Supporting the Horse Immune System

Latest posts by Eleanor M. Kellon, VMD (see all)

How to best support the horse immune system function is a very common concern. There are two mistakes/misconceptions many people have. One is that you should try to “stimulate”. The other is that stimulating is always a good thing.

Stimulating the horse Immune System

Trying to stimulate the immune system is futile if it does not have the basic raw materials it needs to function – nutrients. Hydration, calories, protein/amino acids, fatty acids, vitamin and mineral status all influence immune function. These are the fuels and also regulate and orchestrate right down to the level of DNA.

For example, this recent study in humans found clear effects on immune function with even mildly inadequate zinc intakes that did not produce any change in zinc blood levels:

dietary zinc depletion and homeostasis

I’ll give more details regarding basic nutrients below, but want to emphasize a very important point here. There is nothing that will substitute for inadequate levels of a basic nutrient. Their jobs in signalling between cells, forming antibodies and cytokines, and protecting the immune system cannot be bypassed by any herb, isolated other nutrients, drugs or vaccines. Nutrition must come first.

Two horses together

Major Components of the Equine Immune System


This is the “primitive” immune system. It does not require antibodies or lymphocytes with “memory” or previous invasions to work. Foreign organisms/substances, damaged or abnormal body cells, parasites will all trigger a Th2 response which is primarily inflammatory. Allergies are also predominantly Th2 responses. An integral component is the complement system (below).


Th1 is the adaptive immune response. It involves the formation of various classes of antibodies specific to a particular organism. Memory T cells are also formed, which are cells that will remember this organism and can produce antibodies much faster than possible on the first exposure.

Complement system

A family of circulating proteins that are part of the Th2 immune response. They attract white blood cells to foreign organisms and enhance them being destroyed by white blood cells. Malfunctioning in this system may also be involved in autoimmune diseases.+ Cytokines: Cytokines are small proteins secreted by the nervous system and immune system cells which trigger reactions in other cells types. Some are inflammatory, others counter-regulatory/anti-inflammatory. Antibodies: Antibodies are proteins produced by a specific class of white blood cells. They bind to organisms, or cells identified as infected by organisms, targeting them for destruction.

Protein and Immune Function

Protein plays a critical role in immune function. Antibodies, cytokines and the complement system are all manufactured from protein. For example:

Protein energy malnutrition

In addition to protein in general, specific amino acids (the building blocks of protein) play critical roles Glutamine, an amino acid is both a fuel for immune system cells and a precursor for glutathione, which is a critical intracellular antioxidant that protects the immune system cells from damage from “friendly fire” when they are destroying organisms.

Amino Acids. 1999;17(3):227-41. Glutamine and the immune system. Calder PC,Yaqoob P. Institute of Human Nutrition, University of Southampton, United Kingdom Glutamine is utilised at a high rate by cells of the immune system in culture and is required to support optimal lymphocyte proliferation and production of cytokines by lymphocytes and macrophages. Macrophage-mediated phagocytosis is influenced by glutamine availability. Hydrolysable glutamine dipeptides can substitute for glutamine to support in vitro lymphocyte and macrophage functions. In man plasma and skeletal muscle glutamine levels are lowered by sepsis, injury, burns, surgery and endurance exercise and in the overtrained athlete. The lowered plasma glutamine concentrations are most likely the result of demand for glutaminne (by the liver, kidney, gut and immune system) exceeding the supply (from the diet and from muscle). It has been suggested that the lowered plasma glutamine concentration contributes, at least in part, to the immunosuppression which accompanies such situations. Animal studies have shown that precursors has been provided, usually by the parenteral route, to patients following surgery, radiation treatment or bone marrow transplantation or suffering from injury. In most cases the intention was not to stimulate the immune system but rather to maintain nitrogen balance, muscle mass and/or gut integrity. Nevertheless, the maintenance of plasma glutamine concentrations in such a group of patients very much at risk of immunosuppression has the added benefit of maintaining immune function. Indeed, the provision of glutamine to patients following bone marrow transplantation resulted in a lower level of infection and a shorter stay in hospital than for patients receiving glutamine-free parenteral nutrition. inclusion of glutamine in the diet increases survival to a bacterial challenge.

equine immunity

Supporting the Ill or Injured Horse

For support of the ill or injured horse, 15 to 30 mg/kg of L-Glutamine on top of the maintenance requirement is reasonable.

