In horses, nutritional secondary hyperparathyroidism (NSH) results when an animal consumes a ration that contains an inadequate ratio of calcium to phosphorus. The dietary imbalance of calcium and phosphorus can result in either a decreased serum concentration of ionized calcium in the blood or an increased serum concentration of phosphorus. Consumption of oxalate-containing plants may also cause the disease. Generally, young horses are more frequently affected than are old horses. The clinical signs of NSH include shifting-limb lameness, enlarged facial bones, and pathologic fracture. Diagnosis of the condition is facilitated by dietary history, clinical examination, radiography, serum and urine chemistry analysis, and determination of the serum concentration of parathyroid hormone. The physical examination often does not reveal clinical signs that are specific for NSH. Patients may be referred for other diseases (e.g., hindlimb lameness or sinusitis). In addition, the dietary history can be misleading and may require detailed investigation. The treatment of NSH is empiric and IS directed toward providing a ration that has an adequate calcium:phosphorus ratio. In horses with NSH, with we generally recommend feeding a ration that contains a calcium:phosphorus ratio of at least 4:1. Enforced rest is part of the treatment regimen. Despite the advent of modern ration formulation, cases of NSH continue to occur and require management by equine practitioners. This article discusses the causes, clinical signs, diagnosis, management, and prevention of NSH. Case studies involving two horses with the disease are presented.
