da), enabling rapid serogroup determination from culture [14]. However, MALDI-TOF MS currently does not replace serotyping or WGS for definitive serovar assignment [14].

Septicemia Diagnostics in Foals

In foals with suspected septicemia, blood culture using automated blood culture systems is fundamental [6]. Paired blood samples (aerobic and anaerobic bottles) should be collected aseptically before antimicrobial administration [6]. Hematology and serum biochemistry reveal leukopenia, band neutrophilia, hypoglycemia, and elevated acute-phase proteins [6]. The presence of endotoxin in plasma can be assayed using the limulus amebocyte lysate (LAL) test, though it is not routinely performed [12].

Below is a summarized diagnostic algorithm:

flowchart TD
 A[Equine patient with fever, diarrhea, colic, or sepsis] --> B{Clinical suspicion of salmonellosis}
 B --> C[Collect feces from adult or blood/feces from foal]
 C --> D[Performed?]
 D -- Foal --> E[Blood culture, CBC, biochemistry]
 D -- Adult --> F[≥3 fecal samples for culture and PCR]
 E --> G{Blood culture positive?}
 G -->|Yes| H[Isolate, serotype, AST]
 G -->|No| I[Consider non-septic causes]
 F --> J{Fecal culture or PCR positive?}
 J -->|Yes| K["Serotype confirmation; PFGE or WGS for epidemiology"]
 J -->|No| L["Repeat sampling or consider other enteropathogens: ECoV, Clostridium"]
 H --> M[Antimicrobial therapy guided by AST]
 K --> N["Infection control: isolation, cleaning, surveillance"]

Antimicrobial susceptibility testing (AST) should be performed on all Salmonella isolates, using disc diffusion or broth microdilution methods, with results interpreted according to CLSI veterinary breakpoints [6, 8].

Antimicrobial Resistance and Therapy

The emergence of MDR S. Typhimurium strains in horses complicates therapy. Resistance to ampicillin, tetracycline, chloramphenicol, and doxycycline is common, while fluoroquinolones (e.g., ciprofloxacin) and aminoglycosides (e.g., amikacin, gentamicin) often retain in vitro activity [8, 5]. However, resistance genes are frequently plasmid-borne, facilitating lateral transfer to other bacteria [8]. Whole-genome sequencing of equine S. Typhimurium isolates has identified a high prevalence of AMR genes, including blaTEM, tet(A), and sul2 [2, 3]. In foals with septicemia, prompt administration of broad-spectrum antimicrobials (e.g., a combination of a beta-lactam and an aminoglycoside) is critical, with subsequent de-escalation based on AST results [6]. Supportive care includes intravenous fluid therapy, plasma transfusion for failure of passive transfer, anti-endotoxin therapies (e.g., polymyxin B), and vasopressor support for septic shock [6, 11]. For adult horses with colitis, the use of antimicrobials is controversial because they may exacerbate dysbiosis; however, systemic antimicrobials are indicated if septicemia is suspected or if there is severe leukopenia [7].

Epidemiology and One Health Considerations

S. Typhimurium is a leading cause of equine salmonellosis in many countries. Serotyping and molecular typing have demonstrated that equine isolates often belong to clones that also infect humans, cattle, and swine [4, 5]. In Argentina, PFGE analysis showed that S. Typhimurium from horses were genetically indistinguishable from human isolates, indicating cross-species transmission and potential zoonotic risk [4]. Similarly, in Japan, whole-genome analysis of S. Typhimurium and its monophasic variant (4,[2],12:i:-) from horses and cattle in a mixed farming region identified clusters of sequence type 34 (ST34) with very few single-nucleotide polymorphisms (SNPs) between isolates from different species, confirming interspecies transmission [5]. The monophasic variant S. 4,[2],12:i:- is increasingly reported in horses and shares genetic traits with S. Typhimurium [5]. Carrier horses, especially those in hospital settings, can shed large numbers of bacteria and contaminate the environment, leading to nosocomial outbreaks [4]. Therefore, biosecurity measures including isolation of affected horses, disinfection with products effective against Gram-negative bacteria (e.g., accelerated hydrogen peroxide), and surveillance of in-contact animals are essential [7, 4].

Conclusions

Salmonella Typhimurium remains a significant equine pathogen capable of causing severe enteric fever and colitis in adult horses and life-threatening septicemia in foals. Diagnostic strategies must account for intermittent shedding and the value of molecular methods for rapid detection and epidemiological tracking. The zoonotic potential of equine S. Typhimurium underscores the need for a One Health approach to surveillance and control. Antimicrobial stewardship is critical in the face of expanding MDR clones. Future research should focus on virulence factors that mediate equine-specific pathogenesis and on the development of effective vaccines for horses.

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