J.D. Wheat Veterinary Orthopedic Research Laboratory

Silicosis Associated Osteoporosis: An Emerging Equine Bone Disease

Photo: Silicosis Associated Osteoporosis

A progressive, debilitating, and ultimately fatal bone disease has been recognized in recent years to affect horses in California.  The disease – termed silicate associated osteoporosis (SAO) - has a preference for bones of the upper portion of the limbs (e.g., scapula, pelvis), the ribs, and the vertebral spine.  When mildly affected, horses appear to have an intermittent lameness without an identifiable cause.  The lameness may affect one leg, several legs, or different legs at different times.  When multiple legs and/or the spine are affected, horses can appear to have a generalized stiffness and reluctance to move.  With disease progression, bones of the spine and upper portions of the front and hind legs become weak.  Over the course of months to years the bones deform and sustain incomplete bone fractures that attempt to heal.  Ultimately, a fracture may be severe enough to cause death or necessitate humane euthanasia.

Severely affected horses can be recognized by skeletal deformities.  The scapula (shoulder region) begins to bow outwardly.

Note the prominent lateral (outward) bowing of the left scapula – as viewed from behind the shoulder on the side of a horse with its head lowered to the ground.

Photo: Silicosis Associated Osteoporosis

This usually starts with one shoulder bowing initially, but often both shoulders are eventually affected.  The back becomes markedly swaybacked in a relatively short period of time.   The neck becomes stiff making turning the head and eating off the ground difficult, or ultimately impossible.

Note the marked swayback (and altered shoulder shape) characteristic of horses with advanced stages of silicate associated osteoporosis

Photo: Silicosis Associated Osteoporosis

Diagnosis of Silicate Associated Osteoporosis (SAO) is confirmed by bone scans.  The results of routine blood tests are usually normal.  Radiographs of the legs are not generally helpful in disease diagnosis because the bones in the lower part of the limbs are minimally affected.  Good quality radiographs of the lower cervical vertebrae in the base of the neck may be useful for detection of bone changes in moderately to severely affected horses.

Note lytic lesions (holes) in bones of the neck from affected horse

Photo: Silicosis Associated Osteoporosis

Ultrasound examination of the scapula (shoulder blade) may demonstrate thickening of the scapular spine or evidence of a fracture. 

Note the wider border and rougher edge of the ultrasound image of the scapular (shoulder blade) spine

Photo: Silicosis Associated Osteoporosis

Bone scans are highly useful for verification of the widespread nature of the disease.  Multiple abnormalities are identified in multiple bones of the upper portions of the legs, the vertebral spine, and the ribs. 

A bone scan illustrates multiple foci of high bone turnover (white areas) on the scapula (left) and the rib cage (right)

Photo: Silicosis Associated Osteoporosis

Unfortunately, bone scans are only available in university settings and specialty equine practices because they require expensive, specialized equipment.

Affected horses may have concurrent pulmonary disease.  Many horses with SAO have lung inflammation associated with inhalation of cristobalite, a specific silicate crystal found in some environments.  Horses with moderate to severe lung disease require extra effort to breathe.  These horses may have an elevated breathing rate during rest, accentuated muscles in the chest and abdomen due to increased muscular effort to breathe, and flaring of the nostrils in an effort to obtain more air.

Flared nostrils in a horse at rest are indicative of respiratory disease