Equine Motor Neuron Disease (EMD) is an acquired neurodegenerative disease in horses affecting the somatic lower motor neurons of adult horses. The disease is characterized by the onset of neurologic dysfunction and muscle wasting resulting from the deterioration of motor neurons and myopathy. The typical age of affected horses ranges from 15 months to 25 years and peaks when horses are 16 years old. Both sexes are affected although increased incidents have been documented in Standardbred, Quarter horses, Arabians, and Thoroughbreds. EMD is considered to be a multifactorial disease, however a dietary deficiency in vitamin E is considered to be a major predisposing factor in its development. This is largely related to when horses have a decreased antioxidant capacity, it leads to accumulation of free radicals, and results in oxidative damage to the somatic ventral motor neuron cells. Equine degenerative myeloencephalopathy (EDM) is an idiopathic, diffuse, degenerative disease which primarily causes damage to the horse’s spinal cord. EDM was identified as the second most common cause (24% of cases) of spinal cord damage in horses at Cornell University in 1978 and ranked second in the causes of spinal ataxia at the University of Montreal from 1985 to 1988. EDM is considered to be an advanced form of neuroaxonal dystrophy (NAD). EDM appears to have a genetic basis and frequently affects Lusitano, Appaloosa, Standardbred, Arabian and Paso Fino horse breeds. Research has shown that horses can develop EDM and equine motor neuron disease (EMND) concurrently, in young related horses, and in association with an underlying vitamin E deficiency. EDM and cervical vertebral compressive myelopathy (CVCM) or “Wobblers”, both share clinical signs of a general proprioceptive symmetric ataxia, abnormal base- wide stance while at rest, and proprioceptive deficits in all limbs.
Horses will usually start to show signs of EDM when they are 6 to 12 months of age. Horses with mild cases of EDM may present as performance-related problems. At first the condition produces subtle signs, being nothing more than a "clumsy" foal but slowly progress as clinical signs are usually slow and insidious. Ataxia signs will become more apparent and worsen over time. Paralysis and spastic muscular movements will become more evident, until late stages where the horse is unable to get up from laying down without assistance. The only way to get a definite diagnosis that a horse has EDM is by conducting post-mortem examination shortly after death.
We find that neurofilament light (NfL) is a useful assay to detect axon damage in equine neurodegenerative diseases. As we get more data we will publish the significance of NfL in disease. The NfL value should be 0 and detecting any circulating NfL is an abnormal finding. The NfL value will change quickly with proper treatment, of course the treatment depends on the disease process. The attending veterinarian will make a differential diagnosis, confirm the diagnosis, treat the disease and then use a pre and post NfL to assess the effect of therapy.