Selenium is essential for horses, however too much selenium can be toxic. Selenium exists naturally in the soil, varying in concentration across different geographical regions. Nearly all plants accumulate selenium from the soil. Some plants, known as selenium accumulators, require a large amount of selenium to grow. Their presence in the pasture is a good indicator that the selenium levels are high. However another danger is that certain times of the year, these selenium accumulating plants may appear appetizing to horses. Through consuming these plants, horses ingest varying elevated levels of selenium causing differing levels of toxicity. Horse owners need to be mindful of the selenium concentration in their area, as otherwise they could easily and unknowingly be poisoning their horses.

There are two different forms of selenium toxicity which can occur in horses----acute and chronic form.

• Acute (Blind staggers: This is the least common form. It occurs when horses ingest a high dose of selenium over a short period of time (greater than 500 to 1000 ppm). Symptoms found in affected horses include sudden, unexplained blindness, respiratory failure, kidney failure, and colic.
• Chronic (Alkali or bobtail disease): This is the most common form of selenium toxicity seen in horses. Horses must ingest a minimum of 3.3 mg/kg of body weight of selenium daily. Clinical signs most commonly seen in affected horses include lameness, hoof soreness, reddening and swelling of coronary band, hoof cracking parallel to and below the coronary band, poor hoof quality, and dull haircoat.