Septic arthritis—inflammation of the joints caused by an infectious agent—can cause significant pain and lameness in horses. Although more horses are recovering from infection than in the past, early and accurate diagnosis is still key to successful treatment. This, in itself, can be a challenge, but researchers are making strides in identifying a new way to identify septic arthritis.

“A significant concern regarding the diagnosis of septic arthritis are the clinical similarities between joint infection and aseptic joint inflammation,” said Elsa Ludwig, DVM, MS, CVA, now an associate veterinarian at the Vermont Large Animal Clinic, in Milton.

Normal equine synovial fluid has a total protein of less than 2.0 g/dL and a total nucleated cell count of less than 500 cells/µL, she said. In severe septic arthritis cases, synovial fluid total protein can increase to greater than 4.0 g/dL and total nucleated cell count can rise to more than 30,000 cells/µL; these are typically coupled with severe lameness.

However, Ludwig said, “cases that are early in the disease process, sepsis resulting from intra-articular corticosteroid injection, nonseptic inflammation, or infection with an organism of low virulence may have synovial fluid total nucleated cell counts that are below 30,000 cells/µL and total protein concentrations below 4.0 g/dL, making the distinction between septic and nonseptic inflammation less clear