Equine Degenerative Myeloencephalopathy and Equine Neuroaxonal Dystrophy
What is Equine Degenerative Myeloencephalopathy (EDM) and Equine Neuroaxonal Dystrophy (eNAD)?
It is an inherited degenerative neurologic disease that develops in horses in the first year of life. The genetic basis of this disease is still undetermined. The disease was initially thought to be caused by Vitamin E deficiency. However recent studies have found the disease likely due to genetic mutations in proteins involved in Vitamin E transfer leading to a higher metabolism of Vitamin E. Environmental factors such as lack of pasture, insecticides, wood preservatives/sealers and oxidative stress likely contribute to the disease as well.
What are the clinical signs of EDM/eNAD?
EDM/eNAD occurs equally in both male and female horses. The disease has been identified in multiple breeds of horses including Andalusians, Appalooosas, Arabians, Haflingers, Lusitanos, Mongolian horses, Morgans, Norwegian Fjord Horses, Paso Finos, Paints, Quarter Horses, Standardbreds, Tennessee Walking Horses, Thoroughbreds, Warmbloods and mixed breed horses. The horses develop a symmetric ataxia (walking like they are drunk) in hind and front limbs. The hind limbs tend to be worse than the front limbs with signs slowly progressing over time. The horses may stand in abnormal positions and have a very unusual gait like a marching soldier when the head is raised or have abnormal behavior. Signs develop from a couple weeks of age to 3 years of age. Most horses develop the disease between 6-12 months of age. Diseases with similar presentations are “wobblers” disease/cervical vertebral malformation (spinal cord compression caused by an abnormality of the neck spine, equine protozoal myeloencephalitis (EPM) and equine herpesvirus-1 (EHV-1) myeloencephalopathy.
What tests are required to identify EDM/eNAD?
An internal medicine specialist along with your primary care veterinarian can help diagnose the disease. EPM, wobblers disease and EHV-1 should be tested for so they can be ruled out as the cause of disease. EPM is ruled out by utilizing a test that looks at the antibody ratios in the blood and cerebral spinal fluid (see EPM article for more information). Wobblers disease can be rule out using radiographs (x-rays) of the cervical spine, myelography or computed tomography. Testing Vitamin E can help but is not definitive because many of these horses may have normal Vitamin E concentrations. Sadly, the definitive way to test for EDM/eNAD is via looking at the spinal cord and brain in sections to look for abnormalities. A new test has recently been developed to look at a protein biomarker that is seen to be elevated in horses with this disease.
Phosphorylated neurofilament heavy chain (pNfH) can be found in the blood and cerebral spinal fluid of horses with EDM/eNAD and can help lead to the diagnosis of this disease. Remember however, it can help rule in the diagnosis of the disease but is not definitive.
What are treatment options?
Currently there are no effective treatments for EDM/eNAD. Vitamin E (α-tocopherol) supplementation can help families of horses known to already be diagnosed with EDM/eNAD. Some of these horses stabilize by 2-3 years of age if diagnosed at an early age, however all these cases are abnormal neurologically and are generally unsafe to ride. Until we know more about how the disease is inherited, it is recommended that horses diagnosed with the disease are not bred.
Can this disease be prevented?
It is unclear if the disease can be prevented. However, in families of horses known to have EDM/eNAD, pregnant mares and foals are treated with Vitamin E. It is important to use α-tocopherol (Vitamin E) in the form of RRR α-tocopherol which gets absorbed via the intestine the best. The pregnant mares are treated with Vitamin E the last 4 month of pregnancy. The foals are treated with Vitamin E for the first 2 years of life. It must be realized that despite treatment with Vitamin E, foals can still show neurologic deficits.
Take Away Points
Vitamin E deficiency and genetics play a role in the development of EDM/eNAD.
EDM/eNAD is a common cause of neurologic disease/ataxia in horses.
Definitive diagnosis is looking at the spinal cord and brain after death.