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Salmonella in horses

Prevalence

Infections caused by Salmonella spp are common worldwide, however, salmonella infection in horses is relatively rare in Sweden.

General facts about salmonella

Several salmonella serotypes can affect horses. The most common, S. Typhimurium, is further subdivided into phage types. Some of the phage types have increased resistance to antimicrobial substances, such as phage type DT 104. It has been detected in a few imported horses.

S. Abortusequi is a serotype that is more or less specific to horses, but is unusual and has not been detected in Sweden.

Symptoms

The development of symptoms depends on:
• Dose of infection. It usually takes a large dose of bacteria for the animal to become ill.
• Immune status of the individual.
• What kind of serotype of salmonella it is.

Many infected horses show no symptoms. Infected foals are more prone to clinical disease than adult horses.

Diarrhoea, often severe and watery, is the most common symptom.
Other symptoms are:
- Fever
- Colic
- Poor general condition
The infection is often self-limiting.

If the condition deteriorates and the loss of body fluid becomes too severe it may result in the death of the horse. This will also be the result if septicaemia and/or endotoxic shock occurs.
In severe cases the following symptoms can be seen:
- Dehydration, visible as hollow eyes and dry skin
- Tachycardia
- Dark red and discoloured mucous membranes
- Diarrhoea, which may appear green to black in colour

Septicaemia can, particularly in foals, lead to arthritis/polyarthritis, and/or pneumonia.

Laminitis is considered a possible complication of salmonellosis in horses. There is a suspicion that the bacteria’s endotoxins may play a role in the pathogenesis of laminitis.
Abortion is a further complication mainly associated with S. Abortusequi.

Transmission of the disease

Salmonella is excreted through faeces and the route of transmission is primarily faecal-oral, i.e. through faecal contaminated feed or water, but aerosol infection may also occur. Birds, rodents or other horses are examples of carriers that may contaminate horse feed, water and environment.

Acutely ill animals excrete large amounts of bacteria. The excretion decreases gradually over time and ceases, unless the animal has become a symptomless carrier. It is a general perception that antibiotic treatment increases the risk for symptomless carriers. In addition, there is a risk that the symptoms may worsen since antibiotics disrupt the normal flora of the intestine. Therefore, it is recommended not to treat cases of uncomplicated diarrhoea with antibiotics, but give  supportive treatment (such as fluid therapy) if necessary.

Risk factors for increased excretion of salmonella and subsequent development of disease include stress factors like transport, overcrowding, changes in feed regimen, intense physical activity, antibiotic treatment, surgical treatment etc..

A major problem in the spread of salmonella is its long-term survival in the environment, which means that the infection can reoccur despite eradication efforts.

Pathogenesis

Salmonella bacteria have virulence factors that attach to and invade intestinal mucosa cells. Enterotoxins (toxic proteins that are excreted by the bacteria) increases the excretion of electrolytes and water to the intestinal lumen and affect the local immune defence with cytotoxic (cell killing) effect as a result. Endotoxins (toxins released from the cell surface when the bacteria die) also have some impact but not as potent as enterotoxins. The most important part of the treatment is to maintain fluid- and electrolyte balance at the right level so the horse can have a chance to cope with the infection.

Diagnosis

Positive culture from faeces or organs is needed to ensure the diagnosis.
For the aforementioned S. Abortusequi a blood test can be taken for detection of antibodies in serum. This is mainly an export requirement to some countries and is rarely used for diagnostic purposes. Differential diagnoses to salmonella are diarrhoeas caused by other disease agents such as Clostridium perfringens or rotavirus.

Treatment

Treatment is mainly symptomatic to maintain the right fluid- and electrolyte balance. Plasma may be used if hypoproteinaemia occurs. In some cases nutrient solutions may be of use.

Antibiotics are not recommended if the only symptoms are diarrhoea and a reduced general condition. Septicaemia or other extraintestinal manifestations can however motivate antibiotic treatment. Most strains of salmonella found in horses in Sweden have a resistance pattern that allows the antibiotics recommended for gram-negative bacteria to be used.

Isolation

Sick horses should quickly be isolated from other horses. Since the infection spreads through faeces from sick animals, it is important to consider how to handle faeces, bedding material etc. from premises where excreting animals are kept. Separate boots, clothes, forks, shovels, wheelbarrows etc. should be used in infected and uninfected areas respectively. Tools used in infected areas should also be cleaned/disinfected at regular intervals.

Cleaning and disinfection of stables

It is important to start with a thorough mechanical cleansing, followed by disinfectant with proven efficiency against salmonella.  High-pressure washing should be avoided since it creates aerosols that can spread the infection. Lastly, it is important to let the stable dry up thoroughly since moisture benefit and dryness disadvantage bacteria.

Prophylaxis

In order to avoid introduction of disease it is recommended to have a quarantine stable for all horses brought onto the premises. 

Prognosis

In the case of diarrhoea and a mild to moderately impaired general condition, the prognosis may be considered favourable. The more serious the course of the disease, the worse is the prognosis.

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