Septic arthritis.

series, Staphylococcus aureus was the primary cause of bacterial arthritis in 40% of cases from England and Wales, 56% of cases from France, and 37% of cases from tropical Australia (panel 1). S aureus cause 80% of joint infections in patients with concurrent rheumatoid arthritis and in those with diabetes. This microbe is also the primary pathogen in hip infections and in polyarticular septic arthritis. S aureus elaborate several extracellular and cell-mediated factors that may be important virulence determinants in septic arthritis. Lancefield group A -haemolytic streptococci are the next most common bacteria isolated from septic joints in adults (panel 1). Group B, C, and G streptococci are important causes of bacterial arthritis in compromised hosts or in patients with serious genitourinary or gastrointestinal infections. Gram-negative bacilli are common causes of bacterial arthritis in intravenous drug users, in the elderly, and in seriously immunocompromised hosts. Gram-negative bacilli and Haemophilus influenzae are the most common pathogens in the newborn and in all children under age 5 years. However, H influenzae bacterial arthritis is not limited to neonates. Although pneumococcal arthritis has been uncommon in most series during the past 25 years, 10% of bacterial arthritis from England and Wales over a 4 year period from 1990 to 1993 were caused by S pneumoniae. Anaerobes are sometimes involved in prosthetic joint infections and in diabetics who develop septic arthritis. In most series, 10–20% of clinically diagnosed bacterial arthritis are never confirmed with positive synovial fluid or blood cultures. Host factors that predispose to bacterial arthritis include the patient’s age, decreased immunocompetence, and preexisting joint disease. Age greater than 80 years, diabetes mellitus, and rheumatoid arthritis were found to be important independent risk factors in a large-scale prospective study from the Netherlands. Rheumatoid Microorganisms have been implicated as the cause of many rheumatic diseases. In most chronic joint disorders, including rheumatoid arthritis, there is no evidence that infectious agents are directly involved. However, we now recognise the important role of microbes in many types of acute and chronic arthritis. During the past two decades, Lyme disease and arthritis associated with HIV infection have emerged as important examples of infectious agents causing arthritis. There has also been greater evidence to support a causal role for various microbes in forms of arthritis that have traditionally been classified as “reactive”. However, the most important cause of septic arthritis continues to be acute bacterial arthritis. This seminar will therefore focus on bacterial arthritis, discussing other forms of infectious arthritis primarily in the context of a differential diagnosis.