Shipping fever is a lower respiratory tract infection seen in horses transported over long distances or experiencing unusual or stressful events. [1]

Known causes of shipping fever include prolonged periods of head elevation, strenuous exercise, anesthesia or complications from a viral illness. [2]

These events increase the amount of debris aspirated by the horse and inhibit the horse’s ability to clear debris from the lungs. Stress also compromises the immune system, making horses susceptible to a viral respiratory infection.

Without prompt medical intervention, shipping fever can develop into pleuropneumonia, which is a dangerous form of equine pneumonia. It is caused by fluid build-up in the lungs and pleural cavity, which is the space between the lungs and chest wall.

Early recognition and treatment of shipping fever are key to a good prognosis. If you are planning on shipping or travelling with your horse, preventative steps can support the immune system and reduce the chance of infection.

Shipping Fever in Horses

Shipping fever is a transport-associated syndrome seen in horses and other livestock animals, causing pyrexia (fever) and other respiratory symptoms. [3]

The stress of travel, combined with exposure to unfamiliar pathogens, can cause various symptoms ranging from mild to severe.

Shipping fever is characterized by the presence of bacteria and other irritants (i.e. hay particles, dust, chemicals) in the lower airway. Longer distances and travel times carry a higher risk of lower airway infection. [3]

Most shipping fever cases present with a general systemic inflammatory response. [4] Signs of infection usually appear within 1-3 days following shipment or arrival at destination and can worsen quickly.

Between 9 – 12% of horses transported for distances between 1,000 – 1,300 km experience shipping fever. [3]

The terms shipping fever and pleuropneumonia are often used interchangeably. However, pleuropneumonia is technically a consequence of severe shipping fever.

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Cause & Pathology

Long-distance travel (or other sudden changes in route) can cause significant stress for the horse, with eleveated levels of the stress hormone cortisol.

High cortisol levels affect the body’s ability to fight off infection, in part by reducing neutrophil counts in the blood. Neutrophils are a type of white blood cell or immune cell that help the body fight infections.

As a result, pulmonary defence mechanisms can become overwhelmed, allowing infection to develop in the lungs. [5] Pneumonia or lung abscesses can develop and extend into the pleural cavity (the fluid-filled space surrounding the lungs). [6]

Shipping fever infection is often polymicrobial or caused by many different bacteria, which may be resistant to certain antibiotics. Infection can worsen quickly, making some cases difficult to treat.

The most common aerobic organisms that cause infection in horses are Streptococcus equi zooepidemicus, Escherichia coli, Actinobacillus and Pasteurella. [4][7]

Common anaerobic organisms include Bacteroides, Clostridium, Peptostreptococcus and Fusobacterium.

Clinical Signs

Before travelling with your horse, ensure that you are able to identify the signs and symptoms of shipping fever. Early detection is important to prevent serious infection.

The following signs indicate shipping fever in the horse: [4][7][8]

  • Nasal discharge
  • Thoracic (chest) pain
  • Elevated body temperature (pyrexia)
  • Cough
  • Rapid, shallow breathing
  • Lethargy
  • Depression
  • Abducted elbows
  • Loss of appetite
  • Reluctance to drink
  • Reluctance to move

Many cases of shipping fever do not present with respiratory signs. Severe pain can inhibit the horse’s ability to cough.

Secondary conditions may develop in advanced cases of shipping fever. These include pulmonary abscessation, colitis and laminitis. [9]

Risk Factors

While any horse can develop shipping fever, competition horses and racehorses have a higher risk due to frequent travel and time spent at high-risk areas (i.e. racetracks and competition grounds). [4][10]

Competition horses are also more likely to come into close contact with horses from different origins, increasing their potential exposure to pathogens. [3]

The horses most affected by shipping are athletic and under five years old. [7]

Transportation & Head Position

Head positioning during transport is considered the most important risk factor for developing shipping fever. [11]

Horses that are cross-tied with their heads raised for extended periods (12-24 hours) may be predisposed to shipping fever. [11]

Cilia are finger-like projections in the trachea, responsible for moving inhaled debris and bacteria away from the lungs. [12]

Prolonged upward fixation of the head allows for the accumulation of bacteria and inflammatory cells in the lower respiratory tract, as the cilia cannot effectively remove debris. Consequently, foreign particles cannot easily move out of the trachea. [12]

Strenuous Exercise

Strenuous exercise can contribute to lower respiratory illness by increasing the rate and depth of aspirated debris. [11] Horses usually develop some lower airway contamination following a bout of exercise.

Intense exercise can also increase the release of the stress hormone cortisol, which impairs the function of pulmonary alveolar macrophages. These immune cells are responsible for finding and removing bacteria and debris from the lungs. [11]

For this reason, strenuous bouts of exercise can reduce the horse’s immune response and increase susceptibility to illness. If a horse is exercised soon after travelling, immune protection of the respiratory tract is further diminished. [11]

Exercise-induced pulmonary hemorrhage (EIPH), commonly seen in Thoroughbred or Standardbred racehorses, creates an ideal environment for bacterial proliferation and infection. [7][13]


Management practices can help to prevent shipping fever in travelling horses by supporting respiratory function and the immune system.

Travel is inherently stressful for horses, which can suppress the immune system. You can reduce your horse’s stress by making several stops when travelling long distances to allow the horse to rest, eat, drink and stretch. [11][15]

Horses shipped over long distances should be allowed to lower their head to the ground several times throughout the trip. [12] Lowering the head and stretching the neck promotes drainage and expulsion of foreign particles from the airways.

Ensure adequate ventilation and minimize dust on the trailer, boat or plane. Poor ventilation can lead to irritation of the respiratory tract and increase the risk of infection.

When transporting your horse, monitor t