Salmonellosis refers to infection with Salmonella, a bacterium that primarily causes intestinal infections and diarrhea in mammals. Salmonella can affect numerous species, including horses and humans. Horses typically acquire the bacteria by ingesting contaminated feed or water.

Common symptoms of salmonellosis in horses include diarrhea, fever, and lethargy. In some cases, Salmonella can cause sepsis (blood infection), which can be life-threatening. Veterinarians use a fecal or blood sample to identify the bacteria and make a diagnosis.

Treatment of Salmonella infection often requires hospitalization and intensive care. Fluid therapy, antibiotics, and toxin neutralization products are components of many treatment protocols. The prognosis for horses with Salmonella infection depends on the severity of disease, with outcomes ranging from excellent to very poor.

Preventative measures focus on reducing exposure to Salmonella through environmental management, hygiene, and quarantine of new arrivals.

Salmonella Infection in Horses

Salmonella bacteria are a common intestinal pathogen (disease causing agent) associated with diarrhea and intestinal infections in many species. [1] The most common intestinal strains of Salmonella in horses are Typhimurium, Newport, and Agona. [1]

Horses also have a host-adapted strain of this bacteria, S. Abortusequi, which only affects horses. [1] This strain causes abortions in affected broodmares and sepsis (blood infection) in newborn foals. [1]

Most horses acquire Salmonella from their environment. [1] Possible exposure routes include: [1]

  • Ingestion of contaminated feed
  • Ingestion of feces from mice or birds
  • Consuming contaminated water
  • Contact with contaminated surfaces, including the hands of handlers
  • Contact with livestock species shedding Salmonella

Once ingested, Salmonella bacteria attach to the intestinal lining and inject bacterial proteins into the intestinal cells. [1] These proteins trigger transfer of the bacteria from the intestinal lining into the bloodstream. [1] If not controlled by the immune system, the transfer of bacteria can result in sepsis.

Infection of intestinal cells also triggers inflammation of the intestine, resulting in cell damage. [1] This inflammation triggers fluid secretion into the intestines, causing the increased fecal fluidity associated with diarrhea. [1]

Additionally, damage to the intestinal cells allows protein within the bloodstream to enter the intestines, resulting in hypoproteinemia (low blood protein). [1] One of the major roles of proteins in the body is preventing fluid from leaving the bloodstream, meaning that hypoproteinemia further increases fluid secretion and worsens diarrhea. [1]

Risk Factors

Not all horses develop symptoms after infection with Salmonella. [1] Risk factors for developing infectious symptoms after Salmonella exposure include: [1][2][3][4]

  • Age: young horses have a higher risk of developing symptoms compared to adults
  • Stress: horses experiencing stress are more likely to develop disease; including stressful events such as transportation, surgery, feed changes, or other illnesses
  • Intestinal flora: horses with healthy intestinal flora are less likely to develop symptoms
  • History of antibiotic use: antibiotics may disrupt the intestinal flora and make Salmonella infection more likely
  • History of hospitalization: equine hospitals may have a high burden of Salmonella in the environment, increasing the likelihood of infection; in particular, horses undergoing abdominal surgery have the highest risk of infection after hospitalization
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Types of Salmonella Infection

There are five main syndromes associated with Salmonella infection in horses: [1][3]

  • Fever with mild diarrhea
  • Toxic enterocolitis
  • Sepsis
  • Asymptomatic disease
  • Equine paratyphoid

Salmonella can also cause abortion and liver infections less commonly in adult horses. [1] In foals, Salmonella infection can also target the bones, joints, and umbilicus. [1]

Fever and Diarrhea

Mild diarrhea with fever typically indicates Salmonella infection of the intestinal tract that has not triggered a widespread inflammatory response. [1] These horses typically recover uneventfully when their immune system gains control of the infection. [1]

In this form of Salmonella infection, infected horses initially show no symptoms. [1] Despite not showing symptoms, the horse sheds the bacteria in their feces, resulting in a potential source of infection for other horses. [1]

After a few days to a week, affected horses may develop symptoms such as: [1]

Symptoms typically last 1-3 days. [1]

Toxic Enterocolitis

Toxic enterocolitis is a severe inflammatory disease of the intestine and colon. [1] The main symptom of toxic enterocolitis is large volumes of liquid diarrhea often referred to as “pipe stream” or “fire hose” diarrhea. [1] Diarrhea typically develops 1-3 days after infection. [1] A “septic tank” odor is characteristic of Salmonella infection. [3]

Other symptoms include: [1]

  • Fever
  • Severe lethargy
  • Reduced appetite
  • Increased heart rate
  • Bright red gums and mucous membranes

Without intensive treatment, most horses that develop toxic enterocolitis will develop sepsis. [1] Some horses may develop laminitis due to bacterial toxins entering the bloodstream. [1]

Sepsis

Sepsis refers to bacteria entering the blood stream and causing an infection. [1] When this happens, the bacteria can spread to multiple organs, resulting in a whole-body infection that is difficult to treat. [1]

