horse eating from tub in dry lot

Vitamin E is a potent antioxidant primarily found in green pasture grass that plays a role in muscle atrophy (wasting) and neurodegeneration in horses. As pasture lands become increasingly more limited and more horses are housed on less acreage, vitamin E deficiency becomes a real problem.

Stephanie Valberg, DVM, PhD, Dipl. ACVIM, ACVSMR, Mary Anne McPhail Dressage Chair in Equine Sports Medicine at Michigan State University’s College of Veterinary Medicine, in East Lansing, described conditions linked to vitamin E deficiencies and how to manage them during the 2018 Kentucky Equine Research Conference, held Oct. 29-30 in Lexington.

Veterinarians see three neurologic conditions associated with vitamin E deficiencies in horses, said Valberg, and which one the horse develop depends primarily on genetics.

Neuroaxonal Dystrophy and Equine Degenerative Myeloencephalopathy (NAD/EDM)

This neuromuscular disorder typically appears in horses 6 months to 3 years of age. Clinical signs of ataxia (incoordination) and proprioceptive deficits (awareness of where one’s limbs are) are similar to those of horses with wobbler syndrome (spinal cord compression).

“We believe it results from a genetic predisposition and vitamin E deficiency in utero and the early six months of life,” said Valberg, explaining that the equine nervous system depends on adequate vitamin E to develop normally.

Affected horses’ signs persist through adulthood and can be confused for other forms of neurologic disease. Therefore, said Valberg, NAD/EDM is probably underdiagnosed.

While owners can supplement susceptible horses (i.e., broodmares and foals living on farms that have had cases) with vitamin E to try to prevent them from developing this disease or reduce its severity, once horses develop clinical signs, supplementation has no effect, she said.

“This is a serious disease for breeders,” said Valberg, noting that it typically crops up on farms that have experienced a decrease in pasture quality and green grass.

Vitamin E Deficient Myopathy

This muscle disorder, as the name implies, is due to a vitamin E deficiency and occurs in adult horses around the ages of 7-10. Valberg listed clinical signs such as an inability to lock the stifles, weakness, trembling, a low head position, difficulty lying down, weight loss, and muscle atrophy. In acute cases, she said, the most obvious sign will be trembling, whereas in chronic cases owners typically notice reduced muscle mass in the hindquarters and some trembling.

“We think it takes 20% of the muscle being affected to see atrophy and weakness,” said Valberg.

Veterinarians can test for this condition by taking a biopsy of the sacrocaudalis dorsalis muscle (located above the tailhead on either side of the spine) and looking for abnormal mitochondrial staining. Fortunately, it’s reversible with supplementation.

Equine Motor Neuron Disease (EMND)

This neurodegenerative disorder affects the spinal cord where the nerves come out to control muscle contraction. Horses are typically older (>10), said Valberg, and have been vitamin E deficient for a long time (not every deficient horse show signs, she noted). Affected horses typically display similar signs as horses with the above-mentioned vitamin E deficiency. Additionally, they might have a distinct pigmented pattern to their retinas.

“Once horses reach this stage, they might stabilize with supplementation but might not return to performance,” she said.

Supplementing With Vitamin E

“The impact of vitamin E deficiencies causing subtle but significant muscle atrophy and a decline in performance are under recognized by many performance horse veterinarians,” Valberg said. “It should be on everyone’s radar because it’s easy to diagnose with blood samples for vitamin E and can be readily treated with liquid vitamin E supplements.”

The type of supplement you provide varies by case. For healthy horses in at-risk areas, Valberg suggests supplementing about 1,000-2,000 IU/day of the oral, powdered natural (rather than synthetic) form. If your horse already suffers from EMND or vitamin E deficient myopathy, she recommends supplementing 5,000 IU/day of the natural liquid form until all clinical signs are gone and then transitioning to powder over a series of weeks once the horse returns to normal. Expect it to take several months for the horse’s signs to disappear.

Because horses’ responses to vitamin E supplementation vary, Valberg urges veterinarians and/or nutritionists to measure vitamin E levels before and four weeks after supplementation and to adjust the dose accordingly.

She added that while vitamin E supplementation won’t help resolve other neurodegenerative diseases such as shivers or pituitary pars intermedia dysfunction, owners should know that a deficiency might exacerbate them.

“I think it’s important to maintain horses at normal vitamin E levels,” Valberg said. “I’m a big fan of measuring vitamin E in horses and supplementing as needed. As we have less and less pasture, we’ll see more of these cases.”