Immunity Through Nutrition

If your performance horse does well on a balanced diet, can he do better with some extra, immune-boosting nutrients? What about young horses and aging horses, ill horses and healthy horses, working horses and lightly ridden horses?

For sure, studies show that extra amounts of certain nutrients benefit the immune system--but not in every case. Some horses don't need and can't use extra nutrients, and it's not just a situation of "could help/can't hurt." When administered inappropriately, some immune-friendly nutrients can actually have negative consequences.

Before supplementing your horse, here's what you should know about the immune system, immunity boosters, and the effects they can produce.

System Analysis

The immune system protects the body against infection or disease by producing humoral (antibody) and cellular responses that destroy invading bacteria and viruses. "The immune system is essential for long-term survival," explains David W. Horohov, PhD in immunology, William Robert Mills Chair in Equine Immunology, Maxwell H. Gluck Equine Research Center at the University of Kentucky.

Good immune health results from a balanced diet containing the correct amounts and proportions of antioxidants and nutrients--in particular vitamins C, E, A, and the B complexes; the trace minerals zinc and selenium; and proteins.

"Without a balanced diet, deficiencies occur, adversely affecting the health of immune cells," says Carey A. Williams, PhD in equine nutrition, equine extension specialist and assistant professor in the department of animal science at Rutgers University. "When immune cells aren't healthy, you have decreased immune function."

With decreased immunity, the body is less able to fight infections or other pathogenic agents, resulting in increased risk of disease and infection.

Most healthy adult horses can obtain all the immune-friendly nutrients they need from their forages or feeds, as long as their food sources meet National Research Council (NRC 1989) recommendations.

"The NRC requirements are published at absolute minimums and are currently being updated to possibly include optimum levels," Williams notes. "Most horses classified as maintenance (those that are neither pregnant, lactating, growing, nor exercising) generally don't need a grain supplement, extra antioxidants, or immune boosters as long as they have good-quality forage--pasture or hay--and free-choice salt blocks. These horses have very minimal requirements in terms of vitamins, minerals, proteins, etc., so normally their forages or pastures are going to have amounts adequate for maintaining a balanced diet."

But there are types of horses that are at higher risk for immune deficiencies: Foals, pregnant/lactating mares, and horses that are stressed, aging, ill, or receiving immune-suppressing drugs. In many cases, supplementing these horses with particular vitamins and minerals can aid the immune system, thereby preventing certain problems and helping with others.


Prior to delving into how immune-friendly supplements can help, be aware, first and foremost, of how they can hurt.

Vitamin C--"I want to caution that you don't supplement antioxidants--especially vitamin C, which is synthesized in the body--unless horses are really stressed and really need it," warns Sarah Ralston, VMD, PhD, Dipl. ACVN, associate professor of animal science at Rutgers University. In her studies, Ralston learned that supplementing with vitamin C for too long or unnecessarily apparently interfered with the body's own production of vitamin C, so when supplementation was stopped, vitamin C levels dropped and remained low for up to three weeks.

Weanlings given 5 grams of vitamin C twice a day for two weeks when they were not stressed did not mount a response to vaccines given three days after supplementation was stopped, Ralston found.

Vitamin E--Excess vitamin E competes with vitamin A for absorption, potentially leaving the horse vulnerable to problems related to vitamin A deficiencies, Ralston states.

Vitamin A--Excess levels of vitamin A levels (over 100,000 IUs per day in the average horse) can be toxic, so just increasing the vitamin A intake when supplementing with vitamin E is not recommended.

Zinc--"Zinc and copper have a relationship and need to be kept in a specific ratio, otherwise they're going to interfere with each other," Williams says. "Excess zinc can cause a copper deficiency."

Calcium and Phosphorus--Calcium and phosphorus also need to be kept in balance with each other (2 parts calcium for 1 part phosphorus). "Anything that is thrown out of balance can definitely hurt the immune system," Williams warns.

Selenium--Too much selenium is toxic. "Never supplement with selenium unless you know what the selenium intake is in the total diet," Williams cautions. "A good guideline is to try and maintain 0.2 to 0.3 mg selenium per kg feed intake. Most commercial feeds on the market contain between 0.5 and 1.0 mg/kg selenium. Selenium becomes toxic between 2 and 5 mg/kg intake."