During sepsis, Salmonella bacteria produce endotoxin, a bacterial toxin that triggers vasodilation (widening of blood vessels) throughout the body. [1] Vasodilation results in a massive drop in blood pressure that the heart often cannot compensate for. [1]

Ultimately, this results in inadequate delivery of oxygen and nutrients to the body’s tissues, causing cell death and organ failure. [1] Veterinarians refer to this outcome as toxic shock syndrome. [1]

Symptoms of sepsis include:

  • Fever
  • Severe lethargy
  • Reduced manure production
  • Rapid heart rate
  • Collapse
  • Death

Sepsis is frequently fatal even with appropriate treatment, due to the development of toxic shock syndrome. [1]

Asymptomatic Disease

Asymptomatic horses shed Salmonella in their feces, however they do not show any symptoms of disease. [1] Studies suggest that around 1% of horses shed Salmonella as asymptomatic carriers. [1] Most carriers stop shedding the bacteria by 7 to 31 days after infection. [1]

The reason some horses develop disease while some become carriers is unknown. [1] However, research suggests it may be related to the relative dose of Salmonella that the horse receives upon exposure. [1] Horses that ingest a low dose of bacteria are more likely to become carriers, while higher doses are more likely to cause symptoms. [1]

Equine Paratyphoid

Equine paratyphoid refers specifically to infection with S. Abortusequi, a strain of Salmonella that only affects horses. [3] This strain causes abortion in pregnant mares and illness in newborn foals. [3]

Symptoms in pregnant mares include: [3]

  • Fever
  • Reduced appetite
  • Diarrhea
  • Abortion

Stillborn foals often have multiple small hemorrhages throughout their body and fluid accumulation in the pericardium (sac around the heart) and the abdomen. [3]

Live foals born affected with S. Abortusequi primarily develop pneumonia and intestinal infections, both of which are severely and rapidly life-threatening to newborns. [3]

Diagnosis

There are many potential causes of diarrhea in horses, so diagnosis typically begins by identifying the most likely cause. [1]

Other possible causes that must be ruled out include: [5]

To identify the most likely cause of diarrhea, veterinarians may perform diagnostics such as: [5]

  • Bloodwork
  • Urinalysis
  • Ultrasound of the abdomen
  • Rectal palpation
  • Abdominocentesis, taking a sample of fluid from the abdomen

Definitive diagnosis of Salmonella requires bacterial culture. [1] In most cases, veterinarians submit a fecal sample for bacterial culture. [1][2] This test allows confirmation of the bacterial species, as well as the identification of which Salmonella strain is present. [1]

In horses showing signs of sepsis, veterinarians may submit blood samples instead, to demonstrate that the infection is present in the bloodstream. [1]

Treatment

The treatment plan for Salmonella depends on the type of infection present. [1] More severe infections, such as toxic enterocolitis or sepsis, typically require hospitalization for intensive treatment. [1]

Components of a typical treatment plan include: [1]

  • Antibiotics
  • Fluid therapy
  • Toxin neutralization

These horses also require supportive care including frequent bathing, regular stall cleaning, and dietary support. [1]

Antibiotics

The use of antibiotics in treating Salmonella cases is a matter of veterinary debate, as there is some evidence that antibiotic use does not change the duration of symptoms or bacterial shedding in the feces. [1]

Because of this and concerns about antibiotic resistance, most veterinarians limit use of antibiotics to only the most severely affected horses. [1]

Veterinarians use a susceptibility panel to guide their antibiotic selection process. [1] This laboratory test involves subjecting the bacteria cultured from the feces or blood to several different antibiotics and concentrations of antibiotics, to determine the most effective treatment. [1]

Common antibiotics used for treating Salmonella include: [1][5]

  • Enrofloxacin
  • Ceftiofur
  • Gentamicin
  • Amikacin

Fluid therapy

Horses experiencing diarrhea from Salmonella infection are at a high risk of dehydration due to fluid loss. [1] Therefore, large volumes of intravenous (into a vein) fluids are necessary when treating horses for Salmonella infection. [1]

Horses also lose large amounts of electrolytes in diarrhea, which must be replaced for the horse to recover. [1] Veterinarians can adjust the levels of electrolytes in the intravenous fluids to compensate for the horse’s electrolyte loss. [1]

Making adjustments in electrolyte delivery requires repeated bloodwork to monitor the horse’s blood electrolyte concentrations. [1]

Fluid therapy is also crucial for horses experiencing hypoproteinemia due to their infection. [1] Veterinarians can use colloidal fluids to replace the fluid-retaining effects of protein in the blood. [1] Use of these products can prevent some fluid loss and reduce the amount of intravenous fluids needed. [1]

Toxin neutralization

Endotoxins produced by Salmonella can have devastating effects on the cardiovascular system, potentially resulting in death. [1] Veterinarians managing cases of Salmonella sepsis focus on toxin neutralization to either remove the toxin from the bloodstream or prevent its effects. [1]

