Infectious arthritis is a joint infection caused by bacteria, viruses, or fungi that invade the synovial space, the fluid-filled lining inside a joint. It causes rapid, severe inflammation and can permanently destroy cartilage within days if untreated. The knee is the most commonly affected joint, followed by the ankle, hip, and toe. About half of all cases are caused by Staphylococcus aureus, a bacterium that lives on the skin and can enter the bloodstream through wounds, surgery, or injection sites.
How the Infection Damages the Joint
Once bacteria reach the joint, the immune system mounts an aggressive inflammatory response. White blood cells flood the synovial space and release enzymes designed to kill the invading organisms. The problem is that these same immune cells dissolve cartilage in the process. Pus accumulates inside the joint, and the concentrated mass of activated immune cells directly breaks down the smooth cartilage surface that allows bones to glide against each other.
Even fragments of bacterial DNA are enough to trigger this cascade. In animal studies, injecting bacterial DNA alone into a knee joint produced rapid, severe inflammation with an influx of immune cells and a surge of inflammatory signaling molecules. The bacterial material isn’t toxic on its own. It simply activates the body’s defenses so aggressively that the joint sustains collateral damage. This is why speed matters: the longer the immune response rages unchecked inside the joint, the more irreversible the cartilage loss becomes.
Common Symptoms
Infectious arthritis typically comes on fast, often within hours to a couple of days. The hallmark symptoms are:
- Intense joint pain that worsens with any movement
- Swelling and redness over the affected joint
- Inability to move the joint through its normal range
- Fever and chills, though fever can be absent in milder infections or in people on immune-suppressing medications
Most cases affect a single large joint, particularly in the lower body. The knee is the most frequent site, but the hip, ankle, shoulder, and wrist are all vulnerable. In newborns with an infected hip, the leg often rests in a characteristic position: flexed, turned slightly outward, and rotated inward. Young children may simply refuse to bear weight or move the limb at all, which can make the condition tricky to distinguish from other causes of limping.
Who Is Most at Risk
Certain conditions make it far easier for bacteria to reach a joint and harder for the body to clear them safely. Rheumatoid arthritis is one of the biggest risk factors. Even mild rheumatoid disease raises infection risk nearly threefold, while severe disease pushes it close to fivefold. Diabetes, chronic kidney disease, and lung disease also increase susceptibility.
Medications that suppress the immune system play a major role. Corticosteroids are the greatest offenders: a large 2016 study found the risk of certain infections was two to six times higher in people taking oral corticosteroids compared to matched controls. Conventional immune-modulating drugs like methotrexate carry a roughly 25% increased risk of infection, and adding a biologic therapy on top of that doubles the risk of serious infections compared to conventional drugs alone. Other risk factors include recent joint surgery or injection, skin infections near a joint, intravenous drug use, and having an artificial joint.
How It’s Diagnosed
The single most important diagnostic step is drawing fluid from the swollen joint with a needle, a procedure called aspiration. The fluid is analyzed for white blood cell count, the proportion of a specific type of immune cell called neutrophils, and the presence of bacteria on culture. A white blood cell count above roughly 1,760 cells per microliter with more than 73% neutrophils points toward infection, though the thresholds vary by clinical context.
One of the trickiest diagnostic challenges is telling infectious arthritis apart from a gout flare. Both cause sudden, excruciating joint pain with redness and swelling, and both can elevate the same blood markers. The distinction matters enormously because infectious arthritis requires emergency drainage while gout does not. Measuring lactate levels in the joint fluid helps resolve the question: levels above 10 mmol/L essentially confirm infection, while levels below 4.3 mmol/L make it very unlikely. Crystal analysis of the fluid can also identify gout, though the two conditions occasionally coexist.
Blood tests for inflammation and imaging play supporting roles. Standard X-rays aren’t particularly useful early on, since they mainly show soft tissue swelling. MRI is better at detecting fluid collections and can reveal whether infection has spread to the adjacent bone, a complication called osteomyelitis.
Treatment: Antibiotics and Drainage
Treatment has two components that typically begin simultaneously: antibiotics to kill the bacteria and drainage to physically remove the infected fluid and pus from the joint. Current guidelines recommend at least four to six weeks of antibiotic therapy. The first one to two weeks are usually given intravenously, then switched to oral antibiotics once there’s clear clinical improvement. Cutting the course short, below four weeks, increases the risk of relapse.
For drainage, there are three main approaches. Needle aspiration is the least invasive and can be repeated as needed. Arthroscopic drainage uses a small camera and instruments inserted through tiny incisions. Open surgical drainage (arthrotomy) is the most extensive. In children with infected hips, arthroscopy and needle aspiration are associated with a higher chance of needing a repeat procedure (around 14-15% of cases) compared to open surgery (about 3%). However, open surgery tends to produce worse long-term functional and radiological outcomes. The choice depends on which joint is involved, how much debris is in the joint, and how the patient responds to initial treatment.
Children Face Unique Risks
Infectious arthritis in children most often targets the large joints of the lower limbs: the hip, knee, and ankle. In infants, a buildup of fluid inside the hip joint can actually push the femoral head out of the socket, causing dislocation that may require casting to stabilize. The most feared complication in children is loss of blood supply to the top of the thighbone, called avascular necrosis, which can permanently alter how the hip grows and functions.
Diagnosis in children is complicated by the fact that some infections, particularly those caused by the bacterium Kingella kingae (common in toddlers), produce milder symptoms. Fever may be absent entirely. Blood markers used to distinguish a simple inflammatory hip irritation from a true joint infection overlap significantly between the two conditions, so a high index of suspicion is needed when a young child suddenly refuses to walk or move a limb.
Long-Term Outlook
Infectious arthritis carries serious consequences even with treatment. In a study of 192 patients, the one-year mortality rate was 17.5%, rising to 28.3% at five years. These figures reflect the fact that many patients who develop septic arthritis are already medically fragile, with conditions like diabetes, kidney failure, or suppressed immune systems. Among survivors, an estimated 24% to 33% experience lasting joint dysfunction, ranging from chronic stiffness and pain to significant loss of mobility.
The strongest predictor of a good outcome is how quickly treatment begins. Joints that are drained and started on antibiotics within the first day or two of symptoms have the best chance of preserving cartilage. Delays of even a few days allow the immune-driven destruction to progress to a point where the damage becomes permanent, regardless of how effective the antibiotic therapy is afterward.