Of all the places a horse could develop an infection, a joint is one of the least favorable because it's difficult to treat and can have catastrophic consequences. Veterinarians commonly treat arthritic joints with injections, and even hands well-practiced in joint injection techniques run into the occasional infection due to bacterial contamination.
Stephen Adams, DVM, MS, Dipl. ACVS, of Purdue University's School of Veterinary Medicine, recently described how to select and inject drugs to avoid injection, along with ways to recognize and treat joint sepsis at the 2012 American Association of Equine Practitioners' (AAEP) Convention, held Dec. 1-5 in Anaheim, Calif.
Veterinarians can infuse a variety of drugs into horses' intrasynovial cavities (joints, bursa, or tendon sheaths). Some medications are more likely to cause sepsis than others; the drug polysulfated glycosaminoglycans (PSGAGs), for example, enhances the infectivity of Staphylococcus aureus introduced via a needle in a synovial cavity.
Yet, Adams explained that if the veterinarian administers the antibiotic amikacin in combination with PSGAGs, he or she can all but eliminate the risk of infection. In a survey of equine practitioners who administer intra-articular injections, 46% said they add antimicrobial drugs to their joint injections and 30% add corticosteroids.
Adams suggested it is appropriate to use intrasynovial antibiotics with PSGAGs, for multiple injections in the same joint in one day (e.g., during medication therapy following joint anesthesia) and in immune-compromised individuals.
Another drug combination veterinarians commonly use in joints is corticosteroids and hyaluronic acid. Although researchers have shown this combination to pose a low infection risk, Adams cautioned, "Corticosteroids can delay the onset of septic clinical signs by 9 to 10 days due to its profound anti-inflammatory properties."
For joint anesthesia used during a lameness exam, Adams reported that the local anesthetic mepivacaine is less irritating or toxic to cartilage cells than lidocaine, though neither drug appeared to increase a horse's risk of developing sepsis. He cautioned against refrigerating these drugs, as the cold temperature might inactivate important preservatives that prevent S. aureus from growing.
Adams further warned veterinarians about the danger of using multiple-dose vials that are easily contaminated with bacteria, rubber, and metallic particles when needles are inserted multiple times. He urged practitioners to use only single-dose vials or a brand new vial anytime they inject a drug or anesthetic into a joint.
Adams then discussed how to differentiate a joint flare from sepsis. Some general rules apply: A joint flare usually occurs within 24 hours of injection, whereas sepsis (bacterial infection) usually takes three to nine days to develop. If the joint is sore on palpation and/or lameness is Grade 3-4 (the horse is lame at the trot on straight line or lame at the walk), sepsis has likely set in. "When in doubt, flush it out," Adams recommended.
For a favorable outcome, he suggested analyzing synovial fluid, taking radiographs of suspected septic joints, and aggressively treating septic joints with appropriate systemic antibiotics and with joint lavage. Adams advised continuing sepsis treatment for at least 14 days, but if the horse shows only limited improvement over 48 hours then arthroscopic surgery might be indicated.
As it is with many equine health conditions, the best cure for joint infections is prevention. Veterinarians must consider which drugs are most appropriate to inject and be diligent about preventing needle contamination to minimize the horse's chances of developing a joint infection.
Disclaimer: Seek the advice of a qualified veterinarian before proceeding with any diagnosis, treatment, or therapy.