Time is of the essence when determining whether an equine patient has this potentially life-threatening problem. So, make sure you know what diagnostic clues to look forand what could be a red herring.
Septic arthritis, tenosynovitis or bursitis can be devastating and even life-threatening for equine athletes. Early diagnosis and aggressive treatment are imperative for a successful outcome. Sepsis in synovial structures is generally diagnosed based on the horse's history and clinical signs as well as the results of imaging studies, cytology and synovial fluid culture.
In foals, septic arthritis can occur from the first week to the first few months of life and is typically hematogenous in origin. In adult horses, septic synovial structures most often are due to a penetrating wound but they can be iatrogenic. In one study, postinjection sepsis presented up to 19 days after injection (median, 2.5 days).1 A higher risk for iatrogenic infection after intra-articular injection has been associated with corticosteroids and polysulfated glycosaminoglycans. Also, corticosteroids can delay the onset of clinical signs up to several weeks.1
Typically, horses with septic synovial structures are markedly lame on presentation. The lameness may be less severe if it is acute or the structure is open and draining. Effusion, heat, swelling and pain during manipulation are common findings on physical examination. However, there may be no synovial effusion if the patient has a draining wound.
A complete blood count is usually unremarkable in adult horses with septic synovial structures, but in foals hyperfibrinogenemia is typical. This is especially true with increased chronicity or with bone involvement.
One of the most useful tests for diagnosing septic synovial structures, synoviocentesis should be performed aseptically at a site distant to any wounds or potentially infected tissue. If a wound is present, distention of the joint after fluid collection can help to determine if there is communication.
Place collected fluid into a plain (red-top) tube, an EDTA (purple-top) tube and a blood culture bottle. Normal synovial fluid should be light yellow and clear. See Table 1 for normal values and those suggestive of a septic synovial structure. Many studies have shown that less than half of synovial fluid samples yield a positive culture.2,3 Placing the sample in a blood culture bottle for aerobic and anaerobic culture can help increase the sensitivity. Also, synovial membrane biopsy at surgery can increase the likelihood of a positive culture to up to 70%.4
Table 1. Synovial fluid values suggestive of sepsis2,3
Suggestive of sepsis
Total nucleated cell count
Same as serum
Decreased compared with serum (>2.2 mmol/L difference)
Serum amyloid A
>20 µg/ml (up to 100 µg/ml for five days after arthroscopy)
Serum amyloid A (SAA), a major acute-phase protein that is produced in response to inflammatory cytokines, has a short half-life (24 hours) and can be used to help diagnose and monitor the progress of an infection. In an experimental model of synovitis and septic arthritis, serum and synovial fluid SAA increased significantly in horses with septic arthritis and remained normal in those with synovitis.5 SAA increased more rapidly in serum (24 hours) than in synovial fluid (36 hours).5 Additionally, a correlation has been shown between decreasing SAA and a positive response to treatment.6
Recently, a real-time polymerase chain reaction (PCR) assay of synovial fluid has been investigated and shows promise as a faster (four-hour-turnaround) and more sensitive (87%) and specific (72%) diagnostic test than culture.7 More research is needed to standardize this test for clinical use.
Obtain radiographs to determine whether there is osseous involvement. Radiographic changes are usually not evident for two or three weeks, but it is helpful to have the initial images for comparison later. Contrast radiography, such as fistulography or contrast arthrography, can also be performed to help determine whether there is synovial involvement.
Ultrasonography can be very useful to help differentiate cellulitis from synovial infection, assess joint fluid character, identify foreign bodies and guide synoviocentesis. In a recent study evaluating ultrasonography and septic arthritis or tenosynovitis, the most common findings were marked effusion, moderate-to-severe synovial thickening and fibrinous loculations.8 Echogenic fluid was noted in about half of the cases. The presence of synovial effusion should not be used as the sole diagnostic indicator of septic synovial structures because synovial effusion is common with inflammatory synovitis or tenosynovitis.
Magnetic resonance imaging (MRI) and computed tomography can provide high tissue detail and may be performed in cases in which further definition is required. The most common MRI findings reported are extracapsular and subchondral bone hyperintensity on fat-suppressed images.9
Arthroscopy or tenoscopy can help determine the extent of synovial structure involvement. These techniques also have therapeutic purposes. The presence of osteitis or osteomyelitis and marked pannus have been associated with nonsurvival.10
Synovial sepsis can be a life-threatening condition, but with an early diagnosis and aggressive surgical treatment, the prognosis can be good. If you have a patient with a possible septic synovial structure, consult a board-certified equine surgeon to confirm the diagnosis and help treat the horse.
1. LaPointe JM, Laverty S, Lavoie JP. Septic arthritis in 15 standardbred racehorses after intra-articular injection. Equine Vet J 1992;24(6):430-434.
2. Taylor AH, Mair TS, Smith LJ, et al. Bacterial culture of septic synovial structures of horses: Does a positive bacterial culture influence prognosis? Equine Vet J 2010;42(3):213-218.
3. Schneider RK, Bramlage LR, Moore RM, et al. A Retrospective study of 192 horses affected with septic arthritis/tenosynovitis. Equine Vet J 1992;24(6):436-442.
4. Morton AJ. Diagnosis and treatment of septic arthritis. Vet Clin North Am Equine Pract 2005;21(3):627-649.
5. Ludwig EK, Brandon Wiese R, et al. Serum and synovial fluid serum amyloid A response in equine models of synovitis and septic arthritis. Vet Surg 2016;45(7):859-867.
6. Haltmayer E, Schwendenwein I, Licka TF. Course of serum amyloid A (SAA) plasma concentrations in horses undergoing surgery for injuries penetrating synovial structures, an observational clinical study. BMC Vet Res 2017;13(1):137.
7. Elmas CR, Koenig JB, Bienzle D, et al. Evaluation of a broad range real-time polymerase chain reaction (RT-PCR) assay for the diagnosis of septic synovitis in horses. Can J Vet Res 2013;77(3):211-217.
8. Beccati F, Gialletti R, Passamonti F, et al. Ultrasonographic findings in 38 horses with septic arthritis/tenosynovitis. Vet Radiol Ultrasound 2015;56(1):68-76.
9. Easley JT, Brokken MT, Zubrod CJ, et al. Magnetic resonance imaging findings in horses with septic arthritis. Vet Radiol Ultrasound 2011;52(4):402-408.
10. Wright IM, Smith MR, Humphrey DJ, et al. Endoscopic surgery in the treatment of contaminated and infected synovial cavities. Equine Vet J 2003;35(6):613-619.
Dr. Maggie Peitzmeier is an equine surgeon at Pioneer Equine Hospital in Oakdale, California. Her clinical interests include sports medicine, orthopedics and colic. In her spare time, she enjoys riding her horse, playing volleyball and taking her dog to the beach.
Surgery STAT is a collaborative column between the American College of Veterinary Surgeons (ACVS) and dvm360 magazine. To locate a diplomate, visit ACVS's online directory, which includes practice setting, species emphasis and research interests, at acvs.org.