Researchers at Oregon State University and elsewhere are continuing to examine the role of vitamin E in horse health, including disease prevention and therapy. In particular, vitamin E deficiencies and/or supplementation could be important factors in equine degenerative myeloencephalopathy, equine motor neuron disease, and equine protozoal myeloencephalitis.

Equine degenerative myeloencephalopathy (EDM) is a disease of the spinal cord and brain stem. EDM is one cause of wobbler disease, and it affects young horses of all breeds. Clinical signs of ataxia (incoordination) usually appear by age one, and the signs can be acute in onset or progress slowly from clumsiness to visible ataxia. If the horse does not deteriorate to the point where euthanasia is recommended, progression of EDM usually arrests by age two or three.

The relationship between vitamin E and EDM began to surface 13 years ago when Linda L. Blythe, DVM, PhD, a professor of neurology at Oregon State University, and colleague Morrie Craig, PhD, professor of biophysics/toxicology, observed that a family of Appaloosa horses had a high incidence of EDM.

"In the family, two half-brothers initially had the disease," said Blythe. "Their dam had produced four foals, all wobblers. One of the affected stallions had sired seven offspring, and of those, four were ataxic."

Using one of the Appaloosa foals, a moderately ataxic horse, as a foundation stallion, Blythe and Craig bred him to a number of mares, studied the offspring, and compared them to foals from non-EDM-affected horses on the same pasture.

"We found that the EDM stallion sired nine foals that from ages six weeks to ten months had vitamin E levels that fell far below their control counterparts on the same pasture," said Blythe. "That gave a pretty good indication that vitamin E was involved. It wasn't simply what they were eating in the pasture, because the control foals were eating the same thing."

Blythe also wanted to know if EDM had a nutritional and/or developmental cause.

"We didn't know if the foals had any disease before they started eating food, or if
it was a nutritional problem after birth," she said.

Examining the vitamin E levels in both blood and tissues of foals near the time of birth, Blythe concluded that although EDM has a genetic predisposition, this disease is not something with which foals are born.

In another study, Joe Mayhew, DVM, when he was at the University of Florida, worked with the foals of two Standardbred stallions which had produced EDM in 40% of their offspring. The mares bred to these two stallions and their foals were supplemented with approximately 1,500 IU per day of vitamin E, and the following year, only 10% of their offspring were affected. In subsequent years, none of the offspring of those stallions had EDM.

Subsequently, the OSU researchers designed a treatment study of wobbler and control foals grazing on the same pasture. As the wobbler foals started developing incoordination and their vitamin E levels dropped at about six months of age, Blythe supplemented them with 6,000 IU per day of vitamin E. By the time the horses were two years old, their coordination appeared normal. "All markedly improved, while one or two had some residual neurological deficits," she said.

While massive vitamin E supplements given to EDM foals can halt disease progression and often can return a horse to usefulness, Blythe emphasizes that preventing EDM by supplementing foals, especially those from families known to have a history of this disease, is a far better option.

Equine motor neuron disease (EMND) is a rare, naturally occurring disease of the nerve cells that control the skeletal muscles. EMND is similar to Lou Gehrig's disease in humans, and the affected horse can suffer from weight loss, excessive recumbency, trembling, muscle atrophy, constant weight shifting of the rear limbs, and abnormally low head carriage.

Discovered and studied at Cornell University, EMND has no known cause. However, the most commonly identifiable environmental risk factor for EMND is an absence of grazing for more than a year and poor-quality hay. EMND horses also have low levels of vitamin E in tissues or blood.

Cornell researchers placed five EMND horses on pasture for periods ranging from nine months to more than two years, and vitamin E was administered to two of the affected five. The researchers noted dramatic clinical improvement in four of the five affected horses. Comments Blythe, "They don't know whether the disease caused reduced vitamin E, i.e., used it up, or if low vitamin E was a causative factor. They don't think vitamin E deficiency alone will cause the disease, but they think it will set the stage or make the horse more likely to get this disease when exposed to something, and nobody knows what that something is."

Equine protozoal myeloencephalitis (EPM) is a disease of the central nervous system, brain, and spinal cord, and it is caused by a protozoal parasite. Onset can be extremely rapid, slow and insidious, or subtle. Clinical signs vary and can include weakness, lameness, incoordination, inability to move correctly (especially in the hindquarters), persistent recumbency, seizures, weight loss, blindness, loss of balance, and disuse of a single limb.

Treatment is aimed at killing the protozoa via oral anti-protozoal medications. With treatment, many horses can become useful again, but some will continue to exhibit signs of neurologic disease. Untreated, horses either will stay the same (showing non-life-threatening neurological signs) or, more likely, the central nervous system damage will worsen and the horse will become incapacitated or die.

In addition to using anti-protozoal medications, some experts recommend supplementing with 6,000-9,000 IU per day of vitamin E. Although no control trials have been done to support this, Blythe points out that, "Vitamin E has been shown in people and horses to augment or make the immune system work better. Vitamin E may help the immune system do the final killing (of the parasite) where antibiotics don't destroy all of it."

How Much, and When to Measure

Another OSU study identified normal levels of vitamin E, and when blood samples measuring those levels should be taken. Explained Blythe, "We found that at one time of the day levels would be adequate, but at another time, they'd be marginal. There's an inherent variation of about 12% in these horses."

This means two or three blood samples should be taken over a single day and pooled for the vitamin E analysis in order to get an averaged value.

Additional studies are underway using radiolabelled (deuterated) vitamin E. Lori Walker, a graduate student and a PhD candidate studying with Blythe and Craig, is researching whether horses sired by EDM-affected stallions and normal horses absorb vitamin E at the same rate, and to ascertain if natural vitamin E is different or better than synthetic vitamin E. The data collection is completed, and it is awaiting statistical analysis.

While research continues into vitamin E's potential, one thing is already clear: Sufficient levels of vitamin E are critical when dealing with specific equine health needs, especially in the young, growing horse from one month to two years of age.

The Role of Vitamin E

Vitamin E generally is supplied through green forage, with little added to most concentrate feeds. Storage can destroy some of the vitamin E in foods.

The metabolic role of Vitamin E is not completely understood. It appears to prevent free radical damage to tissues or lipid peroxidation, a process which can result in harmful effects in the body. It also is important for the proper function of nerves and muscles. Deficiencies may impair neuromuscular function.

Who Needs Vitamin E?

Not every horse needs vitamin E supplementation. Blythe says that studies show horses most at risk for vitamin E deficiency include the following:

  • Horses without access to green pastures;
  • Horses fed old hay or pelleted feed;
  • Horses kept where winters are severe and kept in the barn for long periods of time;
  • Horses exposed to creosote-painted fences;
  • Horses sprayed with excessive insecticide;
  • Horses with EDM, EMND, EPM, and foals of EDM horses;
  • Foals up to two years of age;
  • Ill horses, especially those with diseases affecting the immune system;
  • Older horses which might not have the gastrointestinal efficiency for absorbing vitamin E from grasses;
  • Heavily exercised or raced horses.

Says Blythe: "Our research shows you need about 2,000 IU a day to prevent neurologic dysfunction, and 6,000-9,000 IU a day for treating a disease process that damages the nervous system such as EPM, EMND, or EDM."--Marcia King

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.

Stay on top of the most recent Horse Health news with FREE weekly newsletters from Learn More