As mentioned earlier, there are documented conditions for which immune-boosting supplementation is warranted.

Stressed Horses--Horses that work or exercise heavily, show frequently, spend long hours in a trailer, or sustain other physical or environmental pressures usually undergo stress--and sometimes stress is physically detrimental. "We know quite a bit about how stress affects the immune system," says Horohov. "One effect is that stress can alter immune function, such as reducing the ability of macrophages to kill bacteria and viruses. Decreased antibody production to vaccination is another example of a stress-induced alteration in the immune system."

Adds Ralston, "Severe stress also causes increased utilization of antioxidants like vitamins E, C, and the B complexes. For example, vitamin C is synthesized in the liver, but during periods of stress, it's released from its body stores as part of the stress response and gets secreted in the urine; if the stress is long enough or severe enough, it can deplete the stores and the synthesis apparently can't keep up."

Studies have found that:

  • In horses transported for 12 hours, vitamin C levels dropped below normal and stayed down for 24-36 hours afterward;
  • Of weanlings transported for 36 hours under identical conditions from the same farm, those supplemented with vitamins C and E for five days after they arrived remained healthy or became only slightly stressed (no fever), while 80% of the unsupplemented weanlings developed high fevers, loss of appetite, cough, and nasal discharge; and
  • Heavily exercised horses (i.e., endurance, three-day eventing) supplemented with vitamin E had less loss of white blood cells (white blood cells are the main components of the immune system and are important for fighting infection and disease).

Recommendations do vary, though. If you know your horse is going to be stressed--i.e., an upcoming event or long transport--Williams suggests giving the animal 3,000-5,000 IU vitamin E a day about four weeks prior to the event. "Vitamin E is fat-soluble, so it needs to build up stores in the system in order for it to actually be effective at a higher level," she says. Then about five to seven days before the stressing event, Williams says, start your horse on 7 to 10 grams of vitamin C. "If the shows are a month apart, you can supplement for a total of 10 to 14 days around the stressful event. If they are on a high level of vitamin C for an extended period of time, gradually wean them off the supplement--but not during a stressful period--to give the liver time to adjust to making the normal levels of vitamin C again.

"If the horse is continuously being transported, showing, or in intense exercise training, I recommend keeping them on the vitamin E throughout the season," she continues. "My previous research has shown that horses undergoing intense endurance exercise benefit from levels up to 5,000 IU per day (increased immune function, increased antioxidant status, and decreased muscle enzyme leakage)."

Ralston prefers giving these horses vitamin C only during the period of stress, which would be after arriving at the competition and through the period they are away, then for one to two days after they get home. She suggests 10 grams per day for the average adult horse, and 5 grams daily for weanlings.

"If it's a one-day show, they probably won't even need it," Ralston says. "Vitamin E can also be used with vitamin C, but only needs to be given once a day."

She has had good results using 1,000 IU of vitamin E in adults and 400 IU in weanlings. In the horses she studied under this regimen, Ralston reports, there hasn't been a single case of shipping fever.

"Up to 1,000 IU of vitamin E per day for the average horse is OK, especially if they are on pasture or good-quality hay that has high levels of beta carotene in it," says Ralston. "Higher amounts can be used during periods of acute stress, such as competition, but I worry about using more than five times that amount on a daily basis. The long-term studies proving the safety of feeding extremely high levels for long periods of time have not been done, to my knowledge." For more information, see "Vitamin E for Better Health" on p. 20.

Young Horses--Even though foals receive some immunity by drinking colostrum within the first 12-24 hours of life, lack of exposure and the naiveté of the immune system still leave them at increased susceptibility to various disease-causing agents, Horohov states. Some of these deficits can be addressed through vaccination, but the steps involved in full maturation of the foals' immune system remain unknown.

Beyond that, foals going through weaning or those being transported for long hours, especially those without prior handling, suffer the same adverse stress effects as their adult counterparts. Supplement the wee ones with 5 grams of vitamin C a day, as indicated earlier.

Pregnant/Lactating Mares--These conditions are directly harder on the nutritional status of the animal (e.g. the nutrient requirements of a reproducing animal are much greater), Williams says. "If the animal is on a low plane of nutrition, it will indirectly affect the immune system due to the lack of nutritional balance of the above discussed nutrients."

