Equine Bone Fragility Disorder Reported in California

Veterinary researchers at the University of California, Davis, are working to classify an observed bone fragility disorder that might prove to be the culprit behind some cases of intermittent chronic lameness that have no other explanation.

After an extensive review of medical records from horses examined at the university's Veterinary Medical Teaching Hospital between June 1980 and June 2006, researchers found 16 horses with lameness that could not be pinpointed. All 16 horses showed multiple sites of increased radiopharmaceutical uptake (or "hot spots") on scintigraphic images, which indicates increased bone activity (such as bone remodeling or healing after a fracture).

"The horses all have similar clinical symptoms and present with what we think are differing stages of the same disease," said Jonathan Anderson, BVM&S, resident in equine surgery at the hospital and lead author on the study. "It appears that the disease becomes clinically evident as a chronic low-grade lameness that progresses to involve other legs, or deformity of flat bones of the appendicular skeleton...including lordosis (swayback) and extreme scapular bowing (when the scapula, or shoulder blade, arcs out from the horse's body)."

Horse with lordosis and scapular bowing

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

The study notes many similarities between the clinical signs of these horses and those described in a study presented at the 2006 AAEP convention by California practitioners Matthew Durham, DVM, and Coral Armstrong, DVM, (see article #10043). These include the involvement of multiple bones, most often the scapula and ribs, and progression of the disease to lordosis, severe deformation of the scapulae, or degenerative changes of the cervical vertebrae.

All of the horses in Durham and Armstrong's study were also diagnosed with pulmonary silicosis, a mild to severe respiratory disease caused by inhaling certain types of silicate dust found in some geographic regions. They suggested silicosis was the primary cause of the disorders they recorded.

However, in Anderson's study, only three of the 16 horses with inexplicable lameness were definitively found to have pulmonary silicosis, while four others showed some clinical signs of the disease.

"The relationship between pulmonary silicosis and this (fragility) disorder has not been established," Anderson said. "A correlation between the two is based on anecdotal evidence, albeit good anecdotal evidence, but not scientific foundations at this point." He added that the horses affected most by silicosis were not necessarily the ones with the most severe signs of the bone disorder, and he suggested that controlled exposure trials are necessary to establish the validity of a link between silicosis and the disorder.

Treatment of the 16 study horses was limited to pain management through use of non-steroidal anti-inflammatory drugs and exercise restriction until the level of discomfort or pathologic fractures dictated euthanasia. Eleven of the 16 horses examined had been euthanized at the time of the study.

Although no specific treatments can be recommended until the cause of the disease is identified, researchers suggested that some treatments used in human osteoporosis could be effective in these horses. Particularly, the drug Tildren (tiludronate, a bisphosphonate) has been shown to reduce pain and degenerative bone changes in similar conditions in humans. Anderson says that Tildren has been tried in a few equine cases, but follow-up data is not yet available.

According to Anderson, UC Davis is currently undertaking additional research to investigate all aspects of the disorder, including its pathogenesis, relationship to silicates, geographical distribution, the bones affected, and what exactly is happening to the affected bones on both a gross and microscopic level.

The study, "Clinical and scintigraphic findings in horses with a bone fragility disorder: 16 cases (1980-2006)," appeared in the June 2008 issue of the Journal of the American Veterinary Medical Association.

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