Incidence of synovial infections

Septic arthritis/tenosynovitis is common in all types of horse, with the highest incidence in neonates. The tarsocrural joint is most frequently involved, followed by the fetlock, carpus, stifle and pastern joints respectively.

Tendon sheath infections are less common, most frequently involving the digital tendon sheaths, and less frequently the carpal and tarsal sheaths. Adult horses usually have involvement of a single joint or synovial sheath, but polyarthritis can occur.

Neonates frequently have multiple joints affected. Bacterial infection of any synovial structures can result in permanent degenerative changes to the synovium, joint capsule, articular cartilage, tendons and surrounding structures, with crippling lameness. 

Pathophysiology of synovial infection 

Septic arthritis / tenosynovitis / bursitis results from the inoculation of micro-organisms into a synovial structure resulting in an inflammatory response which fails to prevent proliferation of the micro- organisms. The size of the innoculum necessary to overcome local defences and progress to infection varies depending on the organism involved and other local physical and immunological factors. A rapid influx of leucocytes (predeominantly neutrophils) occurs, which are capable of phagocytosing bacteria and releasing destructive enzymes. Disruption of the blood-synovium barrier results in release of multiple inflammatory mediators into the joint and amplification of the inflammatory response. Neutrophil activation modulates the release of enzymes, free radicals, and cytokines including IL-1 and TNF into the synovial cavity.

Release of inflammatory mediators stimulates chondrocytes within the joint to release enzymes causing disruption of the proteoglycan-hyaluronan complex in the articular cartilage. The protection of the articular cartilage is further compromised by changes in the synovial fluid due to changes in viscosity, and fibrin and pus accumulation. Accumulation of debris within the joint also facilitates adherence of bacteria and impedes normal joint metabolism 

Leakage of fluid into the synovial compartment results in joint or tendon sheath distension, and increased intra-articular pressure. This reduces blood flow to the synovium and impairs normal exchange across the synovial membrane. In joints this can further compromise the viability of the articular cartilage. Distension of the synovial compartment activates stretch receptors in the joint capsule or tendon sheath causing pain. 

Adults: Joint, bursa or tendon sheath infections occur most frequently from direct contamination by a penetrating wound and much less commonly through idiopathic inoculation during intra-articular medication or infection after a surgical procedure. Clinical signs (lameness!) may appear 1-19 days following intra-articular injection. Factors which increase the risk of iatrogenic infection include skin or coat contamination, cellulitis, oedema, haematomas, pre-existing synovitis or arthropathy, septicaemia, a lack of aseptic technique or immunocompromised.

Foals: Haematogenous infection is unusual in adult animals but common in foals (neonatal foals up to 3-4 months of age), resulting in multiple joint infections. So any foal with pneumonia, umbilical infection, colitis etc... is at risk of joint sepsis as well. 

Gram-positive isolates are more common in adults with Staphyloccoccus aureus having the highest incidence, and non-haemolytic Staphylococcus, β-haemolytic, and non β-haemolytic Streptococci, Rhodoccoccus equi, and Corynebacterium have also been isolated.
Multiple and gram-negative infections are more frequent in foals with E.coli being the most frequent isolate, and Pseudomonas, Enterobacter, Actinobacillus, Proteus, Klebsiella, and Salmonella have also been isolated. 

Most synovial infections in adults occur from direct penetration and knowledge of the regional anatomy and the extent of the synovial compartments is a prerequisite for recognising the risk of synovial infection from an adjacent wound. Clinical signs of synovitis include pain (that means lameness!), heat and swelling and are similar for joint and tendon sheath infections. Pyrexia is often absent and is a poor indicator of synovial infection in adults. Pain may appear to be sudden in onset and lameness may often be almost non-weight bearing. Careful digital palpation should enable identification of synovial distension and comparison with the contralateral limb may be helpful. The presence of gross cellulitis, bruising or oedema can impede identification of anatomical structures and may be ameliorated by cold hosing or bandaging in some cases. Clear, yellow viscous fluid exuding from a wound may suggest leakage of synovial fluid but could also be serum. In the presence of wounds, hair clipping followed by aseptic wound lavage allows a visual and digital appraisal of the wound. The application of sterile methyl-cellulose gel (K-Y1) to the wound prevents hair contamination during clipping. Clinicopathological parameters are frequently within normal limits, but a high normal white blood cell count with an elevated neutrophil ratio (left shift) may occasionally be observed.

Neonates are more likely to demonstrate a leucocytosis or fever. Many cases will require synovial analysis for confirmation (Table 1) although commonly in foals they are reluctant to rise and are noted to be lethargic or nursing less often. 

