Septic Arthritis, Tenosynovitis, and Bursitis
Infection of synovial structures may involve joints (septic arthritis), tendon sheaths (tenosynovitis) or a bursa (bursitis), which is a fluid-filled cavity situated within tissues exposed to friction. The infection occurs when microorganisms are given time to colonize the space after a penetrating wound, a joint injection, following surgery or following a spread of bacteria via the blood stream (hematogenous infection). Although common in the foal, hematogenous spread is rare in the adult horse but has been reported to occur (< 10% of cases). Therefore, synovial infection should be considered as a potential differential diagnosis in any horse with sudden onset of lameness, synovial effusion (fluid buildup) and fever. Horses with drainage through communicating wounds will generally be less lame than those without drainage. Therefore, it is important to rule out possible synovial structure communication with a wound at the time of injury rather than wait to see if the horse becomes lame. Many times, these horses do not become significantly lame until 7-10 days later when the overlying wound has closed sufficiently to prevent synovial drainage.
A diagnosis of joint, tendon or bursa infection is generally confirmed through fluid analysis and culture. Normal synovial fluid has less than 300 white blood cells/dL and contains less than 2.5 g/dL of protein. More importantly, lymphocytes are the predominant cell type at 90% or greater. Difficulty may arise in differentiating periarticular trauma (inflammation surrounding the joint) and joint infection in early cases of infection when total white cell counts are not significantly above the normal range. However, if the percentage of inflammatory cells (neutrophils) is high infection should be assumed.
Synovial fluid obtained for cell count should also be submitted for bacterial culture. Isolation of the causative agent can confirm the diagnosis of infection but more importantly it can guide antimicrobial treatment. Blood culture bottles with enrichment medium optimize isolation rate (~77 % vs. 37% with agar plates). The medium used in blood culture bottles contain lytic agents which release engulfed bacteria enabling them to be cultured. In addition the medium also contains SPS (sodium polyanethole sulphonate) and/or resins which absorb antibiotics which if they have been previously given to the horse would delay bacterial growth.
The initial treatment of a potentially septic (infected) synovial structure should include broad spectrum antimicrobials directed at both aerobic and anaerobic micro-organisms. The source of the infection should be taken into consideration. In traumatic wounds, members of the family Enterobacteriacaea are most likely to be found but Streptococcus, Staphylococcus, and anaerobes (those bacteria surviving in the absence or oxygen) may also be present. Traumatic wounds generally result in a mixed population of bacteria. Horses which develop infection following synoviocentesis (joint taps) or surgery are likely to be infected with Staphylococcus. It should be remembered though that fungal infections, however rare, do occur in horses and should be considered in patients that are not responding to antibiotic therapy as anticipated.
The treatment of septic arthritis (joint infection), tenosynovitis and bursitis (infection or inflammation of a tendon sheath or bursa) has greatly improved over the last decade which has led to improved outcome. This in large part is due to early aggressive treatment, availability of a greater variety of antimicrobial choices, and improvement in the delivery of antibiotics to the site of infection.