Salmonella in Horses

Salmonella can infect most mammals, birds, reptiles, and even humans with its many strains. Horses generally contract the bacteria through the oral ingestion of contaminated fecal material. Infected horses shed the bacteria through their feces, thereby infecting pastures, stalls, grooming tools, equipment, feed, and virtually anything else they come into contact with. Even carrier horses that show no symptoms whatsoever can shed the bacteria. Human handlers are not only at risk of helping to spread the bacteria, but can also become infected as well. It is important to practice effective management strategies and hygiene to ensure the health of you and your horses.

Salmonellosis is an infection of the salmonella bacteria, and is a commonly diagnosed cause of diarrhea in horses. Symptoms can range from mild fevers and gastrointestinal complaints, to severe pain, dehydration, and systemic failure which can be fatal. While many cases of salmonella can be effectively treated, it is important to recognize the signs and begin treatment early to avoid life threatening complications.

The symptoms of salmonella can vary from carrier horses that show no signs, to systemic disease that can lead to death. Signs include:

• Severe diarrhea that is watery, colored green to black, and sometimes bloody and foul smelling
• Fever
• Weakness
• Lethargy
• Loss of appetite
• Abdominal pain
• Dehydration
• Colic
• Decreased alertness
• Decreased sensation to pain
• Anorexia 
• Depression
• Soft tissue infections 
• Enteritis, or inflammation of the intestines
• Intestinal impactions and obstructions
• Gastric reflux
• Purple or dark red mucous membrane
• Abdominal distention 
• Laminitis, a disease of the hoof
• Increased heart rate at rest
• Hypoproteinaemia, or low levels of proteins in the blood
• Blood clots
• Bone and joint infections
• Fungal pneumonia 
• Coagulation abnormalities
• Blood poisoning, or septicemia
• Renal failure
• Failure of the heart to pump
• Hepatitis
• Abortion

Foals can have addition symptoms of:

• Lameness
• Joint swelling
• Pneumonia
• Meningitis
• Septic arthritis
• Inflammation of the growth plate

Types

There are generally 4 types of salmonella recognized, related to the severity of clinical signs.

• Asymptomatic carrier – This is when a horse shows no symptoms, but may be actively shedding the bacteria, thereby potentially transmitting it to other animals; this carrier may develop symptoms, especially in times of stress
• Mild form – Horses suffer from fever, depression, loss of appetite, anorexia, and soft feces
• Severe form – Horses suffer from colitis, abdominal pain, watery and foul smelling feces, and subsequently, dehydration; electrolyte losses and metabolic imbalances can lead to systemic infections
• Systemic disease – The horse’s body can experience severe colic, severe pain, colon inflammation, septicemia, bone and joint infections, and go into shock; these symptoms can progress into life threatening conditions

The cause of salmonella in horses is from the oral ingestion of the Salmonella bacteria. This bacteria is shed in the feces of contaminated horses, which is usually transmitted through contaminated environments or objects. The salmonella organisms can also become aerosolized. Paths of transmission include:

• Contact with contaminated animals that are actively shedding the bacteria
• Ingestion of contaminated water, hay, grass, feeds, or insects
• Contact with contaminated environment or objects, including water buckets, tack, grooming tools, equipment, stalls, bedding, boots, or handlers
• Contamination through an open wound 
• Aerosol contamination through eyes and nose 
• Foal infection through the umbilical stump 

Stress or illness can promote the shedding of bacteria. These stimuli can include:

• Surgery
• Transportation
• Crowding 
• Changes in feed
• Other diseases, especially GI disorders
• Antimicrobial drug treatment 
• Intensive physical activity  
• Training

Diagnosis is made based on clinical signs, a history of salmonella in other horses in the population, a history of transportation, and the results of testing. Examining multiple fecal cultures can positively identify the presence of the salmonella bacteria in your horse, as well as the particular strain. Due to the similarity of symptoms to other diseases, your veterinarian may also examine cultures taken from the wall or content of the intestines. Your vet may also use a polymerase chain reaction test (PCR) which can yield better and faster results than a bacterial culture, but often cannot identify the particular strain of salmonella affecting your horse. A punch biopsy of the rectal tissue can also be analyzed for the presence of the bacteria. Other tests can include blood counts and cultures, electrolyte analysis, and joint, cerebrospinal or tracheal fluid testing in foals.

Salmonella is treated through aggressive supportive therapies to address the symptoms, namely metabolic imbalances and diarrhea, through the use of drug treatments, and through the isolation of the affected horse and contamination management.

Supportive therapies include nutritional support, fluid and electrolyte administration, plasma therapy, probiotics, and gastrointestinal absorbents and protectorants. Use of these supportive therapies can resolve signs within 7 to 10 days. During any treatments, blood samples should be closely monitored for changes in electrolyte and other metabolic factors. Therapies can be altered based on findings.

Some cases may need drug treatments. Anti-inflammatory drugs are often used to manage abdominal inflammation and discomfort. Anti-endotoxin antibodies are used to neutralize the bacterial endotoxins. Antimicrobial therapy may be recommended, but is often not used due to the possibility of the development of drug resistant strains of the Salmonella bacteria.

Isolation measures includes stall confinement, turnout in an isolated paddock, and repeated cultures to determine the presence of the bacteria in your recovering horse. Making sure the environment is cleaned of the bacteria is essential for the health of the rest of your population. Remove infected bedding, feed, and manure. Use an appropriate disinfectant on stalls, surfaces, water buckets and feed tubs, grooming tools, equipment, tack, and any other contaminated items or areas. Samples of the environment can be used to determine efficacy.

Salmonella is a zoonotic disease, meaning that humans can become infected as well, so take measures to protect yourself. Anyone handing infected horses should always wash hands after contact with the horse or its environment, and wear disposable plastic boots.

Recovery of your horse will depend on how severe the infection is when treatment begins, and on the efficacy of the treatment. Many horses can recover, and discontinue shedding within 4 months. However, recovered horses can continue to shed the bacteria for days to months longer, highlighting the need for infection control. Diarrhea lasting longer than 10 days indicates a severe loss of colonic mucosa, and horses are less likely to survive. 

Management practices can help to minimize the spread and progression of this potentially fatal disease. Avoid stressors for your horse by not overcrowding, minimizing diet changes, isolating new arrivals, thoroughly and regularly cleaning horse trailers, providing individual feed and water buckets for traveling horses, practicing consistent hygiene, and changing clothing and boots after visiting another horse facility or farm.