Does your horse resist going in the trailer or show signs of stress after being unloaded at your destination?
You may need to trailer your horse for many reasons, such as seeing a veterinarian, competing, breeding or for recreational purposes.
Trailer stress can make it more difficult to load your horse, increase the dangers of transit, and negatively impact health and performance. Trailering also increases the risk of gastric ulcers, colic, dehydration, heat stroke, and shipping fever.
Trailer transport is sometimes unavoidable, but trailer stress and the risks that come with it can be minimized. Preparing for a trip should start well before the day you leave.
You can support your horse’s well-being during trailering by ensuring they are comfortable and accustomed to the trailer and by implementing some of the management practices recommended in this article.
Why Won’t my Horse Load in their Trailer?
Horses are locomotory prey animals, meaning they will try to escape unfamiliar or frightening situations.
If your horse isn’t used to riding in a trailer, it can trigger their “fight or flight” response and increase the risk of injury. 
There are many aspects of trailering that your horse may find stressful or uncomfortable, but these triggers are not always intuitive.
- Anxiety from a previous trauma during trailering
- Being isolated or apart from their social group; horses are herd animals and may resist leaving their companions to enter a trailer/li>
- Unfamiliar feel and hollow sound of walking up a ramp
- Dark interior of a trailer, especially on a bright sunny day
- Stress of confinement
- Poor air quality
- Inability to lower their head to clear mucus from the airways if their head is tied
- Lack of water and feed
- Unfamiliar vibrations during transport
- Exertion of maintaining balance
- Uncomfortably high or low temperature and humidity
- Difficulty urinating and defecating while maintaining balance
These stressful circumstances can promote a physiological response in your horse that result in undesirable or dangerous behaviours and an increased risk of injury.
Physiological Stress Responses to Trailering
Defecating, sweating, pawing, head swinging, ear pinning, and restless behavior during loading or transit are all signs of stress.
- Elevated heart rate
- Increased breathing rate
- Increased cortisol levels
- High lactate caused by muscle activity
- Oxidative stress
- High white blood cell count
- Increased serum creatine kinase activity indicating muscle stress
Some of these signs are outwardly apparent, while others require a blood test to assess. If you notice your horse is sweating profusely, is breathing rapidly, or has a fast heart rate, they are likely stressed.
Health Issues Associated with Trailering
The most common risks of trailering include physical injuries, shipping fever, heat stroke and gastrointestinal problems.
While trailering poses some inherent risks, careful management can minimize negative outcome and their severity.
Injuries may be caused by behaviours or movement of the horse during transit, items in the trailer that are not securely fastened, or transport vehicle accidents due to mechanical failure or collision with another vehicle. 
Before beginning a journey with your horse, check the trailer and the hauling vehicle for signs of wear or damage that could cause an accident. 
Self-inflicted trailering injuries, such as kicking or rearing, usually occur within the first hour of travel.  Horses typically become habituated to trailer stressors within the first hour of travel, at which point the risk of injury decreases. 
The most common trailer-related injuries are:
- Joint damage
In some cases, horses may fully recover from transport-related injuries. One survey found that 3.2% of horses injured during trailer transport had to be euthanized. 
It is important to manage horses carefully during transport to minimize the risk of physical injury.
The stress of transportation can lead to gastrointestinal issues such as colic, colitis, and ulcers. One study of Australian horses found that gastrointestinal issues accounted for 27% of cases requiring veterinary attention following transportation. 
The risk of gastrointestinal issues is substantially increased in the springtime and on trips longer than 20 hours. Factors such as dehydration, lack of food and water, and diet change upon arrival at the destination increase gastrointestinal complications.
Colic is a commonly observed gastrointestinal issue associated with transportation.
The combination of stress and dehydration during transportation can decrease blood flow to the gut, and lead to impaction colic of the colon. 
When transporting your horse, be vigilant for signs of colic such as pawing, looking at the flank and kicking at the abdomen. Contact a veterinarian immediately if your horse exhibits colic symptoms after being transported.
Enterocolitis occurs when harmful bacteria and fungi in the gut proliferate rapidly, causing inflammation and death of tissue of the gastrointestinal tract.
Ulcers can develop as a result of stress, changes in stomach pH, and changes in gut motility during and after transportation.
The severity of ulcers is greater in horses that more frequently lose their balance, are more reactive, or are more stressed during transportation. 
Longer transport periods may habituate horses to trailer stressors, but also increase the risk of respiratory issues, including shipping fever. Shipping fever occurs when bacteria and particles from the air get trapped in the mucus that lines the upper airways of the lungs.
Because horses are grazing animals, they are adapted to spending ample time with their heads lowered.
The horse’s airways are lined with small hairs called cilia. When their head is lowered, the cilia push mucus and trapped debris back up the airways. This prevents debris from travelling deeper into the lungs.
Unfortunately, common trailering practices can interfere with a horse’s natural ability to clear her lungs and increase the risk of shipping fever.
Tying a horse’s head during transport inhibits them from lowering their head to clear the mucus from their lungs.
Poor air quality can also contribute to shipping fever. This can arise from:
- Dust from hay
- Exhaust from the vehicle
- Inadequate ventilation in the trailer
This combination of factors allows bacteria and other irritants to accumulate in the lower parts of the lungs, where they cause infection and pneumonia.
The most common form of pneumonia associated with transportation is called pleuropneumonia, and it is often referred to as shipping fever.
Horses shipped for a short duration are unlikely to develop shipping fever, but the risk increases significantly on trips lasting six or more hours