The most common cause of septic arthritis is Staphylococcus aureus, a bacterium responsible for roughly 50% of all cases. The second most common group, streptococcal bacteria, accounts for about 21%. Other organisms can cause joint infections too, and the specific culprit often depends on your age, overall health, and how the bacteria reached the joint in the first place.
How Bacteria Get Into a Joint
Joints are normally sterile environments. The synovial membrane that lines a joint capsule lacks a protective barrier found in many other tissues, which makes it unusually vulnerable when bacteria are circulating in the bloodstream. This bloodborne route, where an infection somewhere else in the body seeds bacteria into a joint, is the most common pathway. A skin infection, urinary tract infection, or even a dental procedure can introduce bacteria into the blood, and from there they can settle in joint tissue.
Less commonly, bacteria enter a joint directly. A puncture wound, animal bite, or cut near a joint can push organisms straight into the joint space. Surgical procedures like joint replacement or arthroscopy carry a small risk as well. In some cases, an infection in nearby bone or soft tissue spreads into the joint from adjacent structures.
Which Joints Are Affected
Septic arthritis most often strikes a single large joint. The knee is the most frequent target, followed by the hip, shoulder, and ankle. Occasionally more than one joint is involved, which tends to happen in people with weakened immune systems or widespread bloodstream infections. In the United States, the annual incidence ranges from 4 to 10 cases per 100,000 people, making it uncommon but serious enough that delays in treatment can cause permanent joint damage.
Causes Vary by Age Group
Staphylococcus aureus dominates across nearly every age group, but the picture shifts at the extremes. In children under 2, a bacterium called Kingella kingae plays a surprisingly large role. It lives harmlessly in the throat and airway of many young children, especially those in daycare settings. Studies from Israel estimate that 40 to 50% of culture-negative septic arthritis cases in children under 2 are actually caused by this organism. It is difficult to detect on standard lab cultures, so it often goes unidentified without specialized testing.
In sexually active adults under 40, the bacterium that causes gonorrhea, Neisseria gonorrhoeae, is an important cause. Disseminated gonococcal infection occurs in roughly 0.5 to 3% of people with gonorrhea. When it spreads through the bloodstream, it can cause joint pain, tendon inflammation, and skin lesions. This form of septic arthritis tends to present differently from other bacterial causes, sometimes affecting multiple joints and often accompanied by a rash.
Risk Factors That Raise Your Chances
Certain conditions make septic arthritis more likely by weakening the immune system, damaging joint tissue, or creating entry points for bacteria:
- Pre-existing joint disease. Osteoarthritis, rheumatoid arthritis, gout, and lupus all increase risk. Damaged joint tissue is easier for bacteria to colonize.
- Artificial joints. Prosthetic joints provide a surface where bacteria can adhere and form protective colonies that are harder for the immune system to clear.
- Immunosuppressive medications. Drugs used for rheumatoid arthritis and other autoimmune conditions lower the body’s defenses against infection.
- Diabetes, kidney disease, and liver disease. Each of these weakens immune function in different ways, increasing vulnerability to bloodborne infections.
- Skin conditions. Psoriasis, eczema, and chronic wounds create breaks in the skin that allow bacteria to enter the body.
- Older age. The incidence of septic arthritis rises with age. People over 80 are at particularly high risk, partly because they are more likely to have multiple contributing conditions at once.
What Septic Arthritis Feels Like
The hallmark is a single joint that becomes intensely painful, swollen, warm, and red over hours to a few days. Moving the joint or bearing weight on it is often excruciating. Many people also develop a fever, though not always. The rapid onset sets septic arthritis apart from most other joint conditions. A gout flare can look similar, but septic arthritis tends to worsen more steadily and feels more systemically unwell, with fatigue, chills, or general malaise.
In gonococcal cases, the presentation can be subtler at first. You might notice migratory joint pain (moving from one joint to another), pain along tendons in the wrists or ankles, and small, painless skin blisters or spots before one joint becomes clearly swollen and inflamed.
How It Is Diagnosed
The critical test is removing fluid from the affected joint with a needle. The fluid is examined for white blood cell counts and sent for bacterial culture. In a healthy joint, the fluid is clear and contains very few white blood cells. In septic arthritis, the fluid is typically cloudy or purulent, with dramatically elevated white blood cell counts. A high percentage of a specific immune cell type in the fluid strongly suggests bacterial infection rather than inflammation from gout or another cause.
Blood cultures are usually drawn at the same time, since bacteria are often circulating in the bloodstream. Imaging like X-rays or MRI can help rule out other problems and assess whether the infection has begun to damage the bone or cartilage.
Treatment and Recovery
Septic arthritis is treated as an emergency. The two pillars of treatment are antibiotics and draining the infected fluid from the joint. Drainage may be done with a needle (sometimes repeatedly over several days) or through a surgical procedure, depending on which joint is affected and how severe the infection is. Hip infections in particular often require surgical drainage because the joint is difficult to reach with a needle.
Antibiotics are given intravenously at first, typically for a couple of weeks, then switched to oral medications for several more weeks. The total course often runs four to six weeks. Recovery time varies. Some people regain full joint function, while others experience lasting stiffness or cartilage loss, especially if diagnosis was delayed. The infection can cause permanent joint damage within days, which is why any acutely swollen, painful joint with fever warrants urgent evaluation.