Bone fragility disorder (BFD) is a progressive, debilitating bone disease of horses that results in severe bone loss and weakening, leading to spontaneous bone fractures. The disease is a type of silicate-associated osteoporosis (SAO) that affects horses that have previous or currently live in the state of California in the United States, particularly the northern coastal mountain range and the Sierra Mountains. The soil in this geographic region contains silica crystals (e.g., quartz, cristobalite and tridymite) that are toxic to horses if they are inhaled while grazing in pastures. Once inhaled, these crystals migrate into the horse's lung tissue and surrounding lymph nodes, causing chronic inflammation.
Early clinical signs of BFD in horses consist of exercise intolerance, body and neck stiffness, and intermittent lameness. As the disease progresses, horses often have difficulty breathing and develop outward bowing of their shoulder blades and swayback. Affected horses are at an increased risk of spontaneous bone fractures, and often die from catastrophic injuries or are euthanized.