Bisphosphonates inhibit bone breakdown/resorption, making them useful for treating bone disorders such as podotrochlosis (navicular syndrome) in horses. Cells called osteoclasts play a key role in breaking down old bone while another type of cell—osteoblasts—creates new bone. This natural process ensures bones remain strong and healthy and allows them to adapt to changes in exercise level or musculoskeletal system stress. Bisphosphonates bind to osteoclasts to block excess bone resorption.
Doctors have used nitrogenous bisphosphonates in humans for many years. Two rare but serious side effects in humans include atypical thigh bone fractures and jaw osteonecrosis (a condition in which, because of disrupted blood supply to the bone, bone breaks down faster than it can be produced).
Bisphosphonates used in horses are non-nitrogenous. Till date, no published studies appear to have identified either femoral fractures or osteonecrosis as potential threats to horses receiving non-nitrogenous bisphosphonates. Some horses can experience certain adverse effects during and after bisphosphonate administration. With clodronate disodium (Osphos) administration those can include clinical signs of discomfort such as pawing, yawning, and lip-licking, as well as colic. Adverse effects associated with tiludronate disodium (Tildren) administration include colic, frequent urination, muscle twitching, and injection site inflammation.
In studies associated with each of the drugs, most of the adverse events were self-resolving. Bisphosphonates should not be administered concurrently with NSAIDs. Bisphosphonates can cause gastrointestinal and renal (kidney) toxicity; NSAIDs also can contribute to kidney dysfunction and gastrointestinal ulceration, particularly of the stomach (both glandular and squamous portions) and right dorsal colon. Practically, this can be tricky, because NSAIDs are popular treatments for many painful conditions, including navicular syndrome and osteoarthritis.
Bisphosphonates aren’t recommended for use in young and/or pregnant horses. The Fédération Equestre Internationale will begin prohibiting nitrogenous bisphosphonate use in horses starting in 2019.
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