Basics of the Equine Immune System

Updated December 20, 2023 | By: Carolyn Hammer DVM PhD

Your horse’s immune system is a complex and highly coordinated system of cells and molecules that work together to defend his body against disease-causing organisms. These disease-causing organisms, also known as pathogens or germs, could be bacteria, viruses, parasites, or fungi. 

How a Horse’s Immune System Works

When faced with a pathogen, there are three levels of immune protection in place to shield your horse from illness: physical barriers, innate immunity, and adaptive immunity.

1. Physical Barriers

This is the first line of defense against pathogen invasion. Physical barriers include the horse’s skin and mucous membranes. Coughing or sneezing (and the resulting mucus) are also physical defenses that protect the respiratory system.

Physical barriers work exactly as the name implies – they serve as a shield to prevent pathogens from entering the body.

2. Innate Immunity

If the physical barriers are breached, the next line of defense is the innate immune system. This consists of immune cells and molecules that non-specifically remove pathogens and damaged body cells.

The innate immune system responds rapidly (minutes to hours after detection of a pathogen) but it does not differentiate between specific types of pathogens. So, it responds the same way every time, no matter what type of germ it encounters. 

3. Adaptive Immunity

The adaptive immune system is slow to get going (days after detection of a pathogen) but is highly specific, effective, and produces antibodies. Antibodies are protective immune proteins also known as immunoglobulins.

The adaptive immune system also generates memory immune cells which allow for a faster response the next time the same exact pathogen enters the body. It is the adaptive immune system that is targeted by vaccines and results in long-term protection.

Several organs and tissues, such as the spleen, bone marrow, and lymph nodes, work together to coordinate the innate and adaptive immune response in horses.

Factors That Influence Immune Function in Horses

It’s important to understand that the immune system’s strength and functioning varies from horse-to-horse. Factors such as age, stress, and nutrition can influence its effectiveness.

Age

Age can significantly impact a horse’s immune system. In foals, the immune system is still developing. Some immune mechanisms do not respond as effectively to pathogens compared to in adult horses.

Foals have limited exposure to pathogens; therefore, their adaptive immunity has not had a chance to generate memory immune cells. This leaves them at an increased risk for illness and infections.

Generally, as a horse ages, his immune system becomes more experienced and effective at recognizing and combating pathogens. However, in older horses, particularly those over 20 years old, immune function may start to decline. This can result in increased inflammation and a reduced ability to respond to vaccines. Age-related changes in the immune system can also be influenced by other factors including genetics, nutrition, stress, and overall health status.

Stress

Chronic and prolonged stress can suppress the immune system. A few examples of stress include:

• environmental changes
• transportation
• intense training schedules
• social hierarchy disruptions
• physical pain or discomfort

When a horse experiences stress, his body responds by releasing a hormone called cortisol. Chronic and prolonged elevated levels of cortisol can inhibit the production and function of immune cells. This can lead to increased susceptibility to disease, slower wound healing, and a reduced response to vaccination. Minimizing stressors is an important part of maintaining optimal immune system function.

Nutrition

About 70% of the immune system is located in the digestive tract. Therefore, providing a well-balanced, high-quality diet that’s tailored to your individual horse’s needs is essential for maintaining a healthy and robust immune system. Here are some ways in which nutrition impacts a horse’s immune response:

• Energy: The immune system requires adequate energy to perform its various functions. Energy is supplied in the diet through the metabolism of carbohydrates (including fiber), protein, and fat. 
• Protein: Proteins are necessary to produce antibodies and other molecules used for immune defense. 
• Vitamins and Minerals: Essential vitamins and minerals, like vitamins C, E, and A, zinc, and selenium, are critical for immune system function. These are antioxidants which help to protect cells from the damaging effects of oxidative stress. Additionally, many vitamins and minerals support the growth and activity of immune cells and help to regulate immune cell responses.
• Omega-3 Fatty Acids: Omega-3 fatty acids have anti-inflammatory properties that can assist in modulating immune responses. A balanced ratio of omega 3 to omega 6 fatty acids promotes a normal response to inflammation in the body and aids in the absorption of fat-soluble vitamins A, D, E, and K.
• Water: Proper hydration is key for maintaining mucus production in the respiratory and digestive tract. Dehydration weakens these physical barriers, making your horse more susceptible to illness.
• Body Condition Score: Maintaining an appropriate body condition score through optimal nutrition is important for an effective immune system. Both underweight and overweight horses can have compromised immune systems.

Tips to Support and Maintain a Healthy Immune System

Remember, every horse is unique, and his specific needs may vary based on age, breed, activity level, and health status. Always talk to your veterinarian for personalized guidance on maintaining a healthy immune system for your horse.

Supplements That May Support Immune Health

Beyond providing high-quality forage with appropriate grain as needed, giving a supplement designed to support immune health may help combat the effects of stress on your horse’s body.