Seniors--"As horses age and progress into their elder years, deficits occur in the immune system that result in increased susceptibility to certain types of infectious diseases as well as disorders like cancers," Horohov says. "The mechanism responsible for that isn't clear. The immune system in the older animal has been primed, so there's probably a different mechanism responsible for age-related decline in immune function compared to the early susceptibility one sees in young individuals."

But aging horses also have a propensity toward developing another type of immune dysfunction--autoimmune disease--in essence having too much immune response.

"Therefore, the approach that one uses to improve immune function isn't as clear and as straightforward as we know with stress," Horohov points out.

Few studies have been done exploring the relationship between immune dysfunction in the older horse and nutrition. Consequently, there is no blanket recommendation because of the complexity of aging and its effect on the immune system, Horohov says. "If the horse seems fine, has a good attitude, is maintaining weight, has a good hair coat, and is being provided with a complete and well-balanced diet, its immune system is probably fine," he states. "If health problems arise, you need to assess what the problem is, then make the necessary adjustments."

Ill Horses--Some disorders are linked to immune dysfunction and might have accompanying vitamin deficiencies. These most commonly include Cushing's disease, respiratory disease, liver disease, and chronic infections, says Ralston.

Cushingoid horses displaying clinical signs of the disease, which often includes chronic infections, seem to be helped with 10 grams of vitamin C twice a day, Ralston reports. Since niacin (a B-vitamin) is synthesized in the liver and other B vitamins are made during the fermentation in the large intestine, B vitamin supplementation might help in cases of liver disease and severe diarrhea.

For any horse coming down with respiratory illness, give him 7-10 grams of vitamin C as soon as clinical signs are seen, Williams says.

For other illnesses that might be related to immune deficiencies, check with a veterinary nutritionist before supplementing to make sure you're not using something that would do more harm than good.

Drugs--"Many performance-enhancing drugs are immune suppressors, so one needs to be careful," warns Williams. "Glucocorticoids, like dexamethasone, cortisone, and prednisone, can decrease levels and inhibit white blood cells and inhibit phagocytosis (the ingestion by a cell of a microorganism, cell particle, or other matter surrounded and engulfed by the cell) that takes place during destruction of the body's toxins. Glucocorticoids can also mask the clinical signs of infection. Many of these effects only occur at high doses, and there is individual variation in this response. Cortisone, used to decrease inflammation, purposely kills immune cells to complete this task. Bronchodilators, used to open airways on horses with respiratory diseases, also inhibit the immune system's function.

"Owners whose horses are on immune-suppressing drugs need to strongly consider some nutrition alternative that can help counteract this suppression," she suggests.

A Wing and a Prayer

Although some areas relating to specific nutrients and immunity have been properly researched, the fact is this field is largely unexplored. "We really can't make many specific recommendations as to what people should or should not be giving their horses, because there hasn't been much investigation into many of these areas," Horohov states. "While the public relies on the people that are selling 'immune supplements' to tell them what the benefits are, there is very little information that really substantiates some of the claims that are made."

Williams agrees, saying, "No real research has been done on the herbal remedies, garlic, bee pollen, ginger, ginkgo, etc. Some companies play up the role of 'crucial enzymes,' but enzymatic problems or enzyme deficiencies are rare. Fatty acids, especially the omega 3 fatty acids, are mostly fish oil products and are similar to supplementing fat in the horse's diet."

In reviewing ingredients of "immune boosters" or "immune system supplements" sold through the Internet, Williams found the vast majority were simply antioxidant cocktails of vitamins and minerals. The difference was in the marketing--and the sometimes steeper price.

Take-Home Message

Before you order an immune enhancer from your favorite source, make sure it is something your horse requires and that you're using the correct nutrients to address specific needs. If you're uncertain, Ralston suggests you have your veterinarian consult with a veterinary nutritionist certified through the American College of Veterinary Nutrition or an American Society of Animal Science certified equine nutrition specialist.

Otherwise, you could be doing more than throwing your money away--you might be causing problems instead of curing them.

About the Author

Marcia King

Marcia King is an award-winning freelance writer based in Ohio who specializes in equine, canine, and feline veterinary topics. She's schooled in hunt seat, dressage, and Western pleasure.

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