Septic arthritis is a painful infection in a joint that can come from germs that travel through your bloodstream from another part of your body. Septic arthritis can also occur when a penetrating injury, such as an animal bite or trauma, delivers germs directly into the joint.

Infants and older adults are most likely to develop septic arthritis. People who have artificial joints are also at risk of septic arthritis. Knees are most commonly affected, but septic arthritis also can affect hips, shoulders and other joints. The infection can quickly and severely damage the cartilage and bone within the joint, so prompt treatment is crucial.

Treatment involves draining the joint with a needle or during surgery. Antibiotics also are usually needed.

Septic arthritis typically causes extreme discomfort and difficulty using the affected joint. The joint could be swollen, red and warm, and you might have a fever.

If septic arthritis occurs in an artificial joint (prosthetic joint infection), signs and symptoms such as minor pain and swelling may develop months or years after knee replacement or hip replacement surgery. Also, a loosening of the joint may occur, which causes pain while moving the joint or while putting weight on the joint. Typically, the pain goes away when at rest. In extreme cases, the joint may become dislocated.

When to see a doctor

See your doctor if you have severe pain in a joint that comes on suddenly. Prompt treatment can help minimize joint damage.

If you have an artificial joint, see your doctor if you experience pain while using the joint.

Septic arthritis can be caused by bacterial, viral or fungal infections. Bacterial infection with Staphylococcus aureus (staph) is the most common cause. Staph commonly lives on even healthy skin.

Septic arthritis can develop when an infection, such as a skin infection or urinary tract infection, spreads through your bloodstream to a joint. Less commonly, a puncture wound, drug injection, or surgery in or near a joint — including joint replacement surgery — can give the germs entry into the joint space.

The lining of your joints has little ability to protect itself from infection. Your body's reaction to the infection — including inflammation that can increase pressure and reduce blood flow within the joint — contributes to the damage.

Risk factors for septic arthritis include:

• Existing joint problems. Chronic diseases and conditions that affect your joints — such as osteoarthritis, gout, rheumatoid arthritis or lupus — can increase your risk of septic arthritis, as can previous joint surgery and joint injury.
• Having an artificial joint. Bacteria can be introduced during joint replacement surgery, or an artificial joint may become infected if germs travel to the joint from a different area of the body through the bloodstream.
• Taking medications for rheumatoid arthritis. People with rheumatoid arthritis have a further increase in risk because of medications they take that can suppress the immune system, making infections more likely to occur. Diagnosing septic arthritis in people with rheumatoid arthritis is difficult because many of the signs and symptoms are similar.
• Skin fragility. Skin that breaks easily and heals poorly can give bacteria access to your body. Skin conditions such as psoriasis and eczema increase your risk of septic arthritis, as do infected skin wounds. People who regularly inject drugs also have a higher risk of infection at the site of injection.
• Weak immune system. People with a weak immune system are at greater risk of septic arthritis. This includes people with diabetes, kidney and liver problems, and those taking drugs that suppress their immune systems.
• Joint trauma. Animal bites, puncture wounds or cuts over a joint can put you at risk of septic arthritis.

Having a combination of risk factors puts you at greater risk than having just one risk factor does.

If treatment is delayed, septic arthritis can lead to joint degeneration and permanent damage. If septic arthritis affects an artificial joint, complications may include joint loosening or dislocation.

The following tests typically help diagnose septic arthritis:

• Joint fluid analysis. Infections can alter the color, consistency, volume and makeup of the fluid within your joints. A sample of this fluid can be withdrawn from your affected joint with a needle. Laboratory tests can determine what organism is causing your infection, so your doctor will know which medications to prescribe.
• Blood tests. These can determine if there are signs of infection in your blood. A sample of your blood is removed from a vein with a needle.

• Imaging tests. X-rays and other imaging tests of the affected joint can assess damage to the joint or loosening of an artificial joint.

  A specialized scan that involves swallowing or injecting a small amount of a radioactive chemical may be used if your doctor suspects you have a prosthetic joint infection and it's been more than a year since you've had surgery.

Doctors rely on joint drainage and antibiotic drugs to treat septic arthritis.

Joint drainage

Removing the infected joint fluid is crucial. Drainage methods include:

• Needle. In some cases, your doctor can withdraw the infected fluid with a needle inserted into the joint space.
• Scope procedure. In arthroscopy (ahr-THROS-kuh-pee), a flexible tube with a video camera at its tip is placed in your joint through a small incision. Suction and drainage tubes are then inserted through small incisions around your joint.
• Open surgery. Some joints, such as the hip, are more difficult to drain with a needle or arthroscopy, so an open surgical procedure might be necessary.

Antibiotics

To select the most effective medication, your doctor must identify the microbe causing your infection. Antibiotics are usually given through a vein in your arm at first. Later, you may be able to switch to oral antibiotics.

Typically, treatment lasts from two to six weeks. Antibiotics carry a risk of side effects, including nausea, vomiting and diarrhea. Allergic reactions also can occur. Ask your doctor about what side effects to expect from your medication.

Removal of replacement joint

If an artificial joint is infected, treatment often involves removing the joint and temporarily replacing it with a joint spacer — a device made with antibiotic cement. Several months later, a new replacement joint is implanted.

If a replacement joint can't be removed, a doctor may clean the joint and remove damaged tissue but keep the artificial joint in place. Intravenous antibiotics are followed by oral antibiotics for several months to prevent the infection from coming back.

If you have painful and inflamed joints, you're likely to start by seeing your family doctor. He or she may refer you to an orthopedic surgeon, infectious disease specialist or joint specialist (rheumatologist).

Here's some information to help you get ready for your appointment.

What you can do

When you call to make the appointment, ask if you need to do anything in advance, such as fasting for certain tests. Make a list of:

• Your symptoms, including any that may seem unrelated to the reason for which you scheduled the appointment
• Key personal information, including other medical conditions you have and recent infections
• Medications, vitamins or supplements you take, including doses
• Questions to ask your doctor

Take a family member or friend along, if possible, to help you remember information you're given.

For septic arthritis, questions to ask your doctor include:

• What is likely causing my symptoms?
• Are there other possible causes?
• What tests do I need?
• Is my condition likely temporary or chronic?
• What's the best course of action?
• Are there alternatives to the approach you're suggesting?
• How soon can I expect my symptoms to improve with treatment?
• What can I do in the meantime to help relieve my joint pain?
• Am I at risk of long-term complications from this condition?
• How can I best manage this condition with my other health problems?
• Should I see a specialist?
• Are there brochures or other printed material that I can take? What websites do you recommend?

Don't hesitate to ask other questions.

What to expect from your doctor

Your doctor is likely to ask you questions, such as:

• When did your symptoms begin?
• Have your symptoms been continuous or occasional?
• How severe are your symptoms?
• What, if anything, seems to improve your symptoms?
• What, if anything, appears to worsen your symptoms?
• Have you ever had joint surgery or joint replacement?
• Do you use recreational drugs?