Selenium Toxicity

by Amy Gray

Yet another one of the challenges to keeping horses in Colorado is the high concentration of selenium found in our soil.  Selenium is a trace nutrient, actually requried by horses for healthy living.  Too little selenium in the diet causes muscular dystrophy in foals, but is not often seen in this part of the country.  However, selenium toxicity is fairly common, particularly in dry years.  The following provides information on the symptoms of selenium toxicity, possible treatments, and how to identify the potential for a problem, before it occurs.

Symptoms of Selenium Toxicity

Horses suffering from chronic selenium toxicity are said to have "alkali disease."  The excess selenium causes the loss of mane and tail, the first indicator that your horse may be suffering.  Many horse owners mistakenly label an innocent pasturemate as a tail-chewer when this occurs.  However, upon further inspection, hair loss areas appear to have had a set of clippers run over them.  A dull coat normally accompanies the hair loss.

Cracking, brittle hooves are another, more severe, sign of toxicity.  A horse with this condition needs to see both a veterinarian and experienced farrier as soon as possible.  Without treatment, the hoof may slough away from the coronet band and drop, exposing the sensitive laminae to the environment.  This causes extreme pain and lameness for the horse.  Even with treatment, the horse may be susceptible to abscesses until the new hoof capsule is regrown.

Finally, excessive selenium has been known to cause bone lesions and twisted legs in foals.  Acute selenium toxicity, which is rare, may cause the death of the horse when massive amounts of selenium are ingested.

Possible Treatments

If you suspect your horse has selenium toxicity, there are two steps that demand immediate attention.  First, remove your horse from pasture.  Second, call your veterinarian.  Your vet will likely take hair and blood samples to confirm diagnosis of the toxicity, although this is not always mandatory if the symptoms are severe enough.  Upon affirmation of the toxicity, your vet can recommend the best course of treatment.  Often this will mean draining any abscesses your horse may have sustained.  Or, if the hoof wall has broken off, special boots may be applied until the hoof has recovered enough to hold hoof repair materials.  Diet changes may be recommended as well.

How to Identify Potential Problems

There is a set of plants called indicator plants.  These plants are known to proliferate in areas with high levels of selenium.  Identification of indicator plants in your pasture means that you will have to closely monitor the levels of nutritious forage available.  When these begin to drop, typically at the end of the summer or during drought, your horses will turn to the normally unpalatable indicator plants for food.  the problem begins here, as the indicators contain high levels of selenium in their cellular structure.  Species of milk vetch, woody aster, salt bush and goldenweed are all indicator plants to watch for.  Information concerning the effects of these plants can be found at Colorado State University's Veterinary Teach Hospital website.

You may avoid a problem with selenium by providing supplemental forage at critical times, irrigating, avoiding overgrazing, and application of pasture-approved herbicides.  Provision of a high-protein, low selenium diet has also proven to be useful.

Sources of Information:

Evans, J. Warren, et. al. (1990).  Nutrients.  The Horse (2nd Ed., pp. 234-235).  U.S.A.:W.H. Freeman and Company.

Frape, David (1998).  The Roles of Major Minerals and Trace Elements.  Equine Nutriton & Feeding (2nd Ed., pp.         65-69).  Blackwell Science.

Stashak, Ted S. (1996).  In Carroll C. Cann, Susan Hunsberger, Peter J. Carley, & Robert D. Magee (eds.), Practical Guide to Lameness (pp. 223-225).  U.S.A.: Williams & Wilkins