Chronic selenosis in horses fed locally produced alfalfa hay

Chronic selenosis (alkali disease) was diagnosed in horses of western Iowa, a region associated with marginal to adequate soil selenium. Two locally produced alfalfa hays (Medicago sativa L) were identified as the primary source. Difficulty in selecting diagnostic specimens to evaluate potential chronic selenosis cases is complicated by the wide range of tissue concentrations reported in previous cases, conflicting correlation of sample selenium concentrations in the literature, and different recommendations on specimen selection and diagnostic value. These problems arise form the similarity in clinical signs for mild and severe chronic selenosis cases, and inability to establish the disease time course. Therefore, collection of multiple samples to confirm high selenium concentration in various tissues and identify selenium sources is recommended. Value of samples believed to reflect historical exposure vs current status may be questionable. Interpretation of results by practitioners and diagnosticians should consider selenium exposure level and duration, antagonistic or ameliorating factors, and postexposure excretion.