"Osteoarthritis is the most common human joint disorder in the world, and in the equine industry it’s the most economically important disease," said Ashlee Watts, DVM, a graduate student at Cornell University, during the 2007 American Association of Equine Practitioners Convention, held Dec. 1-5 in Orlando, Fla. "It’s the primary cause of decreased athletic function and wastage in racehorses."

Many medications have been tried to treat this common condition, and Watts discussed the relatively new option of doxycycline for arthritis. Doxycycline is a semisynthetic antibiotic that’s related to tetracycline and has been used in horses since the 1990s. It’s also used to treat Lyme disease, and one study on that disease noted that horses given the medication along with other antibiotics "never went better," despite being negative for Lyme disease, suggesting a possible anti-inflammatory effect. Therefore, researchers decided to investigate its possible use against osteoarthritis. (Lisa Fortier, DVM, PhD, Dipl. ACVS, associate professor of veterinary clinical sciences at Cornell University, was the principal investigator on the study.)

Previous laboratory research found that at concentrations as low as 0.0462 microliters (�g/mL) in synovial (joint) fluid and plasma, doxycycline could inhibit the expression of matrix metalloproteinases 3 and 13 (MMP 3 and 13). Various MMP enzymes have been implicated in osteoarthritic and laminitic disease processes, so inhibiting them might help slow these disease processes. However, antibiotics shouldn’t be used at antimicrobial levels (which are substantially higher than 0.0462 microliters (�g/mL) without good reason, because this might promote antibiotic-resistant bacteria strains