Wobbly Horse: What’s Wrong with Him?

When horse owners in the United States imagine a horse that is off-balance, stumbling, and showing other signs of neurological impairment, a diagnosis of equine protozoal myeloencephalitis (EPM) often springs to mind. Surprisingly, EPM is not a top cause of spinal cord disease in one recent study.^*

Scientists from the University of California-Davis reviewed medical records of 316 horses diagnosed with spinal ataxia between 2005 and 2017. The most common causes for spinal ataxia in those horses were:

• Cervical vertebral compressive myelopathy (CVCM, wobbler syndrome) in 29%;
• Equine neuroaxonal dystrophy-equine degenerative myeloencephalopathy (eNAD-EDM) in 14%;
• Trauma in 9%; and
• Unknown cause in 17% of ataxic horses.

The remaining 30% of examined horses were diagnosed with a variety of conditions, including EPM, equine herpesvirus myeloencephalopathy (caused by equine herpesvirus-1), West Nile virus, neoplasia, and others.

This study also found that Warmblood horses commonly suffered spinal ataxia, especially compared to Arabians. Thoroughbreds were at an increased risk of CVCM but had a much lower chance of having eNAD-EDM. Quarter Horses had a higher risk of eNAD-EDM than other breeds.

The research team suggested that knowing the leading causes of spinal ataxia together with breed distribution may facilitate swift diagnosis and, if available, treatment. This is particularly salient for eNAD-EDM, where neither an antemortem diagnostic test nor effective therapy exists.

As a progressive neurologic disorder, eNAD-EDM involves generalized degeneration of axons, myelin, and neurons in the spinal cord. This disease most commonly presents in young horses, approximately six to 12 months of age.

Vitamin E deficiency appears to play a role in eNAD-EDM, presumably because a young horse’s developing nervous system relies on adequate vitamin E levels. However, low circulating levels of vitamin E are not consistently identified in all cases of eNAD-EDM.

“Measuring plasma vitamin E along with patient age and clinical signs of neurologic disease may be enough for a tentative diagnosis, but a necropsy is necessary to definitively determine disease,” explained Kathleen Crandell, Ph.D., a nutritionist for Kentucky Equine Research.

Because eNAD-EDM appears to be a disease of young horses deprived of adequate vitamin E early in life, disease prevention may be achieved by ensuring foals, weanlings, and yearlings have access to fresh pasture. Vitamins A and E are highest when the fresh forage is greenest.

“Up until weaning, foals get most of their vitamin E from their dam’s milk. This is why it is so vital that the dam have adequate vitamin E in her diet, especially in horses genetically predisposed to eNAD-EDM,” Crandell advised.

She added, “Most commercial concentrates include vitamin E, but many times it is a synthetic formulation included in minute quantities—60 to 100 IU/kg. If the mare is deficient in vitamin E because of her diet, the foal will not obtain much from her. Even if a foal receives a creep feed or a commercial concentrate after weaning, there is still a chance that it will not consume enough vitamin E, especially if the foal is maintained on a drylot and being offered preserved forages.”

While veterinarians often treat suspected cases of eNAD-EDM with vitamin E, there are actually no proven therapies for this condition, and spontaneous resolution has not been reported. Even with vitamin E supplementation, neurologic deficits do not appear to improve once the horse is more than six months old. In some cases, the neurologic abnormalities may stabilize by about two or three years of age; however, these horses remain neurologically abnormal and are often unfit for use.

“Kentucky Equine Research recommends supplementing mares, particularly those in genetically susceptible herds, with Nano-E. This water-soluble formulation has superior absorption and utilization over synthetic sources of vitamin E, which can be very important for horses challenged by vitamin E deficiencies,” recommended Crandell.

^*Hales, E.N., M. Aleman, S.A. Marquardt, S.A. Katzman, K.D. Woolard, A.D. Miller, and C.J. Finno. 2021. Postmortem diagnoses of spinal ataxia in 316 horses in California. Journal of the American Veterinary Medical Association 258(12):1386-1393.