Horses can develop oxidative stress that can lead to tissue damage and illness due to reduced immune function.  Dr. Carrie Finno from the University California at Davis says that oxidative stress occurs when free radical production exceeds the animal’s ability to detoxify these highly reactive, unstable molecules.  Fortunately, the body has several naturally occurring antioxidants in place to squelch the damaging molecules and stop the chain reaction of destruction they cause. 

Dr. Stacey Oke indicates in the Horse publication that vitamin E is one of the most potent antioxidants protecting various cells from harmful effects of free radicals.  As long as there is sufficient vitamin E in the diet, it can stop the oxidation from occurring. One of the most common areas in which damage occurs is in cells involving the immune system.  There is evidence in many domestic animals that vitamin E supplementation can improve immune function, but few studies have been performed in horses.  Research shows that horses that were initially fed diets low in vitamin E and selenium had increased immune responses after vaccination when vitamin Ewas supplemented.  Vitamin E supplementation has also been shown to increase the bacterial-killing capacity of the infection-fighting white blood cells.  However, you can’t just supplement every horse with vitamin E because over-supplementation can lead to bleeding problems and inappropriate metabolism of other vitamins.  To determine if your horse even needs vitamin E supplementation, your vet can check the vitamin E level in your horse’s blood.  Also, there are a lot of differences in the type of vitamin E in supplements so always ask your equine vet about the best type of vitamin E supplement for your horse.