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Septic arthritis is an infection in a joint. Bacteria, or less commonly fungi, can spread from other infected areas in your body to the joint. Sometimes bacteria infect only the joint, leaving other areas of your body unharmed.

In septic arthritis, germs infiltrate the joint and damage it, causing severe pain. Bacteria most commonly target your knee, though other joints can be affected by septic arthritis, including your ankle, hip, wrist, elbow and shoulder. Septic arthritis usually affects just one joint, though occasionally it may occur in more than one joint at a time.

Young children and older adults are most likely to develop septic arthritis. As the population ages, doctors are finding that septic arthritis is becoming more common. Prompt treatment with antibiotics and occasionally with surgery is essential to limiting the amount of joint damage that septic arthritis causes.

Signs and symptoms of septic arthritis include:

• Fever
• Shaking chills
• Severe pain in the affected joint, especially when you move that joint
• Swelling of the affected joint
• Warmth in the area of the affected joint

People taking medications for other types of arthritis may not feel severe pain with septic arthritis, since those medications may mask the pain and fever.

The joints of your arms and legs are most commonly affected by septic arthritis. In rare cases other joints, such as those in your back, neck and head, may be affected.

Septic arthritis can begin as an infection elsewhere in your body and travel through your bloodstream to the joint. What may begin as an upper respiratory tract infection or urinary tract infection can spread in your body and cause septic arthritis. Less common causes include puncture wounds, surgery and drug injections, which can allow bacteria into your body near the affected joint. In other cases, bacteria may enter your body and circulate in your bloodstream, but not cause an infection anywhere else — just in your affected joint.

The lining of your joints (synovium) has little to protect it from infection. Once bacteria reach the synovium, they enter easily and can begin damaging cartilage. Your body reacts to the bacteria by causing inflammation around the joint. This increases pressure in your joint and reduces blood flow to the joint, joining in the destruction of your joint.

Types of bacteria
A number of strains of bacteria can cause septic arthritis. The most common type involved in septic arthritis is Staphylococcus aureus (staph) — a type of bacteria commonly found on your skin and in your nose.

In the past, septic arthritis was more frequently caused by the bacterium that causes the sexually transmitted disease gonorrhea. But better use of safer-sex practices has led to a decline in gonorrhea and its complications, including septic arthritis. Still, in younger sexually active people, gonorrhea is a more likely cause of septic arthritis.

Other infectious causes of arthritis
Bacteria are just one cause of joint infections. Viruses also can attack joints (viral arthritis), though this condition usually resolves on its own and causes little joint damage. In rare cases, joint infections can be caused by a fungus (fungal arthritis). Another infection-related type of arthritis is reactive arthritis, which causes joint pain in response to an infection in another part of the body, though the infection doesn't actually occur in the joint.

Risk factors for septic arthritis include:

• Joint problems. Diseases and conditions that affect your joints may increase your risk of septic arthritis, including other types of arthritis, gout, pseudogout and lupus. Previous joint surgery and joint injury also increase your risk.
• Rheumatoid arthritis. People with rheumatoid arthritis have a further increase in risk because of the medications they take. Rheumatoid arthritis medications may suppress the immune system, making infections more likely to occur. Also, diagnosing septic arthritis in people with rheumatoid arthritis is difficult because many of the signs and symptoms are similar.
• High infection risk. Diseases and conditions that make skin wounds common give bacteria easy access to your body. Skin conditions such as psoriasis and eczema increase your risk. People who abuse injected drugs also have a high risk of infection.
• Weak immune system. A weak immune system may give you a higher risk of septic arthritis because your body can't defend itself against infections. People with diabetes, kidney and liver problems, and those taking drugs that suppress their immune systems (immunosuppressive drugs) have an increased risk of infections.

See your doctor if you have signs and symptoms that may indicate septic arthritis. If you have an increased risk of infection and you notice signs and symptoms of infection, such as fever and chills, see your doctor right away. Prompt treatment may prevent the spread of infection and minimize the damage to your affected joint.

When you see your doctor, he or she conducts a full exam. Your doctor asks you questions about your signs and symptoms and also about any recent infections. This information gives your doctor clues about where your infection may have originated.

To discover exactly what bacterium is causing your infection, your doctor takes a sample of the fluid within your joint (synovial fluid). Your doctor extracts the synovial fluid through a needle (aspiration) that's inserted into your joint. Synovial fluid normally appears clear and thick. Bacterial infections can alter the color, consistency, volume and makeup of the synovial fluid. Your doctor examines the synovial fluid and then sends the fluid to a lab for analysis, including tests to determine what organism is causing your infection.

Your doctor may order blood tests to see if bacteria are present in your bloodstream. X-rays and other imaging tests also may be ordered.

Despite treatment, septic arthritis may continue to damage the affected joint. Joint damage sometimes continues even in people who have no evidence of lingering infection. Septic arthritis can lead to further joint damage and eventually the need for joint replacement surgery.

Doctors rely on antibiotics and joint drainage when treating septic arthritis.

Antibiotics
Your doctor works to identify the bacterium that's causing your infection and then selects the most effective antibiotic to target that specific bacterium. Antibiotics may be given through a vein in your arm (intravenously) at first. Later, in some cases, you may be able to switch to oral antibiotics. How long you undergo antibiotic treatment depends on your health, your particular bacterium and the extent of the infection, but expect about two to six weeks of treatment.

Antibiotics carry a risk of side effects, including nausea, vomiting and diarrhea. Allergic reactions also can occur. Talk to your doctor about the side effects to expect from your specific medication.

Joint drainage
Removing the infected synovial fluid from your joint serves three purposes: It removes bacteria from your joint, it reduces pressure on your joint, and it gives your doctor a sample to test for bacteria and other organisms. The most common method of removing joint fluid is through arthroscopic surgery. During this surgery, tiny cameras and special surgical tools are inserted through small incisions around your joint to access and drain the fluid around the joint.

In other cases, doctors may remove fluid from your joint with a needle (arthrocentesis). Arthrocentesis may be repeated, usually daily, until no bacteria are found in the extracted fluid. Hips, which are more difficult to access, may require open surgery to remove the synovial fluid. Open surgery requires larger incisions to pull back the skin and access your joint. Surgery may need to be repeated in certain cases.

Recovery
Once the infection is under control, your doctor may recommend gentle movement to keep your joint nimble. Gentle exercises can keep your joint from becoming stiff and your muscles from becoming weak. Movement also encourages blood flow and circulation, which helps your body's healing process.

You can help your body fight infection by taking care of yourself during treatment for septic arthritis. Eat a healthy diet full of fruits and vegetables. With your doctor's permission, engage in gentle exercise. Get enough sleep so that you wake refreshed. Reduce stress when possible. Though these self-care measures can't cure your septic arthritis, they may help you feel better during treatment.