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.
“Additionally, nonseptic inflammation can result in synovial fluid cytologic values similar to the classically elevated septic synovial fluid parameters,” she continued. “The ambiguity that can result from synovial fluid analysis may prevent a horse from receiving a timely diagnosis and treatment.”
While she was completing her residency at the Virginia-Maryland College of Veterinary Medicine, in Blacksburg, Virginia, Ludwig and colleagues sought to determine whether a biomarker—serum amyloid A, or SAA—might help diagnose septic arthritis. She presented the results of their study at the 2016 American Association of Equine Practitioners’ Convention, held Dec. 3-7 in Orlando, Florida.
Serum amyloid A is a protein produced in the liver in response to inflammation or infection. It increases and decreases quickly, which means it can give veterinarians nearly real-time information about what’s going on in the horse’s body. Additionally, SAA levels can be measured rapidly and easily using stall-side assays.
In their study, Ludwig and her fellow researchers induced septic arthritis and synovitis (noninfectious form of joint inflammation) in nine healthy adult horses. They hoped to determine whether measuring horses’ SAA concentrations could help distinguish between the two conditions, which can appear clinically similar.
The team collected blood and synovial fluid samples at several timepoints following arthritis or synovitis induction. They performed synovial fluid cytology (measuring the number and types of inflammatory cells and amount of protein in the synovial fluid) and measured the SAA levels in both types of samples. (Traditionally, these tests have been run on blood, but the researchers wondered if synovial fluid could yield a helpful result.)
They found that:
- In synovial fluid, the total nucleated cell count and total protein increases on the cytologies were similar following both septic arthritis and synovitis induction;
- SAA levels remained normal in both serum and synovial fluid samples from horses with synovitis;
- SAA levels in blood and synovial fluid increased significantly, however, in horses with septic arthritis; and
- When comparing commercially available SAA assays, there was good agreement between the tests, which provided a convenient and rapid way to measure SAA.
“Interestingly,” Ludwig added, “serum SAA concentrations increased earlier than synovial fluid SAA in the septic arthritis horses in our study.” The exact mechanism behind this lag in synovial fluid SAA response remains unclear.
“While the delayed SAA response in synovial fluid is not ideal for the timely diagnosis of septic arthritis, the early increases in serum SAA may be supportive of a diagnosis of synovial sepsis,” she continued. “The early elevation in serum SAA may be beneficial for veterinarians. It is much easier to collect blood for SAA analysis than to perform synoviocentesis (joint fluid collection), especially in the field. And this ability to use serum for immediate SAA quantification on the farm may aid in a more timely referral.”
These experimental findings suggest that serum SAA could serve as an earlier indication of joint infection than synovial fluid SAA, the team concluded.
“Ultimately, any way to aid in the early differentiation between aseptic synovitis and septic arthritis has the potential to result in more rapid therapy and improve the prognosis for a favorable outcome,” Ludwig said. “Serum and synovial fluid SAA are promising additions to the diagnostic tools available for the identification of joint infections in horses, and SAA warrants further investigation in clinical cases of suspected synovial sepsis.”