Nutrition is an important part of immunity and immune function. Research on the effects of nutrition on equine immune response is fairly limited, but the field has been researched heavily in humans and other species over the past 15 years. Inadequate intakes of key vitamins and trace minerals may lead to immunosuppression, increasing the risk of infections which may further reduce nutrient intake, creating a vicious cycle (Maggini, 2007, Wintergerst et al, 2007).
THE IMMUNE SYSTEM HAS TWO BRANCHES
The immune system is a complex network of specialized tissues, cells, organs and chemicals that protect all species from bacterial, viral and environmental insults. There are two branches to the immune system (see chart on page 63): that which we are born with (the innate, natural or non-specific immune system) and that which we acquire (the specific or adaptive immune system). They are made up of three forms of defense: physical barriers, immune cells and antibodies.
- Present from birth, innate immunity is the first line of defense against invading pathogens. Providing a barrier in the form of skin, mucus membranes and stomach acidity, it works to prevent entry. Should it fail in its attempts to hold off threats, it rapidly attempts to eliminate them.
- Acquired later in life, adaptive immunity is highly specific and involves lymphocytes that have receptors for specific antigens. Slower to respond than the innate immune system, adaptive immunity may take several days from initial insult to activation, but it remains active after the initiating antigen is removed. This gives rise to an important difference between the two branches: the acquired immune system has memory, leading to a more powerful response should the body face the same antigen again.
PERFORMANCE HORSES AND IMMUNOSUPPRESSION
Suppressed immunity may be an issue in performance horses because the effects of exercise can stress the immune system and potentially increase the risk of disease susceptibility. While regular low to moderate exercise can have a beneficial impact, acute bouts of high intensity exercise or exercise that continues for long durations may have negative consequences on the innate immune system (Warren, 2008). Additionally, the travel incurred by performance horses adds further stress, putting them at greater risk.
KEY NUTRIENTS IN IMMUNE FUNCTION
Some of the important nutrients for a properly functioning immune system include amino acids such as methionine, arginine and glutamine; trace minerals zinc, copper and selenium; water soluble vitamins (folate, B6, B12, vitamin C), fat soluble vitamins (A, D and E); and polyunsaturated fatty acids. (See below for more information). The typical equine diet provides ample precursors to vitamin A, and a healthy equine digestive tract generates a plentiful source of B vitamins, while the liver generates vitamin C. But the equine diet may not provide adequate sources of copper, zinc, selenium, vitamin E, Omega-3 fatty acids or some essential amino acids.
ARE THEY GETTING ALL THE NUTRITION THEY NEED?
The diet of all horses should have a solid foundation in forage, since approximately 60% of the digestive tract’s volume is dedicated to the fermentation of complex carbohydrate material. Forages vary in form and quality. Good quality pastures provide ample quality protein and therefore essential amino acids. They are also an abundant source of natural vitamin E and Omega-3 fatty acids. However, when pasture quality drops in the winter, or if forage is put up for hay, the levels of vitamin E and Omega-3 fatty acids may no longer be adequate.
Levels of zinc and copper tend to be low in forages, due to low levels in soil. Selenium availability depends on geographic location, with coastal states (especially the Pacific Northwest) having low levels, and some Mid-Western states having very high levels. To complicate matters, the balance of trace minerals is unlikely to be optimal for absorption within the horse’s digestive tract. Zinc should be three to five times higher than copper for optimal absorption. Other relationships (for example, between iron and copper, and between manganese and copper) may impact the availability of copper and subsequently zinc in the ration. Iron levels in forages tend to be very high (and rarely require supplementation) so even a forage that provides adequate copper and zinc to meet the horse’s requirements may need supplementation.
“…the equine diet may not provide ADEQUATE sources of copper, zinc, selenium, vitamin E, Omega-3 fatty acids or some ESSENTIAL amino acids.”
While hay generally provides more than adequate amounts of crude protein, there may be a low concentration of essential amino acids (those that must be provided in the diet). Higher quality sources of protein may need to be added to meet a horse’s needs for lysine, methionine and threonine. Soybean meal is a common ingredient in many commercial feeds, but some owners choose to avoid soy due to concerns over GMOs and pesticide use. When they stop soy, however, they need to use other equally good sources of amino acids (or find organic or locally raised clean soy).
NUTRIENT FUNCTIONS WITHIN THE IMMUNE SYSTEM
Water soluble vitamins
Vitamin B6 – In combination with B12 and folate, it plays a role in the biosynthesis of nucleic acids and proteins. Adequate intake maintains a Th1 immune response.
Folate – Works to maintain innate immunity in concert with vitamins B6 and B12.
Vitamin B12 – Impacts biosynthesis of nucleic acids and proteins together with folate and vitamin B6
Vitamin C – Stimulates leukocyte functions and is an effective antioxidant, protecting cells from reactive oxygen species generated during inflammatory response. Vitamin C acts to regenerate other antioxidants such as vitamin E. Deficiencies have been shown to impair leukocyte function in other species.
Fat soluble vitamins
Vitamin A – Essential for normal differentiation of epithelial tissue, which means deficiency impairs innate immunity. Vitamin A contributes to Th2 anti-inflammatory response. Deficiencies may lead to excessive pro-inflammatory response with a reduced ability to defend against extracellular pathogens.
Vitamin D – Most cells within the immune system, except B cells, express vitamin D receptors. When metabolized to 1,25(OH)2D3, vitamin D acts as an immune system modulator. Vitamin D also helps enhance innate immunity by increasing differentiation of monocytes to macrophages. Deficiencies lead to an increased risk of infections due to reduced innate immunity and antigenspecific cellular immune response.