Treatments may include: [1]

  • Oral toxin binders: compounds such as di-tri-octahedral smectite (Biosponge®) may bind toxins released in the intestine and prevent their absorption into the bloodstream
  • Antiendotoxin antibodies: these are blood products gathered from horses with antibodies against endotoxins and administered to an affected horse; the additional antibodies in the blood product bind the endotoxins and prevent its effects
  • Polymyxin B: this antibiotic binds to lipid A, a major component of endotoxins, and prevents it from affecting the blood vessels
  • Non-steroidal anti-inflammatory drugs: endotoxins cause vasodilation through activating inflammation, meaning that NSAIDs may prevent some of their vasodilatory effects

Prevention

Preventing Salmonella infection in your horse relies on reducing exposure to the bacteria. [1] Strategies include: [1]

  • Quarantine: all horses arriving at a farm should be quarantined for 14 to 21 days in case they are carriers of Salmonella or other infectious diseases
  • Pasture management: managing pastures to prevent overgrazing and overcrowding reduces the fecal load on the soil, and allows horses to avoid grazing in areas where horses have defecated
  • Feed management: facility owners should implement rodent and bird control practices on their farms, including storing grain in rodent-proof containers
  • Water testing: water testing for specific pathogens can identify Salmonella contamination in the water source
  • Manure management: facility owners should consider placement of manure disposal sites to ensure that runoff does not contaminate pastures or paddocks
  • Hygiene: regular cleaning can remove Salmonella in the environment to reduce exposure risk for horses. Cleaning is particularly important in the housing areas of any horses with a confirmed Salmonella infection

Humans are also susceptible to Salmonella infection, and can become exposed handling Salmonella-infected horses. [1]

Individuals handling affected horses should use personal protective equipment and wash their hands regularly after horse handling. [1] Large operations might consider having dedicated handlers for sick and quarantined animals.

Prevention in Hospitals

Equine hospitals have a high risk of contamination by Salmonella, due to a constant rotation of horses on the property who may be carriers. [6]

Additionally, many horses with Salmonella infection require hospitalization, resulting in a high degree of environmental contamination in the hospital environment. [6] Studies show that between 3-9% of horses hospitalized for colic symptoms shed Salmonella in their feces at least once during their hospital stay. [7][8]

Hospitals attempt to mitigate Salmonella infection risk by: [6]

  • Strict biosecurity and isolation protocols for horses with diarrhea
  • Thorough cleaning protocols between hospitalized horses, particularly for horses who had diarrhea
  • Personal protective equipment for individuals handling diarrhea cases
  • Testing the environment for Salmonella, including testing stall walls, veterinary equipment, feed, and water

Summary

Salmonellosis describes infection with the bacterium Salmonella, which can cause intestinal infections and diarrhea in many species, including horses.

  • Horses typically acquire Salmonella through contaminated feed or water.
  • Symptoms include diarrhea, fever, and lethargy.
  • Salmonella can cause life-threatening sepsis if the bacteria enters the bloodstream.
  • Diagnosis of Salmonella involves a fecal or blood sample for bacterial culture.
  • Treatment can involve a combination of antibiotics, fluid therapy, and toxin neutralization.
  • The prognosis ranges widely depending on the severity of infection.
  • Prevention focuses on reducing exposure through environmental management, cleaning strategies, and quarantine.

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References

  1. Sellon, D. C. & Long, M. T., Eds., Equine Infectious Diseases. Second edition. Saunders/Elsevier, St. Louis, Missouri. 2013.
  2. Burgess, B. A. Salmonella in Horses. Veterinary Clinics of North America: Equine Practice. 2023. View Summary
  3. Barrow, P. A. & Methner, U., Salmonella in Domestic Animals. 2nd ed. CABI, Wallingford. 2013.
  4. Ekiri, A. B. et al., Epidemiologic Analysis of Nosocomial Salmonella Infections in Hospitalized Horses. Journal of the American Veterinary Medical Association. American Veterinary Medical Association. 2009. View Summary
  5. Lavoie, J.-P., Ed., Blackwell’s Five-Minute Veterinary Consult. Equine. Third edition. Wiley-Blackwell, Hoboken, NJ. 2019.
  6. Burgess, B. A. & Morley, P. S., Managing Salmonella in Equine Populations. Veterinary Clinics of North America: Equine Practice. 2014. View Summary
  7. Kim, L. et al., Factors Associated with Salmonella Shedding among Equine Colic Patients at a Veterinary Teaching Hospital. Journal of the American Veterinary Medical Association. American Veterinary Medical Association. 2001. View Summary
  8. Kilcoyne, I. et al., Prevalence of and Risk Factors Associated with Salmonella Shedding among Equids Presented to a Veterinary Teaching Hospital for Colic (2013–2018). Equine Veterinary Journal. 2023. View Summary