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

Introduction

Salmonellosis caused by Salmonella enterica subsp. enterica serovar Typhimurium is a significant bacterial disease of horses worldwide. The clinical spectrum ranges from subclinical shedding to acute enteric fever in adults and rapidly fatal septicemia in neonatal foals. The pathophysiological distinction between enterocolitis in immunocompetent adults and systemic invasion in immunologically naive foals reflects differences in host immunity, gut microbiota maturation, and bacterial virulence factor expression. This article provides a detailed clinical reference for veterinary practitioners and diagnostic microbiologists, emphasizing the biological mechanisms of infection, molecular characterization of isolates, and evidence-based management strategies.

Etiology

Salmonella Typhimurium is a Gram-negative, facultatively anaerobic, motile bacillus of the family Enterobacteriaceae. The serovar is defined by somatic (O) antigen O:4,5 (serogroup B) and flagellar (H) antigens phase 1 (i) and phase 2 (1,2). The bacterium possesses multiple virulence determinants encoded on pathogenicity islands (SPI), including the type III secretion system (T3SS) and effector proteins that mediate invasion of intestinal epithelial cells, survival within macrophages, and induction of proinflammatory cytokine cascades. Lipopolysaccharide (LPS) is a major outer membrane component that triggers Toll-like receptor 4 (TLR4) mediated innate immune responses.

Studies employing whole-genome sequencing (WGS) have identified conserved and accessory virulence genes in equine Salmonella isolates. Uprety et al. characterized the genomic architecture of Salmonella Saintpaul isolates from horses, revealing an array of fimbrial operons, iron acquisition systems, and antimicrobial resistance determinants that may contribute to equine adaptation [1]. Basso et al. performed WGS on Salmonella serovars recovered from diarrheic and non-diarrheic foals, demonstrating that S. Typhimurium isolates harbor a consistent set of SPI-1 and SPI-2 genes necessary for enteric and systemic pathogenesis [2].

Epidemiology

Salmonella Typhimurium is a host-generalist serovar that infects a broad range of mammals and birds. Horses acquire infection through the fecal-oral route, often by ingestion of contaminated feed, water, or fomites. Adult horses can become asymptomatic carriers, shedding the bacterium intermittently, particularly during periods of stress such as transport, surgery, or concurrent disease. Foals are more susceptible to clinical disease due to naive immune systems and an immature intestinal microbiome. The highest incidence of septicemic salmonellosis occurs in foals between 2 and 14 days of age.

Risk factors in equine populations include hospitalization, antimicrobial therapy (which disrupts the gut microbiota and reduces colonization resistance), overcrowding, and poor biosecurity. Outbreaks in veterinary teaching hospitals and breeding farms have been well documented, with S. Typhimurium being one of the most frequently isolated serovars from clinical equine cases.

Clinical Signs

Enteric Fever in Adult Horses

In adult horses, S. Typhimurium infection typically manifests as an acute enterocolitis characterized by fever (38.5-40.5 degrees C), depression, anorexia, and profuse, watery diarrhea that may contain blood or mucus. Abdominal pain (colic) due to intestinal cramping is common. The diarrhea can lead to rapid fluid and electrolyte losses, hypovolemia, metabolic acidosis, and endotoxemia. In severe cases, systemic inflammation progresses to disseminated intravascular coagulation (DIC), laminitis, and multiple organ dysfunction. Peracute cases may present with sudden death before diarrhea develops, due to overwhelming endotoxic shock.

Septicemia in Foals

Neonatal foals are at high risk for septicemic salmonellosis. The clinical signs differ from those in adults because of the foal's immature immune system and the tendency for bacterial translocation across the intestinal barrier. Affected foals exhibit:

• Acute onset of fever (often >39.5 degrees C), lethargy, and profound weakness
• Poor suckling reflex and progressive dehydration
• Diarrhea may be absent initially; the first sign may be septic shock with tachypnea, tachycardia, and cold extremities
• In some cases, S. Typhimurium can localize to joints (septic arthritis), the respiratory tract, or the central nervous system
• Rapid clinical deterioration and high case fatality rate without aggressive intervention

Pathology

Gross pathological findings in adult horses with enteric fever include thickening and hyperemia of the colonic and cecal mucosa, multifocal hemorrhages, fibrinonecrotic pseudomembranes, and serosal edema. The mesenteric lymph nodes are often enlarged and congested.

In septicemic foals, lesions may be more subtle. Multifocal petechiae and ecchymoses on serosal surfaces and in the renal cortex are common. The liver and spleen are frequently enlarged and may contain small areas of necrosis. Histologically, Gram-negative bacteria are visible within Kupffer cells in the liver and within splenic macrophages. Intravascular thrombi in the kidneys and lungs reflect DIC. Immunohistochemistry for Salmonella LPS can confirm bacterial localization.

Diagnostics

A definitive diagnosis requires isolation or molecular detection of S. Typhimurium in fecal specimens, blood cultures, or postmortem tissues.

Culture

Selective enrichment media such as tetrathionate broth or selenite cysteine broth, followed by plating on MacConkey agar, Hektoen enteric agar, or xylose-lysine-deoxycholate (XLD) agar, are standard. Colonies with typical morphology (colorless with black centers on XLD agar) are biochemically confirmed and serotyped with O- and H-specific antisera.

Molecular Detection

Real-time PCR assays targeting the invA gene (invasion protein A) provide rapid, sensitive detection directly from feces. PCR is particularly useful for identifying carriers or early infections before culture positivity.