There’s a time and a place for arginine supplementation as well, but it isn’t always easy to know when. Arginine is contraindicated in horses that are septic from bacterial infections, i.e. have fevers related to a bacterial infection.

Arginine can also “feed” some viruses, (e.g. Herpes) and make the symptoms of both viral and bacterial infections far worse in the initial stages. However, because of arginine’s key role in fueling basic immune system functions, you do want to make sure to provide at least what the horse’s baseline intake would be.

The arginine level in grass hays averages 4.5% of the crude protein. Grains and by-products contain a maximum of 1% of the crude protein. Using these figures, you can determine approximately how much arginine your horse would be taking in at maintenance and how far short the horse is if he is not eating a full ration.

For example, a horse eating 10 kg of a 10% protein hay is taking in 1000 grams of protein, 45 grams of which will be arginine. If the horse is only eating half that much because he is ill or injured, you can supplement with 22.5 grams of arginine divided over the day. Always provide arginine in divided doses and with a meal or syringed in with protein. This will avoid a “rush” of nitric oxide production.

Once the fever has broken or the horse is past the first 3 to 7 days of an injury, an additional 10 grams of arginine per 500 lbs of body weight/day, divided between meals, is reasonable to support healing.

Dehydration has a negative effect on the immune function when the body is subjected to stress, such as by exercise:

Effect of hydration status

The Effect of Minerals on the Horse Immune System

Getting back to minerals, as with zinc, virtually all minerals have documented effects on the immune system. In this study, magnesium and manganese had anti-inflammatory effects while enhancing other aspects of the immune response:

manganese and magnesium on immune function

There are almost 100 formal studies on the effects of magnesium status alone on inflammation and allergy. Both copper and zinc are essential cofactors in the important antioxidant enzyme system superoxide dismutase. As with zinc above, copper also has a direct effect on immune function:

induced copper deficiency

Essential Fatty Acids in a Healthy Immune System

Essential fatty acids are also key players. This is the omega-3 and omega-6 group of fatty acids. They are called “essential” because the horse’s body cannot manufacture them and they must be present in the diet. Although a bit of an oversimplification, the omega-6 fatty acids are instrumental in inflammatory/innate immune reactions while the omega-3s support the counter-regulatory and anti-inflammatory responses that keep those in check.

Inadequate omega-3 intake compared to omega-6 has been implicated in everything from problems in fetal brain development to heart disease in humans. It’s a human problem because of inadequate intakes of fresh vegetables/fruits and whole grains, as well as meats and eggs from intensively raised animals that contain omega-6 to omega-3 ratios as high as 20:1 compared to the natural levels of about 4:1 in naturally fed animals.

The situation is similar in horses. Fresh grass/vegetation has an omega-6 to omega-3 ratio of about 1:4. However, when the grass is cut and cured for hay, the omega-3s are lost. Grains, vegetable oils and popular seed meals like soybean or sunflower also have high ratios of omega-6 to omega-3 fats.

haylage in the field

While horses and people have different natural intakes from their natural diets (horses 1:4, humans 4:1), in both cases there is an overabundance of omega-6. Several equine studies have shown how omega-3 fatty acids can modify inflammatory reactions:

Effects of the omega-3 fatty acid
Effect of intravenous infusion of omega-3 and omega-6

Selenium and Vitamin E for a Healthy Immune System

Selenium and vitamin E are best known for protecting cells from oxidative stress induced by exercise, immune system destruction of organisms, or environmental toxins. However, again there is also another direct role in supporting the activity of the immune system, as in these studies on selenium and vitamin E:

Effect of selenium deficiency
Vitamin E and immunity

These are just a few of hundreds of studies linking nutrients to the normal performance of the immune system. As researchers uncover more sensitive ways of testing, such as DNA expression, it becomes obvious that even relatively minor shortfalls of essential nutrients can impact the immune system.

Iron and the Immune System

Iron on the other hand can have very negative effects on inflammation, malignancy and in fighting infections. In fact, the body’s natural response to these conditions is to rev up the production of its iron-trapping protein, ferritin, and lowering levels of circulating iron.