Vitamin E – Vital to protecting cell membranes from oxidative damage, vitamin E also helps reduce production of immune suppressive factors such as PGE2 in macrophages. Vitamin E also enhances Th1 response. While rare in humans, vitamin E deficiencies are becoming more commonly identified in the equine population, likely due to reliance on dried forages.
Copper – Provides defense against reactive oxygen species and in maintaining intracellular antioxidant balance through the key enzyme Cu/Zn-superoxide dismutase. Copper also supports Th1 response, which is impacted by both excesses and deficiencies.
Zinc – Has an impact on both innate and acquired immunity and is essential for highly proliferating cells, especially in the immune system. It plays a crucial role in immune cell signaling, gene expression, protein synthesis and apoptosis. Deficiency results in increased oxidative stress and reduced Th1 response.
Selenium – Maintains immune cell integrity via glutathione peroxidase and other selano-proteins. Selenium is involved with B cell differentiation, antibody production and expression of some cytokines, and as a result influences both innate and acquired immunity.
Iron – Involved in the regulation of cytokine production and action as well as being essential for cell differentiation and the growth of cells critical for immune function. While deficiencies can be common in the human population, high levels of iron in forages means deficiency is rare in equines.
(Maggini et al, 2007, Wintergerst et al, 2007, and Warren 2008)
MEETING NUTRITIONAL NEEDS IN TIMES OF STRESS
As workloads increase, or physiologic demands such as lactation and growth occur, stressors increase and dietary needs change (NRC, 2007), so forage alone may not be adequate. Increasing the level of stress placed on the horse decreases the dietary margin of error. The impacts of an unbalanced or deficient diet are more likely to be seen in all areas of performance, including immune function. Due to the potential deficiencies that may arise from an all-forage diet, a supplemental source of key nutrients is advised. This may be achieved through the provision of good quality supplements and/or fortified feed.
Selecting the correct commercial feed for the individual horse is key to meeting adequate dietary requirements. If less volume is fed than recommended, key nutrients may not be provided in adequate quantities. For example, complete senior feeds are commonly fed by owners at a rate of one to two pounds per day. These have a very dilute mineral and vitamin package because they are formulated to be given as the horse’s sole feed, typically at a minimum of 1.5% of body weight per day. Feeding only one to two pounds a day leaves the horse short of key essential nutrients. Ration balancers with a daily intake of one to two pounds may be a better choice. They have protein levels of around 30%, and are highly fortified with essential vitamins and minerals, since they’re designed to provide the nutrients typically missing or out of balance in forage without providing large amounts of additional calories.
If clients are unable to maintain condition on forage alone, a commercial feed providing a good source of calories, such as a performance feed, is a good choice. Again, it is important to feed recommended quantities to ensure all nutritional requirements are met.
A qualified equine nutritionist can help ensure the correct feeds are selected by determining the horse’s nutritional requirements, and selecting the correct products to meet those requirements and work with the specific forage being fed. Blood work to determine selenium and vitamin E status is beneficial. Assimilation of vitamin E varies greatly between individual horses and therefore certain horses may remain deficient. Test results will show whether additional supplementation is necessary. Supplemental vitamin E should be fed in the easily absorbable d-alpha tocopherol form, in a water soluble supplement.
Thanks to the microflora living in the equine hindgut, no deficiencies of B vitamins have been identified in the horse other than thiamine and riboflavin (NRC, 2007). However, common sense suggests that supplementation with a B vitamin may be beneficial when a horse is showing symptoms suggestive of hindgut disruption or immune compromise.
Unlike humans, horses have no nutritional requirement for vitamin C; they appear able to produce the enzyme L-gulonolactone in the liver, which enables conversion of glucose to vitamin C (NRC, 2007). Limited research shows that senior horses may have a reduced ability to create vitamin C and may benefi t from supplementation. Certainly, horses showing signs of immune compromise may also benefit. Vitamin C is not easily absorbed in the equine intestinal tract, so over three grams per day are necessary to make a difference in serum ascorbate. There is some concern that supplementation may downregulate natural production over the long term, and for this reason supplemental sources should be reduced gradually.
Vitamin D is typically not given much consideration due to its synthesis in the epidermal strata and its addition to commercial feeds. However, horses kept in stalled environments, turned out for short periods, wearing blankets, or living in northerly latitudes are being diagnosed with low blood vitamin D levels. Therefore, in certain management situations, supplementation may be necessary. The daily requirement is set at 6.6 IU per kg body weight per day, with an upper limit of 22 IU per kg body weight per day (NRC, 2007).
Ensuring a horse’s diet is balanced and meeting all requirements will help develop a strong immune system. Nutritional support may aid horses suffering from conditions that negatively impact the immune system. Working with a knowledgeable and qualified equine nutritionist can help insure your clients’ horses are receiving fully integrated care and that nutritional needs are being met and optimized for health and performance.
Maggini S, Wintergerst ES, Beveridge S, and Hornig DH. “Select vitamins and trace elements support immune function by strengthening epithelial barriers and cellular and humoral immune responses”. British Journal of Nutrition (2007), 98, Suppl. 1, S29-S35.
National Research Council (NRC) Nutrient Requirements of Horses, Sixth Revised Edition, 2007, The National Academies Press, Washington, DC.
Warren LK. “Potential immune-stimulatory nutrients for the equine athlete”. Proceedings of the 4th European Equine Nutrition & Health Congress, April 18-19, 2008, The Netherlands, 28-45.
Wintergerst ES, Maggini S, Hornig DH. “Contribution of selected vitamins and trace elements to immune function”. Annals of Nutritional Metabolism, 2007, 51, 301-323.