Feeding for Immunity

Feeding for Immunity

Photo: Anne M. Eberhardt/The Horse

Craft your horse's diet to help ward off infection and illness

The environment is brimming with bacteria, viruses, and other microorganisms that can negatively impact your horse's health. If your horse is in peak condition, he should be able to withstand most assaults. But stressors such as chronic pain, long-distance travel, or environmental changes can compromise his immunity. Allergies, heaves, thrush, insect bite reactions, and hives all can reflect a struggling immune system, as can the more obvious signs of respiratory disease.

Some infections and diseases are beyond your control, but you can influence the outcome by boosting your horse's immune response nutritionally. Working with a veterinarian and nutritionist to determine what and how to feed--while minimizing animals' stress--goes a long way toward keeping horses hale and hearty.

Stress and the Immune System

One of the best things you can do for your horse is pay attention to his stress level and adjust activities accordingly. Chronic stress, whether physical or mental, weakens his immune system. When cortisol, the stress hormone, becomes elevated, disease agents that don't normally faze your horse might make him sick--for example, equine herpesvirus (EHV). Did you know that your horse might already be infected? This opportunistic organism can remain latent until something (e.g., stress) triggers it, giving it the "opportunity" to cause clinical signs.

Encountering some stressors is inevitable: Horses travel long distances, congregate in strange settings with unfamiliar horses, and are subject to performance stress and other factors. But one of the most common stressors--an empty stomach--is easy to fix. Horses are "trickle feeders" that evolved to roam and graze freely. Not being permitted to graze at will works against a horse's physiology. This is because the horse's stomach, unlike our own, secretes acid continuously, even when empty. Chewing produces saliva, a natural antacid, but it can't help neutralize stomach acid if the horse is left with nothing to chew. Exercising on an empty stomach causes acid to slosh onto the unprotected areas of the stomach's lining, potentially causing ulcers.

Nutrients to Protect Your Horse

Healthy, well-managed pastures supply your horse with many important nutrients, including vitamins E, A (as beta carotene), and C. Grasses are also high in omega-3 fatty acids in the proper proportion to omega-6s. And if a variety of grasses and clover grow in your pasture, you can expect the protein quality to be good.

While free access to pasture often provides all the nutrients a horse at maintenance requires to remain healthy, many horses rely on hay as their main forage source during winter. Hay loses some of its vitamins and omega-3s in storage, so horses consuming only hay for prolonged periods of time (more than three or four months without fresh pasture) might require additional feed supplementation. Most horses do well on a fortified commercial feed in recommended amounts. But if your horse exhibits signs of suppressed immune function (e.g., signs of respiratory infections including a runny nose, coughing, and fever; recurrent thrush; bacterial infections; fever; allergic reactions; and general malaise), it's time to boost his diet's nutrient content.

Your horse might need the following vitamins and minerals to fill nutritional gaps and maintain a healthy immune system:

Vitamin E is a powerful antioxidant. Antioxidants protect against disease by neutralizing the damaging free radicals (unstable molecules with an unpaired electron that pull electrons from other molecules) that mental and physical stress produce.

Katherine H. Petersson, MS, PhD, assistant professor in the University of Rhode Island's Department of Fisheries, Animal and Veterinary Sciences, and colleagues examined the impact of vitamin E dosages 15 times the National Research Council's recommended levels on aging horses' immune systems. They found that horses supplemented with vitamin E were more capable of fighting bacteria and produced increased amounts of the antibody immunoglobulin. However, Carey Williams, PhD, associate extension specialist and associate professor at Rutgers University's Department of Animal Science, discovered that just 10 times the recommended dose of vitamin E in otherwise healthy horses interfered with Vitamin A absorption. Gross excess supplementation is not recommended, therefore, even in chronically stressed horses.

"The benefits of vitamin E as an antioxidant and its ability to improve immune function have been known for years--vitamin E impacts several key components of immune function," says Stacy Oke, DVM, MSc, president of Rolling Thunder Scientific, in Ontario, Canada. "There are four different types of vitamin E: α, β, γ, and ? tocopherols. What remains to be known is which types are the most effective."

Many different tocopherols exist in forages and other sources. The form manufacturers most commonly add to commercial feeds and supplements is α- tocopherol. When added in its natural state manufacturers note it as d-α tocopherol in the ingredients, and in its synthetic version they list it as dl-α tocopheryl acetate. Natural vitamin E is more effective as an immune system booster than the manufactured vitamin; however, the synthetic form is more stable and, therefore, has a longer shelf-life.

