Both equine degenerative myeloencephalopathy (EDM) and equine motor neuron disease (EMND) in horses are rare conditions associated with a deficiency in alpha tocopherol (vitamin E). EDM develops in genetically susceptible horses if the deficiency occurs during the first year of life. Once affected the course of the disease is relatively stable throughout the horses lifetime. At Cornell, it is the second most common diagnosis for neurologic disease after a diagnosis of EPM. EMND occurs in adult horses after an extended period of vitamin E deficiency and occurs in 0-2 horses in 100,000.
The EDM-affected horses are ataxic (symmetric) in all four limbs that starts when the foal is a few months old. Mildly EDM-affected horses trip often. Once affected the course of the disease is relatively stable after 3 years old and doesn’t change throughout the horses lifetime. At UC Davis they are suspicious of EDM if the onset of signs is 6-36 months, there is a history of low vitamin E levels, signs are slowly progressive or stabilize, have negative EPM and West Nile titers, have normal neck radiographs. It is also highly suspicious if there are relatives that also show similar signs. Vitamin E supplementation will not help these horses.
The clinical signs of EMND are different that EDM. EMND-affected horses have generalized lower motor neuron weakness, muscle atrophy, trembling, low head carriage, and they carry their tails higher and a bit off center. These horses have reduced exercise tolerance and can sweat profusely with minimal exercise. Horses continue to have a good appetite but suffer extensive weight loss. The EMND horses are teenagers but the disease may not be recognized before one third of the motor neurons are lost. The diagnosis of EMND can be confirmed by skeletal muscle biopsies and electromyography.
Some researchers postulate that there is an overlap in genetic susceptibility to developing both EDM and EMND in certain families when a nutritional vitamin deficiency is present. It was shown that only genetically susceptible foals develop EDM when they were maintained on a deficient diet. And this occurs during the first year of life to produce the neurologic disease. To get fancy, EDM is inherited in the Quarter Horse as an autosomal, dominant gene with incomplete penetrance.
Where does a horse get vitamin E? Green grass! If the horse has access to green grass then vitamin E deficiency may not be an issue. How much vitamin E does a horse need? The National Research Council suggests 1-2 IU/Kg body weight. That could be lower than forage or most commercial feeds can provide. Also, inflammation in the gut can decrease the absorption of fat soluble vitamins. There are tests that can show if the horse has decreased absorption. The horse is given vitamin E after 12 hours of fasting. A blood sample is then taken after another 12 hours. It is very important that the serum is removed from the clot before shipping the sample to the laboratory. Leaving the clot and serum in contact will artificially lower the vitamin E levels.
There are laboratories that measure the serum alpha tocopherol levels. Testing is generally less than the cost of one months supplementation. The same sample handling applies to measuring levels in serum, remove the clot from the serum. Before you run out to test your horse or purchase expensive supplements remember the incidence rate of EMND in the northeastern US (1985-1995) was only 0-2 horses in 100,000! There was a threefold spike in the number of cases in 1991 but that was attributed to publicity, EMND was described as a new disease entity affecting horses. The same thing happened with EPM, there was a spike in the number of cases associated with the Western Blot test and the release of licensed medications from 1995-1998, but aside from that spike, the number of cases are steady for the last 18 years.