Competition-Ready Equine Immunity: What Weakens Your Horse’s Immune System


You just unloaded your horse from a long trailer ride before putting him in a show stall next to new horses from all over the country. Your horse has days of new stressful situations, temperature changes, schooling, and competing ahead of him. 

You came prepared though, of course. You packed your calming supplement to keep him level-headed, poultice and liniments for after you ride, a joint supplement, and every other horse health product you could think of.

But one thing that’s commonly forgotten about in the hustle and bustle of horse show prep is that your horse’s immune system is easily compromised in these situations. Things like extreme temperatures, stress (emotional and physical), shipping, dust, and horse shows can negatively impact your horse’s immunity.

The Equine Immune System: How it Works

Your horse’s immune system’s main function is to fight infections and reject, or isolate and eliminate foreign substances that come its way.

A horse’s immune system will distinguish between substances that are either “self” or “not-self” so it can protect itself from attacks by said foreign invaders. Your horse’s first response to eliminate foreign substances, or pathogens, and keep them from multiplying is via the innate immune response. Inflammation, and heat without pain, are some common signs of this immune system response. 

The slower, and second line of immunity defense, is known as the adaptive immune response. This is the immune response that is triggered when the innate immunity fails. Adaptive immunity will remember pathogens, such as viruses or bacteria, and will counter-attack these pathogens when a horse is re-infected. Antibodies and lymphocytes come into play with this type of immunity as well.

Equine Genetics: Primary vs Secondary Equine Immunodeficiency

While most horses are born with normal immunity, in rare cases, horses can be born with severely weakened immune systems. These are called primary immunodeficiencies. Certain breeds and genetics play a role in why these horses are born with such weakened immune systems. For example, purebred or part-bred Arabians are prone to a type of immunodeficiency called Severe Combined Immunodeficiency Disease (SCID). Horses with genetically weak immunodeficiencies will be much more susceptible to things like viruses, infections, or EPM. It can also affect the structure, development, and function of a horse’s immune cells. 

Secondary immunodeficiencies can also occur and develop at any point in a horse’s life and are acquired – unlike primary immunodeficiencies, which are genetic. Like primary immunodeficiencies, affected horses will have a difficult time fighting infections, or have recurring infections, including opportunistic organisms – such as the ones that cause EPM. 

External Factors that Can Weaken a Horse’s Immune System 

Stress commonly contributes to a compromised immune system. What may seem minor to you, such as a change in routine, a new rider, a new location, or even a new pasturemate, can be significantly stressful to your horse. The new stress factor may not be readily obvious to you as the owner since horses are prey animals and therefore good at hiding stress. 

Other factors beyond stress– like improper nutrition, age, and climate change— can also weaken a horse’s immune system, especially when combined with a poor body score and old age. 

Even if a horse doesn’t become physically ill from a weakened immune system, performance both inside and outside of the show ring is another thing that can be affected when a horse’s immune system is compromised.



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