Septic or infectious arthritis can be devastating to a horse's athletic career and can be life-ending. Normal joint and tendon sheath function is disrupted by the associated pain and inflammation occurring in the acute stages of the disease. Should prompt treatment of the infection not be performed, normal joint function may be deleteriously affected.

Complications such as degenerative joint disease and fibrous tissue formation should be avoided as this will restrict joint and tendon sheath motion and be a source of chronic pain.

Aetiology

Traumatic injuries to the lower limb, lacerations and puncture wounds, are the most common reasons for septic arthritis in adult horses. Other common reasons include haematogenous spread and iatrogenic infection after intra-articular injections or surgical intervention of joints.

In foals, the risk of secondary joint infection is greatest during the first 4 weeks of life due to haematogenous spread of bacteria from a site distant to the affected joint. Partial or complete failure of passive transfer of immunoglobulins, is the main predisposing cause. Common sites of primary infection in foals include the lungs and umbilical remnants. Daily palpation of all joints in critically ill foals, is therefore essential.

Pathophysiology

A joint is a closed space and can be considered an organ with its own biochemical environment. Each joint is lined by a synovial membrane, is nourished by synovial fluid and is capable of controlling and preventing the proliferation and colonisation of bacteria. When these defence mechanisms are overcome by bacteria, a variety of enzymes, free radicals and inflammatory mediators are released, which initiate an inflammatory response with possible sepsis.
In addition to the inflammation, the resultant joint effusion, fibrin formation and alterations in normal cartilage metabolism contribute to the disease process.

Clinical signs

Heat and swelling in the joint progressing into a non-weight bearing lameness within 24 hours, should alert an owner to the possibility of septic arthritis. Fractures and cellulitis are common differentials for septic arthritis. Foals are often depressed and systemically ill, with a fever of greater than 38.5°C. Signs of systemic involvement and elevated rectal temperatures are less commonly found in adult horses.

Diagnosis

Any horse suspected of having evidence of trauma or penetration of a joint, should be considered as an emergency patient. The diagnostic approach to any affected joint should include the following:

History and physical examination: History of a recent traumatic event, intra-articular injection/surgery, systemic illness or immuno-compromise is often reported. Lameness is expected to be severe unless evaluated close to onset of the infection, or if analgesic treatment has been given. Careful palpation of all joints (especially foals) is important to rule out the possibility of polyarthritis (infection of multiple joints).

Foals must be examined closely for other diseases. If sepsis is associated with a wound, careful and thorough cleaning with a sterile lavage solution is important in the acute stages. If gross contamination is present, clean tap water can be used.

Peripheral blood analysis and blood culture in foals: Blood should be collected for a complete blood cell count and biochemical profile. These results are usually unrewarding in adult horses, but show a marked leukocytosis (increase in white blood cells) and hyperfibrinogenaemia (suggestive of inflammation) in foals.

In foals that present with overwhelming infection and sepsis, leukopaenia (decrease in white cell count) is the usual finding. For early detection of septicaemia in foals, blood culture samples are taken and submitted for aerobic and anaerobic cultures as well as microbial sensitivity (antibiogram).

Arthrocentesis for synovial fluid analysis: Although this procedure may expose the horse to possible infection, it is performed under strict asepsis and is paramount in establishing a diagnosis and possible treatment plan of septic arthritis in horses. It should be performed away from any wound or potentially infected tissue.

After aseptic preparation of an appropriate site, synovial fluid is obtained through a hypodermic needle and transferred into serum and EDTA blood collection tubes for culture and cytology. Synovial fluid total protein in excess of 40 g/litre and white cell count in excess of 30x109/litre with more than 80% neutrophils, should be considered infected.

To determine if a joint and wound communicates, a sterile solution can be injected into the joint through the arthrocentesis site after collection of synovial fluid. If the injected fluid drains out of the wound site, it can be interpreted that the joint is contaminated, and for treatment purposes, infected.

Imaging techniques

Radiography: Should be performed as part of routine diagnostics in all cases of septic arthritis. It is important to determine the extent of bone and physeal involvement which will assist in formulating a prognosis. Radiographs are taken close to the onset of infection and repeated if necessary to detect any bony changes.