Serotyping and Subtyping

Serotyping based on the Kauffmann-White scheme remains the gold standard for epidemiological classification. Molecular subtyping methods include:

• Pulsed-field gel electrophoresis (PFGE): Generates DNA fingerprint patterns for outbreak investigation. Dudek et al. demonstrated that PFGE profiles can distinguish between clinical and reptile-associated Salmonella strains, with implications for source tracking [3].
• Whole-genome sequencing (WGS): Provides the highest resolution for phylogenomic analysis and can predict antimicrobial resistance genes and virulence factors. Basso et al. used WGS to examine genetic diversity among Salmonella serovars recovered from foals, identifying clonal relationships and resistance determinants [2].

Diagnostic Workflow

The following Mermaid diagram illustrates a recommended diagnostic decision tree for suspected equine salmonellosis.

flowchart TD
    A[Equine patient with fever, diarrhea, or septic shock], > B{Foal or Adult?}
    B, >|Foal <14 days| C[Collect blood culture + fecal swab]
    B, >|Adult| D[Collect fecal sample (rectal swab or feces)]
    C, > E[Perform real-time PCR for invA from feces]
    D, > E
    E, > F{PCR positive?}
    F, >|Yes| G[Inoculate selective enrichment broth for culture]
    F, >|No| H[Consider other enteric pathogens; repeat PCR if clinical suspicion high]
    G, > I[Plate on XLD agar; incubate 24h at 37°C]
    I, > J[Pick suspect colonies; perform biochemical confirmation + serotyping]
    J, > K[Optional: PFGE or WGS for epidemiological typing]
    K, > L[Antimicrobial susceptibility testing (disk diffusion or broth microdilution)]
    L, > M[Report results and guide therapy]
    G, > N[If blood culture positive in foal, identify isolate directly from blood culture bottle]
    N, > K

Treatment

The cornerstones of treatment are aggressive supportive care and targeted antimicrobial therapy.

Supportive Care

• Intravenous fluid therapy to correct dehydration and electrolyte imbalances. Balanced crystalloid solutions (e.g., lactated Ringer's solution) with additional potassium and sodium bicarbonate as dictated by serial electrolyte and blood gas analysis.
• Colloidal support (e.g., hydroxyethyl starch or plasma) for hypoproteinemia.
• Anti-endotoxic measures: hyperimmune plasma containing anti-LPS antibodies may be administered in septic foals.
• Non-steroidal anti-inflammatory drugs (NSAIDs) such as flunixin meglumine to reduce fever, endotoxemia, and abdominal pain. However, careful monitoring for gastrointestinal and renal adverse effects is essential.
• Intestinal protectants such as di-tri-octahedral smectite (a clay-based adsorbent) can bind bacterial toxins and reduce diarrhea severity.

Antimicrobial Therapy

Selection of antimicrobials should be guided by culture and susceptibility results. Common empirical choices for septic foals include:

• Ceftiofur (third-generation cephalosporin) at 4-6 mg/kg intravenously every 12 hours
• Gentamicin (6.6 mg/kg intravenously every 24 hours) in combination with ceftiofur, with careful monitoring of renal function
• Enrofloxacin (5-7.5 mg/kg intravenously every 24 hours) in adult horses, although its use in foals is limited due to potential arthropathy

Antimicrobial resistance in Salmonella Typhimurium from horses is an emerging concern. Uprety et al. identified multiple resistance genes in equine Salmonella isolates, including blaTEM, tet(A), and sul2 [1]. Routine susceptibility testing is critical to avoid therapeutic failure.

Control and Prevention

Control of S. Typhimurium in equine populations relies on strict biosecurity and hygiene.

• Quarantine of new arrivals for at least 7-14 days with repeated fecal cultures or PCR to detect carriers.
• Isolation of diarrheic horses and those with confirmed salmonellosis in dedicated infectious disease stalls.
• Use of footbaths, dedicated equipment, and barrier nursing for isolation units.
• Environmental disinfection with agents effective against organic matter, such as accelerated hydrogen peroxide or sodium hypochlorite (1:10 dilution).
• Implementation of an infection control plan that includes regular surveillance cultures of high-risk areas (e.g., neonatal intensive care units).
• Vaccination is not routinely practiced in horses due to limited efficacy of available vaccines; however, autogenous vaccines have been used in some outbreak settings.

Conclusions

Salmonella Typhimurium remains a major cause of enteric fever and septicemia in horses, with distinct clinical presentations in adults and foals. Advances in molecular diagnostics, including WGS, have improved our understanding of the genetic diversity and virulence potential of equine isolates. Rapid diagnosis, aggressive supportive therapy, and judicious antimicrobial use guided by susceptibility testing are essential to reduce mortality. Effective biosecurity measures are critical to prevent hospital-acquired infections and farm outbreaks. Future research should focus on the role of the equine gut microbiome in colonization resistance and the development of novel intervention strategies such as probiotics or bacteriophage therapy.

References

[1] Uprety T, Shaffer CL, Loynachan A, et al. Genomic characterization of Salmonella enterica serotype Saintpaul isolates from horses. Vet Microbiol 2026. https://pubmed.ncbi.nlm.nih.gov/41653782/

[2] Basso RM, Cerri FM, Possebon FS, et al. Whole-genome sequencing of Salmonella serovars isolated from diarrheic and non-diarrheic foals. J Vet Diagn Invest 2025. https://pubmed.ncbi.nlm.nih.gov/39930357/

[3] Dudek B, Książczyk M, Krzyżewska E, et al. Comparison of the phylogenetic analysis of PFGE profiles and the characteristic of virulence genes in clinical and reptile associated Salmonella strains. BMC Vet Res 2019. https://pubmed.ncbi.nlm.nih.gov/31477105/