There are many ways to stimulate your horse’s immune system. It can be done with drugs or natural substances. But the success of either depends on his immune system having all the nutrients it needs to operate.

All nutrients have important functions within the immune system. Even a calorie deficit causes metabolic stress, with adverse effects. The satiety hormone, leptin, is very low in undernourished horses. Drops in leptin occur in malnutrition and lead to severe suppression of cell-mediated immunity and the ability of T lymphocytes to proliferate. This occurs long before nutrients are depleted, literally within a day or two, and is just as rapidly reversed by feeding or leptin administration.

Deficiencies of total protein intake have very negative effects on immunity but are relatively uncommon, though specific amino acid deficiencies might not be. The issue is greatly complicated by the fact that we know very little about the horse’s requirements for specific amino acids.

Understanding amino acids

• Arginine is a conditionally essential amino acid in humans, meaning it can be both synthesized and obtained from the diet, but that diet and/or synthesis may not always be enough to support normal development, or in disease states. Very little is known about arginine requirements in horses, but it is interesting to note that mare’s milk is high in arginine and increases in the later stages of lactation, suggesting dietary arginine from grasses is likely not enough for horses under one year old. Arginine in hays averages in the neighborhood of 4.5% of the crude protein, so a horse eating a 10% protein hay would be getting 0.4% arginine in the diet. Grains and grain products contain 0.5% to 1% arginine. The arginine requirement of horses is unknown. The rabbit is used as a model for non-ruminant herbivores, and rabbit requirements have been estimated at 0.6% to 1.2%.

In the immune system, a key role of arginine is in the generation of large amounts of nitric oxide. This uses up the local arginine, depriving the cells lining the blood vessels and causing vessels in the area of injury or infection to contract. With circulating organisms (e.g. bacteremia, bacteria in blood or toxemia, toxins in blood – aka “blood poisoning”), the reaction is body-wide. Severe blood flow restriction occurs, which can cause shock or laminitis. Because of this, arginine should not be supplemented when there is active infection or inflammation. Arginine also feeds herpes viruses and cancers.

• Glutamine plays several important roles in the immune system. Because glutamine can easily be converted back to glutamate, it is also an important substrate for the synthesis of the antioxidant glutathione. Glutathione is synthesized from glutamate, cysteine and glycine. Selenium is incorporated in the enzyme glutathione peroxidase, which allows the glutathione to neutralize oxygen and hydroxy (OH) free radicals. It is one of the ways in which the white blood cells and other body tissues are protected from damage while the immune system is chemically attacking invading organisms, cancer cells or dead tissues.

Supplementing for immune function

• The best way I have found to support immune function from the protein end is with supplemental whey protein concentrate or isolate. This is the highest quality protein on the face of the earth. If there is a known protein deficiency in the diet, from diet analysis, feed 50% of the calculated deficit (the high bioavailability lets you feed less). Otherwise, for general support, feed 30 to 50 grams/day.

• Antioxidants protect immune system cells and tissues from damage when the system goes to work to destroy organisms or dead tissues. The antioxidant nutrients most likely to be deficient in equine diets are vitamin E (horses not on fresh pasture), selenium, copper and zinc.

• Supplement vitamin E at 1 to 2 IU/lb of body weight when not on pasture, and selenium 1 to 2 mg/day in areas known to be borderline or deficient. Copper and zinc are best supplemented based on hay analysis, but if that is not available, 150 mg/day of copper and 375 mg/day of zinc per 22 lbs of hay is an acceptable minimum.

• Adequate intake of essential fatty acids (EFAs) is critical to good functioning of the immune system. They are used in the manufacture of inflammatory mediators as well as the anti-inflammatory compounds that inhibit them. The relative amounts of Omega-3 versus Omega-6 fatty acids incorporated into lymphocyte cell walls even influences how sensitive they are to signals for secreting inflammatory cytokines.

• Omega-6 is rarely deficient, being abundant in grains, commonly used fats, and surviving the drying of hay better than the anti-inflammatory Omega-3 fats do. To help restore balance for horses not on fresh pasture, feed flax seed at 4 to 6 oz/day.

• Normal functioning of the immune system is highly dependent on the ability of lymphocytes and other white blood cells to rapidly increase their numbers by cell multiplication, and also to generate energy to produce cytokines, antibodies and cytotoxic substances. This is where the B vitamins come into play. Their pivotal roles in these functions lead to a suppression of the immune system’s ability to respond if they are deficient; for example, the dramatic effects on lymphocytes with folate deficiency. B vitamin deficiencies are highly unlikely with horses on generous amounts of hay or pasture, but could be a factor with high grain feeding or in horses with intestinal tract upsets.

What this all boils down to is that adequate and balanced nutrition, which guarantees that all classes of nutrients are present and in the correct proportions, forms the scaffolding for good immunity. However, that does not mean more is better. There is really no convincing evidence in any species that routine mega-dosing improves the immune response. On the other hand, what constitutes optimal intake for immune function is a bit unclear for horses since so little work has been done specifically on equines. Adequate protein with 150% of the NRC recommended minimums for vitamins and minerals is a good first step in supporting your horse’s immunity.