Vitamin E works parallel to selenium compounds that also serve as antioxidants. University of Kentucky researchers found that a low blood selenium status experienced a delayed immune response to vaccinations. However, selenium can be toxic in high amounts, so horses should only be supplemented if veterinarians document a deficit, and supplement levels should be kept low (below 5 mg/day for an average-sized horse). Vitamin E and selenium are commonly packaged together; evaluate the diet's total selenium content before adding more. For therapeutic doses, it might be better to purchase vitamin E alone.

Vitamin A, in humans, is necessary for normal regeneration of mucosal barriers damaged by infection, and it enhances the function of neutrophils, macrophages, and natural killer cells--three types of white blood cells critical to the immune system. Plants contain beta carotene, which the body uses to produce vitamin A. Beta carotene, vitamin E, and vitamin C work synergistically to reduce inflammation and protect lipids in cell membranes against free radicals' damaging effects.

Boon Chew, PhD, of Washington State University's Department of Animal Science, explains that beta carotene has been reported to modulate the immune system in humans and animals. It increases the ability of specialized white blood cells to damage harmful cells. The resulting inflammation helps eradicate bacterial and viral pathogens and stimulates production of various cytokines, which are messenger cells in the immune system. Beta carotene also stimulates blood neutrophils' phagocytic ability, by which they engulf and eliminate bacteria.

Since most vitamin/mineral supplements and feeds are fortified with vitamin A, it is important to monitor levels before supplementing. The active form of vitamin A (Retinol, retinyl compounds) is toxic at five times the National Research Council's recommended level.

Vitamin C neutralizes free radicals through its ability to donate electrons. The horse's liver normally synthesizes vitamin C in more than adequate amounts and releases and excretes it during periods of prolonged stress. Sarah L. Ralston, VMD, PhD, Dipl. ACVN, associate professor in Rutgers' Department of Animal Science, theorizes that older horses' increased susceptibility to infections might be due to the chronically elevated cortisol secretion associated with pituitary dysfunction (equine Cushing's disease). Ralston finds vitamin C to be an effective supplement in reducing the effects of stressful travel. "We verified that after prolonged transportation stress, oral supplementation (with) vitamin C twice a day was beneficial for the horses the first few days after arrival," she explains.

Omega-3 fatty acids have been shown to reduce inflammation as well as allergic inflammatory response in other species. Researchers at the University of Guelph's Equine Research Centre found evidence that the fatty acid source flaxseed can potentially reduce the allergic inflammatory response in horses susceptible to biting midges (Culicoides). Omega-6 fatty acids can exacerbate inflammatory responses if fed in large amounts, but omega-3s do just the opposite--they help inflammation subside. Omega-6 fatty acids are the predominant form in edible oils such as corn and soybean that are commonly used as calorie supplements. If given in reasonable amounts (less than 2 cups per day to an average-sized horse), they should not pose a threat to a horse's inflammatory system. Some omega-6 activity is beneficial, since inflammation is another way the body combats infections.

High-Quality Protein

At least 8% of a horse's diet should be high-quality total crude protein. A high-quality protein has all the essential amino acids in their proper proportion to allow for adequate body protein synthesis. This enables the immune system to produce antibodies and enzymes and to repair tissues. High-quality protein sources for horses include legumes such as alfalfa, clover, and soybeans. For horses fed lower-quality grass hays, add legume hay pellets or cubes to provide high-quality protein. If you feed a commercial product, check for alfalfa or soybean meal on its list of ingredients.

Other Supporting Nutrients

Some veterinarians and manufacturers have said the following supplements further boost immune support, though these claims haven't been scientifically proven:

  • Bioflavonoids These supposedly work with vitamin C to enhance antioxidant capability.
  • Coenzyme Q10 Used primarily in humans, nutritionists are just starting to recognize this substance as a potent player on the antioxidant team.
  • Water soluble vitamins Thiamine, niacin, panthotenic acid, B-6, riboflavin, and cyanocobalamin work together to support digestion, skin, hooves, hair, blood vessels, protein synthesis, nervous system health, and energy production in all species, and their use and excretion is enhanced during periods of stress. However, in horses only thiamine has been clinically proven to be necessary during periods of prolonged stress. Scientists have not documented deficits of the others in horses.
  • Spirulina Blue/green algae reportedly reduces allergy signs and improves respiratory function in other animals.

Take-Home Message

To keep your horse healthy or help him heal, start by calming the stress response: Provide access to forage day and night, and never let him perform without some forage in his digestive tract. Work with a nutritionist to fill in other nutritional gaps that might exist.


About the Author

Juliet M. Getty, PhD

Juliet M. Getty, PhD, is an equine nutritional consultant based in Bayfield, Colo., with more than 20 years of experience. She authored Feed Your Horse Like A Horse.

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