Nutritional Support of the Immune System in Horses

Mature, idle horses on a sound nutritional program are likely to be getting everything they need to keep their immune status strong enough to protect them against disease. However, some groups of horses are at increased risk of illness because of challenges to their immune systems. Young horses, weanlings, senior horses, undernourished or starved equines, horses in heavy training, those that travel frequently, and those that are exposed to various other types of stress may need some help in avoiding infections. For these horses, certain nutrients can give the immune system a boost. Among well-known supporters of immunity are zinc, selenium, and omega-3 fatty acids.

Newborn foals have no protection against disease until they absorb antibodies from the dam’s colostrum. The immune system develops as the foal grows and matures. In a recent study conducted at Buenos Aires University, researchers measured serum protein, zinc, and indicators of antibody  levels in the blood of  pastured weanlings, yearlings, and two-year-old horses of race-type breeding on several farms in Argentina. Each horse consumed a commercial horse feed in an amount consistent with NRC recommendations for age, weight, and activity level. The feed was fortified with zinc sulfate at a rate of 40 parts per million.

Results showed that zinc levels were statistically higher in weanlings and two-year-olds than in yearlings. Young horses with low zinc levels and high ratios of protein to globulin had indications that antibody status was below normal, possibly leaving these individuals at some increased risk of illness. Based on these findings, the researchers suggested that feed amounts were not sufficient to provide adequate zinc to young horses, or else the form of zinc used for fortification might not have been a type that was easily absorbed.

In addition to its antioxidant functions, selenium also supports the equine immune system. In areas of the country where soil levels of selenium are high enough, this mineral is ingested when horses graze. Around the world, some regions of New Zealand, Europe, South Africa, Egypt, and northeastern China are deficient in selenium. Areas of the U.S. that tend to be low in selenium include parts of some eastern and northwestern states. The horse-producing region of central Kentucky is also marginal in selenium.

In a study conducted at the University of Kentucky, mature horses grazing low-selenium pasture foliage were given a selenium-free supplement for 28 weeks. For the next 28 weeks, the horses were divided into three groups with one group receiving 0.3 mg Se/kg dry matter. Horses in the second group received the same amount of selenium as sodium selenite, and those in the third group stayed on the original unsupplemented diet. A fourth group of horses got the NRC-recommended amount of selenium, about 0.1 mg/kg dry matter.

Selenium status declined in all horses during the initial 28-week period. The status improved to correct levels in supplemented horses within 60 days but continued to decrease in unsupplemented horses. Assessment of immune function indicated that below-normal levels of selenium were detrimental to immunity. Also, when low-selenium horses were given mild exercise, they showed a decrease in glutathione peroxidase activity, an indication of selenium status, and this activity did not recover within 24 hours. In contrast, horses supplemented with organic selenium yeast had an increase in glutathione peroxidase activity after exercise. It is suggested that horses grazing on soils that are borderline or deficient in selenium should be supplemented with selenium, especially if they are not consuming a commercial feed that includes this nutrient. Toxicity is a danger if selenium intake is too high, so horse owners should check feed labels before arbitrarily using a selenium supplement.

With horses frequently living into their mid-twenties, many senior equines are plagued with chronic inflammatory conditions such as arthritis, obesity, and metabolic syndrome. Inflammation is often treated with long-term administration of non-steroidal anti-inflammatory medications (NSAIDs) that can negatively affect health. Older horses have also been shown to have declining responses to vaccination and an increased susceptibility to some infectious diseases. Some work has been done to determine whether natural substances such as omega-3 fatty acids, beta-carotene (responsible for the orange color in carrots), and curcumin (related to ginger) might decrease inflammation in older horses as effectively as NSAIDs. Experiments showed that these substances and a number of others were at least as effective as some NSAIDs like phenylbutazone (bute). Studies are continuing to evaluate the effect of these dietary components on inflammation and immune function in senior horses.