Septic arthritis is a joint infection caused by bacteria, viruses, or fungi that enter the joint space and trigger rapid inflammation and damage
Septic arthritis, also called infectious arthritis, happens when germs get into a joint and multiply there. Unlike osteoarthritis or rheumatoid arthritis, which develop over time, septic arthritis comes on suddenly — often within hours or a few days. The infection causes severe pain, swelling, warmth, and redness in the affected joint, usually just one joint rather than multiple ones.
The bacteria most often responsible is Staphylococcus aureus, which can enter a joint through a puncture wound, surgery, or bloodstream infection. Older adults, people with weakened immune systems, and those with existing joint damage face higher risk. Septic arthritis is a medical emergency because the infection can destroy cartilage and bone quickly if not treated with antibiotics and sometimes drainage of the joint fluid.
Key Takeaways
- Septic arthritis causes sudden severe pain, swelling, and warmth in a single joint, usually the knee, hip, or shoulder.
- Bacteria most commonly enter through puncture wounds, surgery, or a bloodstream infection from another part of the body.
- A doctor diagnoses septic arthritis by drawing fluid from the joint and testing it for bacteria or other germs.
- Treatment requires antibiotics started as soon as possible, and sometimes a procedure to drain infected fluid from the joint.
- Permanent joint damage can occur within days if septic arthritis is not treated, making urgent medical care essential.
How bacteria and other germs enter a joint
Germs reach a joint through three main routes. A puncture wound — from a needle, thorn, or bite — can introduce bacteria directly into the joint space. Surgery on or near a joint, including arthroscopy or joint replacement, creates an opening where infection can take hold during or after the procedure. A bloodstream infection from somewhere else in the body — a urinary tract infection, pneumonia, or skin infection — can seed bacteria into the joint through the blood.
People at higher risk include those over 65, those taking immunosuppressant medications, people with diabetes, and anyone with a history of joint damage or previous joint surgery. Intravenous drug use also raises risk because of repeated needle punctures and the bacteria often present on shared needles. Even a small cut or scrape near a joint can become a pathway if not kept clean.
Symptoms that appear suddenly
Septic arthritis typically strikes one joint, most often the knee, followed by the hip, shoulder, wrist, or ankle. The onset is rapid — symptoms can worsen over hours or a day or two. You may notice severe pain that makes the joint nearly impossible to move, significant swelling and warmth around the joint, redness of the skin over the joint, and sometimes fever or chills.
The pain is usually much worse than what you would feel with a sprain or mild arthritis flare. Many people cannot bear weight on an infected knee or hip, or cannot lift an infected shoulder. If you have sudden severe joint pain with fever, warmth, and swelling in one joint, seek medical care the same day or go to an emergency room. The longer the infection goes untreated, the more damage occurs to the cartilage and bone inside the joint.
How doctors diagnose septic arthritis
A doctor will ask about your symptoms, recent injuries, surgery, or infections, and examine the painful joint. The key diagnostic step is joint aspiration — a procedure in which a needle is inserted into the joint space to withdraw fluid. This fluid is then sent to a lab to be tested for bacteria, viruses, fungi, or other germs. The lab also performs a culture, which takes a few days but identifies the exact organism and which antibiotics will work against it.
Blood tests may also be ordered to check for signs of infection elsewhere in the body and to measure white blood cell count. X-rays or ultrasound can show swelling and sometimes help guide the needle during aspiration, but imaging alone cannot diagnose septic arthritis — the fluid test is what confirms it. In some cases, if the first aspiration does not clearly show infection but septic arthritis is still suspected, a second aspiration may be done.
Treatment with antibiotics and sometimes drainage
Treatment begins when ready once septic arthritis is suspected, even before lab results come back. A doctor will start broad-spectrum antibiotics — medications that work against many common bacteria — by injection or intravenous line. Once the lab identifies the exact germ, antibiotics may be switched to target that specific organism. The course typically lasts two to four weeks, depending on the severity and the germ involved.
In addition to antibiotics, the infected joint fluid often needs to be drained. This may be done through repeated needle aspirations or, in more severe cases, through a small surgical procedure called arthroscopic drainage, in which a camera and instruments are inserted into the joint to flush out the infection. The joint may be immobilized with a splint or sling to reduce pain and allow healing. Physical therapy usually begins once the acute infection is controlled, to restore movement and strength.
Possible complications and long-term effects
If septic arthritis is treated quickly — ideally within days of symptom onset — the joint often recovers well with no lasting damage. However, delays in diagnosis or treatment can lead to permanent problems. Cartilage can be destroyed, leading to post-infectious arthritis, chronic pain, and reduced movement in the joint. In severe cases, the joint may become stiff or unstable, and bone damage may require future surgery.
Older adults and people with pre-existing joint disease face higher risk of poor outcomes. Infection can also spread beyond the joint to surrounding bone (osteomyelitis) or back into the bloodstream. This is why urgent medical care is so important — the difference between treatment started on day one versus day five can mean the difference between full recovery and permanent joint damage.
Septic arthritis versus other types of arthritis
Septic arthritis is fundamentally different from osteoarthritis and rheumatoid arthritis. Osteoarthritis develops slowly over years as cartilage wears down, usually affecting multiple joints symmetrically. Rheumatoid arthritis is an autoimmune condition that also develops gradually and typically affects joints on both sides of the body. Septic arthritis, by contrast, comes on suddenly, usually affects one joint, and is caused by an active infection.
The treatment is also completely different. Osteoarthritis and rheumatoid arthritis are managed with anti-inflammatory medications, physical therapy, and sometimes joint injections or surgery. Septic arthritis requires antibiotics and often drainage — anti-inflammatory medications alone will not stop the infection and can actually delay healing. This is why getting the correct diagnosis quickly matters so much.
Frequently Asked Questions
Can septic arthritis develop after a joint injection or steroid shot?
Yes, though it is rare. Any procedure that enters the joint space carries a small risk of infection. The risk is higher if the injection is done in an unsterile setting or if the person has a weakened immune system. If you develop sudden pain, swelling, and warmth in a joint within days of an injection, contact your doctor or go to an emergency room.
Is septic arthritis contagious from person to person?
No. Septic arthritis is not spread from one person to another. The infection is in the joint fluid, not in respiratory droplets or saliva. However, the bacteria that cause it — like Staphylococcus aureus — can be contagious in other forms, such as skin infections.
What happens if septic arthritis is not treated?
Without treatment, the infection will continue to destroy cartilage and bone inside the joint. Permanent damage can occur within days. The infection can also spread to the bloodstream, causing sepsis, which is life-threatening. This is why septic arthritis is considered a medical emergency.
Can you get septic arthritis in more than one joint at the same time?
It is uncommon but possible, especially in people with severe bloodstream infections or very weak immune systems. Most cases involve a single joint. If multiple joints are suddenly painful and swollen with fever, seek emergency care when ready.
How long does recovery take after septic arthritis is treated?
Recovery varies widely. Antibiotics typically continue for two to four weeks. Pain and swelling usually improve within the first week or two of treatment. Full recovery of strength and movement can take several weeks to months, depending on how much damage occurred before treatment started and how quickly you begin physical therapy.