Key Ingredients in Immune Supplements

• Probiotics: These "good bugs" are live microorganisms that support healthy digestive function. Probiotics help break down foods that your horse’s body can’t (like the fiber in hay) to fuel and nourish your horse.
• Prebiotics: As sources of soluble fiber, prebiotics are food for the good bugs in the hindgut that help keep the microorganisms healthy and able to perform essential digestive processes. Since probiotics and prebiotics are vital in supporting a healthy GI tract, they also play a role in strengthening immunity.
• DVEquine™ Postbiotic: Has been shown to improve the digestion of fiber and assist the beneficial bacteria of the cecum in adjusting to changes in a horse’s diet. An in vitro study showed that DVEquine™ Postbiotic produced significantly more of beneficial volatile fatty acids (VFAs) than a control group. VFAs are the main product of fermentation in the hindgut. They provide a major source of energy for the horse and help maintain the health of the cecum and colon.
• CLOSTAT®: A proprietary, patented strain of the naturally occurring microorganism Bacillus subtilis. CLOSTAT® helps improve intestinal health and integrity by maintaining a healthy balance of microflora in the gut. It has demonstrated a record of safe use in university field trials with foals and adult horses, as well as published laboratory studies showing its effect on the body’s natural response to pathogens.
• ButiPEARL® Z EQ: Provides two powerful nutrients - butyric acid and zinc - directly to the cells that make up the intestinal barrier. This combination helps strengthen the lining of the gut, improves nutrient absorption, and helps provide a stronger barrier against pathogens, parasites, and toxins. Research in the lab shows it supports equine intestinal cell structure and function under stressful conditions.
• Mushrooms: Mushrooms have immune-regulating properties that can help keep the immune system in balance. When your horse’s immune system is in homeostasis, all the complex systems of cells inside his body are better able to keep him healthy. Further, mushrooms are made of beta glucans which can enhance the immune response by increasing production of disease-fighting white blood cells [1] as well as support the digestive microbiome.
• Adaptogens: Adaptogenic botanicals, such as American ginseng, support a balanced immune response by stimulating the body’s natural production of antibodies in response to foreign pathogens [2].
• Vitamin E: This powerful antioxidant supports the health and function of the immune system and helps neutralize free radicals. Horses without access to pasture and those under stress or in heavy work should receive supplemental vitamin E.
• Vitamin C: Another powerful antioxidant that protects against cellular damage. Vitamin C helps support immunity especially during periods of prolonged stress, such as intense work, sickness, travel, injury, etc.
• Selenium: This trace mineral helps defend the body’s cells from damaging oxidative byproducts known as free radicals and supports a healthy immune response. Supplemental selenium is a smart way to ensure your horse receives his daily requirement as deficiency from low levels in the soil has been reported in 46 states.

Regular Veterinary Care

Schedule routine veterinary check-ups to catch and address health issues early. Regular dental care and parasite control are also essential components of overall health.

Proper Vaccination Program

Work with your veterinarian to develop a vaccination schedule tailored to your horse’s needs and geographic location. Vaccinations are essential for preventing certain diseases. 

Good Hygiene

Maintain clean living conditions in stalls, paddocks, and pastures by regularly removing manure and soiled bedding. Be sure to clean your horse’s water buckets regularly. Even a well-functioning immune system can be overwhelmed if it is constantly exposed to pathogens in a dirty living environment.

Proper Biosecurity

Minimize pathogen exposure and spread by implementing good biosecurity practices. Isolate new arrivals before introducing them to your herd. Quarantine any sick horses away from healthy horses. These practices will help to limit the spread of infectious diseases.

Immune Deficiencies and Autoimmune Diseases

Immune deficiency in horses is a disorder of the immune system that results in its inability to protect against pathogens. For owners, the common symptoms you might see are a horse that suffers from recurring infections and when you try to treat it, the typical course of antibiotic treatment doesn’t help. These horses often become ill from pathogens that don’t normally cause disease.

Primary Immune Deficiencies

Primary immune deficiencies are caused by genetic defects which lead to a horse being born with a missing or underdeveloped immune system. Fortunately, primary immune deficiencies are rare in horses.

If a horse’s immune system is compromised from birth, he is more susceptible to infections. In some cases, these horses can be managed, but since this type of disorder is in the genetic make-up, it cannot be cured.

Secondary Immune Deficiencies

In contrast, secondary immune deficiencies are more common and acquired later in life. These can result from stress, poor nutrition, metabolic illnesses, or certain drugs (like steroids). Equine Metabolic Syndrome and Cushing’s Disease are two illnesses that can result in secondary immune deficiency.

Unlike primary immune deficiencies, secondary immune deficiencies can be reversed or improved by addressing the underlying cause.

Autoimmune Diseases

Autoimmune diseases are the opposite of immune deficiencies and result when the immune system is overactive. The immune system begins to mistakenly attack and damage its own body tissues.

Some autoimmune diseases have a genetic component and others have no known cause. The most common autoimmune diseases in horses affect the muscles, skin, and eyes. 

Consult your veterinarian for assistance if you suspect your horse may have an immune deficiency or autoimmune disease.

Key Takeaways

• The equine immune system is a complex defense mechanism against disease-causing pathogens.
• Factors such as age, stress, and nutrition can influence the efficacy of a horse’s immune system.
• A high-quality diet, proper management practices, and regular veterinary care are essential for maintaining a healthy immune system.
• Ingredients such as vitamins, minerals, omega-3 fatty acids, mushroom extracts, and more can support a healthy immune system and help the body manage stress.
• Vaccinations, good hygiene, and biosecurity practices are vital for disease prevention.
• Immune deficiencies in horses can be primary (genetic) or secondary (acquired) and may result in recurring infections, while autoimmune diseases occur when the immune system mistakenly attacks the body’s tissues.

Evidence-Based References

1. Picetti TS, Soveral LF, Miotto R, Erpen LMS, Kreutz Y, Guizzo JA, Frandoloso R, Kreutz LC. Orally administered β-glucan improves the hemolytic activity of the complement system in horses. Vet World. 2021 Apr;14(4):835-840.
2. Pearson W, Omar S, Clarke AF. Low-dose ginseng (Panax quinquefolium) modulates the course and magnitude of the antibody response to vaccination against equid herpesvirus I in horses. Can J Vet Res. 2007 Jul;71(3):213-7.