Common signs associated with synovial sepsis: 

•   Severe lameness

•   Wound nearby

•   Joint effusion 

0. Not useful for diagnosing joint sepsis: 

• Pyrexia

•   Peripheral blood analysis

•   Local skin temperature

•   Radiography (unless there is air in the joint after a wound) 

0. Significant changes in synovial fluid parameters occur within hours of joint contamination. Synovial fluid should be obtained adhering strictly to aseptic technique. Those horses with cellulitis around the joint present a diagnostic dilemma, and in some such cases synoviocentesis is delayed if the risk of iatrogenic joint contamination is considered too high. Synovial fluid should be collected in EDTA for cytology and a separate sample collected for aerobic and anaerobic culture. Synovial membrane culture in combination with synovial fluid may result in an increased chance of obtaining an isolate, but positive cultures are only obtained in approximately 50-70% of cases. Normal synovial fluid is clear, yellow and viscous. Gross changes indicating synovial fluid abnormality include increase in turbidity, red or dark yellow colour and reduced viscosity (Table 1). Total protein values >25g/l are indicative of synovitis, however iatrogenic haemorrhage may result in slightly elevated levels. Leukocyte counts in synovial fluid rise quickly in infection, with most clinical cases having count of >30 X 109/l and most cases having >80% neutrophil 

Septic arthritis in foals is usually haematogenous in origin and should be considered as part of a more generalised disease, e.g. septicaemia or failure of passive transfer of immunity. Foals with Rhodococcus equi infections can develop an immune-mediated synovitis or a septic physitis affecting multiple joints. All foals with signs of septicaemia are at risk of developing arthritis. Infections frequently involve bony tissue, and osteomyelitis is frequently present in or near the physis, and in the cuboidal carpal and tarsal bones. Foals of any age from a few days to four months may be affected, and multiple joints are frequently involved, especially in younger foals. Epiphyseal infections may present with hot swollen joints, whereas physeal infections may display more subtle swellings at the physis, accompanied by severe lameness or recumbency. Aspiration at the physis may yield positive culture. Radiographic changes may be very subtle initially and radiography can be repeated after 7 days. 

Lavage through needles is ineffective for removal of solid fibrin or pus in adults, but through and through lavage through needles and catheters is useful in foals and may be performed in the standing animal in field situations. Arthroscopy is the gold standard of treatment and allows visual assessment of the synovium and articular cartilage, lavage and surgical debridement of fibrin or inflamed synovium, which may contain adhered bacteria, and damaged articular cartilage. Lavage solutions should consist of large volumes (5L – 30L) buffered physiological polyionic solutions e.g. Hartmann’s solution (pH7.4) delivered under pressure. Drainage from the digital flexor tendon sheath may be impaired by constriction as the sheath passes through the palmar annular ligament of the fetlock. Improved drainage may be achieved by desmotomy of the palmar annular ligament. 

Systemic antimicrobial therapy

Appropriate antimicrobial therapy is a critical component in treatment of septic arthritis/tenosynovitis. Selection of the drug should be based on results of culture and sensitivity tests but since there may be a delay in culture results, and culture is frequently negative, antimicrobial therapy should be initiated at the outset of treatment with drug(s) selection based on previous clinical experience and the nature of the condition. The common protocol at many equine hospitals is gentamicin 6.6 mg/kg. S.i.d. (i.v.) combined with Sodium Benzyl penicillin at 20,000i.u./kg q.i.d. (i.v.). Neonatal infections often involve gram negative or mixed infections, broad spectrum drug combinations are used. 

Intra-articular medication
Gentamycin (300-500mg), amikacin (500 mg- 1g) and cephazolin (500mg) have been used intra-articularly without adverse reactions. 

Regional limb perfusion

The application of a tourniquet is followed by the administration of the antibiotic into a peripheral vein, distal to the tourniquet, allowing distribution of the drug by diffusion. The tourniquet is left in place for 15 – 20 minutes and the treatment is usually repeated every two days. 

Antibiotic-impregnated polymethylmethacrylate or gelatin sponges.

Two methods giving higher articular levels of antibiotic than can be achieved by safe systemic administration are PMMA beads impregnated with gentamycin, and collagen sponges impregnated with gentamicin (“Collotamp”) 

Some NSAIDS may be delivered initially during treatment but should be used with caution so that an infection is not masked by high doses. However some anti-inflammatory action is beneficial to the joint. 

The prognosis for a successful outcome is highly variable depending on the virulence of the infection agent, age of the patient, number of joints involved and duration of the condition. Published figures for prognosis for survival range from 70-92 % for horses with infected joints, and appear to be slightly better for infected tendon sheaths. The prognosis for a return to athletic soundness was 56% in one study. Early diagnosis (<24h) and aggressive treatment are important factors in maximising the probability of a successful outcome. 

Synovial sepsis will eventually extend to the subchondral bone, but in the adult horse this will take a long time (radiography is 80-90% diagnostic after 21 days of infection) however usually arthroscopy will demonstrate involvement much earlier. Foals will show subtle subchondral lucencies much earlier in joint sepsis. In either case the prognosis is very poor for any high- motion joint once radiographic changes are present.

Any bone can become infected and septic osteitis is seen most commonly in the pedal and splint bones, sometimes in the form of a sequestrum.