Bone fragility disorder (BFD) is a progressive, debilitating bone disease that causes horses to develop lameness, swayback (not age-associated), neck stiffness, and outward bowing the shoulder blades. The disease progressively worsens until horses develop spontaneous bone fractures and die or are humanely euthanized.

Although the cause is unknown, horses affected by BFD also have pulmonary silicosis, which is a chronic lung disease. Pulmonary silicosis is caused by inhaling toxic silica crystals from the soil. Once horses' inhale the crystals, they migrate into the lung tissue and surrounding lymph nodes, resulting in chronic inflammation.

BFD 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 regions contain toxic silica crystals (e.g., quartz, cristobalite and tridymite). Horses inhale or ingest them when feeding on pasture or eating hay off of the ground.