Higher Silicate-Associated Osteoporosis Risk in Some Regions

Silicosis in horses is a respiratory condition caused by exposure to certain types of silicate dust found in some geographic regions. While silicosis is not a commonly diagnosed equine ailment in most areas of the country, Matthew Durham, DVM, of Steinbeck Country Equine Clinic in Salinas, Calif., said he sees cases far too often in his practice. Durham presented research he and co-author Coral Armstrong, DVM, conducted on 18 horses with clinical signs of silicosis, including numerous fractures, and bone deformities 2006 American Association of Equine Practitioners convention in San Antonio, Texas, held Dec. 2-6


A 19-year-old Arabian gelding used in Durham's study, showing scapular bowing and lordosis.

Silicosis is typically a chronic condition in humans causing mild to severe respiratory disease that's often accompanied by rheumatoid arthritis and lupus (a chronic autoimmune disease). In horses it causes respiratory disease, and it appears to be associated with osteoporosis (reduction of bone mass, and its associated fractures) and bone deformity. Silicosis is caused by the inhalation of certain types of silicate dust, including quartz and cristobalite. While quartz does not tend to be dusty, cristobalite is chalky and its dust becomes airborne easily, making horses in areas with high amounts of cristobalite rock particularly susceptible to the condition.

"We see silicosis in our area relatively frequently because we have the right soil type for it," Durham said. "All of the horses we have seen with these bony abnormalities live or have lived in an endemic (prevalent at all times) area."

Durham and Armstrong performed exams on 18 horses in their practice area with signs of silicosis. Of these, 17 had bone deformities including bowed scapulas (in which the scapula, or shoulder blade, arcs out from the horse's body). Fractures among these horses were also common: There were three vertebral, two scapular, five pelvic, and one maxillary (upper jaw) fracture. Additionally, eight horses had rib fractures, including one mare with 22 fractured ribs. The horses also showed significant bone remodeling typical of osteoporosis.

All of these horses were exercise intolerant. Durham said five showed respiratory distress, and researchers felt the others were exercise intolerant due to bone pain.

Durham said that all of the horses in the study group were kept in dry lots or in areas with recent construction, both of which could have caused the horses to inhale silicate dust.

These horses were all from areas of the Monterey Formation, which is a wide swath of soil containing a high level of cristobalite. Durham noted that the study cases lined up right along a ridge--including four cases from one farm--suggesting that the geographic location is the strongest risk factor for silicate-related osteoporosis.

As for treatment, Durham has found that steroids are sometimes more effective for treating the horses' bone pain than NSAIDs. Joint treatments Legend (hyaluronate sodium) and Tildren (tildronate) have also been well-received. As for the respiratory signs, steroids and bronchodilators are typically helpful, but environmental control (decreasing the horse's exposure to silicate dust as much as possible) is key, Durham said.

Durham's research on silicosis is ongoing, and he said upcoming UC Davis projects include detailed bone study, including in vivo bone density measurements, research on the U.S. Geological Survey's maps, and continued examination of the route of exposure.

Get research and health news from the American Association of Equine Practitioners 2006 Convention in The Horse's AAEP 2006 Wrap-Up sponsored by OCD Equine. Files are available as free PDF downloads.

About the Author

Erin Ryder

Erin Ryder is a former news editor of The Horse: Your Guide To Equine Health Care.

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