Bit & Bridle

Equine Salmonellosis

Other Names

  • Salmonellosis
  • Salmonella colitis
  • Equine typhoid

What is Salmonellosis in Horses?

Salmonellosis is a bacterial infection of the gastrointestinal tract caused by Salmonella enterica, a Gram-negative rod-shaped bacterium. It is one of the most clinically significant infectious diseases affecting horses, capable of causing anything from mild, subclinical shedding to severe, life-threatening haemorrhagic diarrhoea and septicaemia. It is also a zoonotic disease, meaning it can be transmitted from horses to humans — making it an important public health consideration as well as a clinical one.

Horses can carry Salmonella without showing any clinical signs and intermittently shed bacteria in their faeces. This “carrier state” is a significant source of infection for other horses, and potentially for humans who handle affected animals or their environments without adequate precautions. Salmonellosis is a notifiable disease in many jurisdictions, and outbreaks at equine hospitals and competition venues have occurred.

In the UK, Salmonella serovar Typhimurium and enteritidis are among the most commonly isolated, though many serovars affect horses. The disease is of particular concern in hospitalised horses under stress or receiving antibiotics, as these factors can precipitate shedding and clinical illness.

Causes

Salmonellosis develops when a horse ingests sufficient numbers of Salmonella bacteria:

  • Contaminated feed or water: Feed contaminated with animal-derived products (such as bone meal in feeds, though rare in equine diets), or water contaminated by run-off from livestock operations
  • Fomites: Shared equipment, hands, boots, and vehicles can transfer bacteria between horses
  • Direct contact: With infected horses or their faeces
  • Stress: Stressful events (transport, surgery, change of management, concurrent illness) can trigger clinical disease in carrier horses or reduce immune resistance
  • Antibiotic use: Broad-spectrum antibiotics disrupt the normal gut microbiome, making horses susceptible to Salmonella overgrowth
  • Hospitalisation: Hospital-acquired (nosocomial) salmonellosis is a recognised risk in equine referral hospitals

Symptoms and Signs

Clinical signs vary with the severity of infection:

Subclinical infection: No obvious illness; horse sheds bacteria in faeces. This is the most common presentation and a significant biosecurity concern.

Mild to moderate disease:

  • Fever (38.5–40.5°C)
  • Mild colic and abdominal discomfort
  • Soft faeces progressing to profuse watery diarrhoea
  • Depression and reduced appetite
  • Dehydration

Severe disease:

  • Bloody or haemorrhagic diarrhoea
  • High fever and severe depression
  • Laminitis (from endotoxaemia)
  • Septicaemia — particularly in foals, immunocompromised horses, or neonates
  • Multi-organ failure in the most severe cases

Diagnosis

Diagnosis requires laboratory confirmation because other causes of colitis produce similar signs:

  • Faecal bacteriological culture: The gold standard. Multiple samples over consecutive days may be needed as shedding can be intermittent. Culture results take 24-72 hours.
  • PCR testing: Faster and more sensitive than culture for detection of Salmonella DNA in faeces. Increasingly used in practice.
  • Blood culture: For cases with suspected septicaemia, blood culture may identify bacteraemia
  • Haematology and biochemistry: Leucopaenia (low white cell count) is a common early finding. Electrolyte disturbances guide fluid therapy.
  • Differentials to exclude: Potomac Horse Fever, Clostridial colitis, cyathostominosis (encysted small redworm emergence), and NSAID-induced colitis

Any horse with suspected salmonellosis should be isolated immediately pending results, regardless of clinical severity.

Treatment

Treatment is primarily supportive; antibiotics are used selectively:

  • Fluid therapy: Intravenous fluids to correct dehydration and electrolyte imbalances are the cornerstone of treatment in moderate to severe cases
  • NSAIDs: Flunixin meglumine helps control fever, pain, and the effects of endotoxin
  • Plasma transfusion: In foals or horses with severe protein loss, plasma provides albumin and immunoglobulins
  • Antibiotics: Not routinely indicated for uncomplicated salmonellosis, as they can prolong shedding and do not shorten the course of disease in mild-moderate cases. Reserved for cases with confirmed septicaemia, severe immunosuppression, or foals with failure of passive transfer. Trimethoprim-sulfadiazine or enrofloxacin (under veterinary direction) may be used.
  • Nutritional support: Maintaining caloric intake with palatable, easily digestible feed
  • Laminitis management: Deep bedding, frog support, and close monitoring in horses with endotoxaemia

Prevention

  • Biosecurity: Isolate any horse with diarrhoea pending diagnosis. Use separate footwear, clothing, and equipment for isolation areas.
  • Hand hygiene: Wash hands thoroughly after handling affected horses, their faeces, or their environment. This is critical given the zoonotic risk.
  • Stress reduction: Minimise unnecessary stress, particularly around transport, surgery, and changes in management
  • Judicious antibiotic use: Only use antibiotics when clinically justified, and always under veterinary supervision
  • Clean water supply: Ensure water sources cannot be contaminated by livestock faeces or run-off
  • Quarantine of new arrivals: New horses should be isolated for a minimum of two weeks before mixing with the existing population

Prognosis

The prognosis depends heavily on the severity of disease, the age and health status of the horse, and how quickly treatment is initiated. Horses with mild to moderate disease that respond to fluid therapy generally do well. Severe haemorrhagic colitis, septicaemia, or cases complicated by laminitis carry a guarded to poor prognosis. Neonatal foals with Salmonella septicaemia have particularly high mortality rates. Subclinical carrier horses may resolve their infection over time or remain long-term carriers. Long-term management of carrier horses is a complex biosecurity challenge.

When to Call the Vet

  • Any horse with watery or bloody diarrhoea lasting more than 24 hours
  • A horse showing diarrhoea alongside fever or depression — especially if recently transported, hospitalised, or treated with antibiotics
  • Suspected diarrhoea in a foal of any age — treat as urgent
  • Multiple horses on the same property developing diarrhoea — implement isolation and contact your vet immediately
  • Any person who handles a horse with diarrhoea and then develops gastrointestinal illness themselves should seek medical advice and mention the horse contact

Frequently Asked Questions

Can I catch Salmonella from my horse?

Yes. Equine salmonellosis is a zoonotic disease. Transmission from horses to humans occurs via the faecal-oral route — typically through contaminated hands that then touch the face or food. The risk is highest when handling horses with active diarrhoea. Wearing gloves, washing hands thoroughly after contact, and avoiding touching your face are effective precautions. Immunocompromised individuals and young children should take particular care.

Why aren’t antibiotics always used for Salmonella in horses?

Antibiotics can paradoxically prolong Salmonella shedding in the faeces in some cases and do not shorten the course of uncomplicated disease. Additionally, antimicrobial resistance is a growing concern, and inappropriate antibiotic use in horses contributes to this. Antibiotics are reserved for cases with bacteraemia, severe immunosuppression, or neonatal septicaemia where the risk-benefit clearly favours treatment.

How long does a horse shed Salmonella?

Shedding duration varies widely. In acute clinical cases, horses typically shed for one to four weeks. Some horses become intermittent or persistent subclinical shedders for months. Multiple negative faecal cultures over consecutive days are required before a horse can be considered cleared for mixing with other horses.

Is there a vaccine for Salmonella in horses?

There is no commercially available licensed vaccine for Salmonella in horses in the UK. Prevention relies entirely on good management, hygiene, and biosecurity practices.