Iron supplementation has no place in the support of an ill or injured horse.

As above, there is no substitute for skipping the step of an adequate diet. Hay analysis to guarantee adequate protein and calorie intake, plus balancing of minerals, is an important first step. If you absolutely can’t analyse, but only very high quality, green, fragrant hay and consider feeding 5 to 10% alfalfa as well.  For most of the UK and parts of Europe (information from Forageplus), high manganese and/or iron are likely to be problems, meaning you should avoid supplementing those.

Levels of Copper and Zinc to Feed Horses

Copper 300 to 400 mg/day and zinc 900 to 1200 mg/day, as sulphate or proteinate forms, are reasonable levels although, again, no guarantees that is correct without analysis. In many cases, higher levels of copper and zinc are needed to balance the high levels of the antagonist minerals iron and manganese.

Forageplus Horse
Horses not on pasture need supplemental vitamin E (minimum 1000 IU/220 kgs in healthy horses) and micronised linseed for correct omega-6 and omega-3 ratios (100 – 200 grams per day).

Selenium Supplementation for Horses

It is unusual to find adequate levels of selenium. You can best check for this with a whole blood selenium assay. In other areas, supplementation of 1/2 mg/day from selenium yeast is advisable depending upon horse work levels.

Supplementation of iodine at 2 to 2.5 mg/500 lbs bodyweight rounds out the picture.

The Correct Nutrition for your Horse

Correct nutrition can reduce the risk or severity of infections, improve immune responses to vaccines and eventually modulate inflammatory reactions and allergies.

No horse will ever be completely immune to infections, let alone free from injuries, so the next issue to tackle is how to support the horse if he/she does become ill or injured.

The first step for doing that is to have a nutritionally adequate and properly balanced diet with ‘forage focused’ minerals at least 150% of NRC (National Research Council). Keeping the horse’s body adequately nourished is extremely important to support the immune system. If the horse is already ill or injured and is off feed, do a calorie and protein count, and be sure to scale back your balancing minerals to match intake.

If you are meeting mineral needs but still coming up short on protein, I would advise using a whey protein concentrate or isolate to make up the difference. Sixty grams of whey per day has generous branched-chain amino acids (BCAAs) to support glutamine and good levels of glutamine itself. Or you can feed an Essential Amino Acid mix.

Equine Infection Fighting Support Summary

  • If the horse is not eating a normal amount, calculate calorie, protein and mineral intake
  • Make up at least the protein and minerals using a palatable balanced protein and mineral supplement.
  • Calculate arginine intake and supplement up to maintenance requirements. Feeding additional arginine (up to 10 grams daily), may be useful after fever (infections) or initial inflammation (wounds) has resolved.
  • Support glutamine levels with 10 to 20 grams of mixed BCAAs (450 kg horse) on top of maintenance requirements or 15 to 30 mg/kg of L-glutamine.

Herbs to Support the Horse Immune System

It would be impossible in the framework of this article to do a thorough discussion of herbs with effects on the immune system. But as a brief overview, with a few exceptions, the herbs used for immune system effects, such as Echinacea, work by themselves triggering an immune reaction, usually of the innate, Th2 immune system.

Although often used in hopes of helping the horse to fight an infection, it is important to remember that the infection itself is already “stimulating” the immune system. An enhanced Th2 response may help clear an extracellular bacterial infection, but not viral or intracellular bacteria (e.g. Lyme).

Adaptogens to Support the Horse Immune System

The adaptogens, in particular Ginseng, show the most promise in terms of capacity to stimulate both arms of the immune system, but much of the research is done with isolated ginsenosides rather than whole herb. We also need to remember that the ginsenoside profile of any given batch of the herb will depend on where it was grown and the growing conditions. This makes the use of herbs a bit of a crapshoot, especially with intracellular organisms where overactivation of the Th2 system may suppress the Th1, the cell-mediated response needed to actually clear the organism.

All horses with immune system issues will benefit from balanced minerals being added to their diet. This ‘forage focused’ approach will support and maintain health in an intelligent way. To find out more about the benefits of feeding ‘forage focused’ minerals to your horse read this article.

Take a look at our article on sarcoids and immunity here.

For more of our Horse Health articles, take a look here.

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Last Updated on December 23, 2021 by Forageplus Team