by Heidi Smith, DVM
What is selenium? It is just one of many trace minerals required by the body. It serves as what is called a "co-factor" in many of the enzyme reactions within the body. It is especially important in the proper function of cell membranes. At the cellular level, a lack of selenium can make it more difficult for muscle cells to transport "good" things in and "bad" things out, or for white blood cells to engulf and destroy bacteria or other invaders. It is also closely related to proper calcium metabolism, which can be important in athletic events, healing of bony injuries, and control of arthritic conditions.
What are some of the signs of selenium deficiency? One of the most dramatic syndromes in which selenium deficiency is often a factor is tying-up syndrome, in which horses experience mild to severe muscle cramping and profound metabolic changes as a result. This can involve small sets of muscles, or many muscles over the entire body. Less dramatically, deficient horses often exhibit poor performance, poor appetite, rough hair coat, and general unthriftiness. The immune system is also affected; foals with low selenium suffer more frequently and more severely from pneumonias and diarrheas. Adult horses may have a more difficult time combating diseases such as influenza or strangles. Deficient broodmares have a more difficult time conceiving; this may be due to an inability to combat the normal contamination of the uterus that occurs at breeding, or may have other causes as yet undocumented. Foaling mares that are deficient are far more apt to retain their placentas. Newborn foals from deficient dams have a much higher incidence of contracted tendons and related problems. In extreme cases, foals can die of the classic "white muscle" condition (more common in calves) in which the heart muscle becomes increasingly streaked with non-functional tissue and eventually goes into acute arrest.
Can selenium levels be tested? Yes! However, the test is NOT a part of a standard profile, and must be requested separately. Normal selenium levels range from .20 ppm to .25 ppm. The sample required is at least 5 ml of whole blood in an EDTA tube (purple top). Although I have been using and recommending Schering Animal Health Laboratory for several years, they have recently quit offering testing for selenium and copper, and are referring all selenium tests to Michigan State University.
The address there is:
MSU Animal Health Diagnostic Laboratory
P.O. Box 30076
Lansing, MI 48909-7576
What can I do if my horse is deficient? Mild deficiencies can be addressed with feed supplements. Several commercial vitamin products (such as Horse Guard, Northwest Supplement, and others) provide approximately a quarter of the horse's daily requirement. More severe deficiencies usually need to be treated initially with injectable products (such as E-Se) to get the levels up initially. Ironically, horses that are severely deficient also have a difficult time absorbing adequate levels from the feed. Once levels are approaching normal, these horses can be maintained on oral supplementation. I use a product called EqU-SeE, which contains selenium and vitamin E, in addition to one of the above supplements, and in many cases, provide the entire daily requirement (8-10 mg per day) through supplementation. Vitamin E levels are also important for the horse to properly utilize selenium; hence, many products contain both, which is desirable.
What about toxicity? Yes, it is true that selenium is potentially toxic. Some areas of the country have very high levels in the soil, which are then incorporated into the feeds grown there. The toxic level of selenium in feed over a period of time is 60-90 mg per day. This is slightly less than 10 times the daily requirement. This small margin between the requirement and the toxic level is far more typical of a drug than a nutrient, and is why many people are shy about aggressively supplementing for selenium. However, with testing available, it is very easy to monitor one's supplementation program to see whether there is any potential for danger. Although toxic cases occur from time to time, they are far outnumbered by deficient horses.
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