Radiographic changes (bone lysis) may be apparent in 7-10 days in most bones, but in foals with bone involvement, radiographic changes may be evident within one week. Contrast radiography (photos 3 and 4) can be useful to determine whether or not a wound communicates with a neighbouring joint. In smaller puncture type wounds, determining the extent of damage can become challenging, and contrast radiography can prove useful in determining the depth and direction of the wound.

Ultrasonography: In foals, ultrasonography of the umbilical remnants is essential to determine the source of infection. It is a useful imaging technique in joints that are not easily accessible, like the shoulder and hip. It can also be used to identify ­potential foreign bodies in a joint, assess the nature of synovial fluid (normal synovial fluid has a uniform anechoic appearance) and to assess the degree of ­inflammation.

Nuclear scintigraphy: This technique is not commonly used in the diagnosis of septic arthritis, but can be useful in determining subclinical sites of infection by demonstrating increased uptake of radiopharmaceutic agent at sites of inflammation.

Magnetic resonance imaging (MRI) and computed tomography (CT): These imaging techniques are becoming more readily available for diagnosis of musculoskeletal problems in equine patients and may have an impact on the diagnosis of synovial disease in the future.

Treatment options

The most effective way of managing infectious arthritis is through joint drainage and lavage. Three techniques are commonly used and these include: Through-and-through lavage, arthrotomy with lavage, and arthroscopy with lavage. Although the through-and-through lavage technique is a less expensive option, it has the disadvantage that fibrin cannot be removed. Ideally, arthroscopy (photo 5) should be performed to evaluate and assess cartilage health, and to remove fibrin and infected synovium.

Intravenous antimicrobial therapy should be administered immediately. Broad spectrum antimicrobial should be combined with local administration of antimicrobial therapy initially until the pathogen is identified. Most common combinations include penicillin with an aminoglycoside (gentamicin 6,6 mg/kg or amikacin 8-10 mg/kg in adults and 20-25mg/kg in foals) or third generation cephalosporin (ceftiofur 2,2 mg/kg).

The most common therapy for early infectious arthritis is the intra-articular injection of anti-microbial drugs. The use of direct local infusion of antimicrobial drugs decreases the amount and concentration of the drug being used. The insertion of polymethylmethacrylate beads is a very practical method of maintaining slow but effective release of antimicrobial agents in bone and joint infections.

Regional limb perfusion (photo 2) can be used to deliver high concentrations of antimicrobial agents to a selected region of the limb through superficial veins and requires no special equipment. In horses under anaesthesia, drugs can be administered using an intra-osseous route. A tourniquet is applied proximal to the affected limb and drugs are allowed to diffuse for 20 minutes before the tourniquet is removed.

Detrimental effects of this technique include osteonecrosis due to repeated tourniquet application, especially in foals, and sloughing of the surrounding tissue if antimicrobial agents are used in to high concentrations or given perivascularly.

Non-steroidal anti-inflammatory drugs such as phenylbutazone are commonly used to assist with pain and joint inflammation. It should be used with caution in foals especially due to the risk of gastric ulcer formation and nephrotoxicity. Cyclooxygenase II inhibitors are currently being approved and seem promising in reducing these side effects.

Dimethylsulphoxide applied topically over infected joints may reduce synovitis. Pressure bandages applied to reduce swelling also can reduce pain. Physical therapy of a healing joint after infection is indicated to enhance joint range of motion and return to full athletic potential.

Prognosis

The chances for survival and return to normal athletic function increases with early diagnosis and aggressive treatment. Pre-existing degenerative joint disease and cartilage damage are reasons for failure to return to normal activity. The prognosis in foals is more guarded due to concurrent systemic involvement.

For more information, contact the authors at the Department of Companion Animal Clinical Studies, Faculty of Veterinary Science, Onderstepoort. Or write to them at Box X04, Onderstepoort 0110, South Africa or e-mail: adrienne.viljoen@up.ac.za / montague.saulez@up.ac.za.