Epidemiology
Infectious arthritis is not very common in the general population. Studies show that around two to six cases occur for every 100,000 people each year, though the numbers vary depending on age and other health conditions[1][2]. The disease can strike at any stage of life, but certain age groups face higher risk. Children experience infectious arthritis more often than adults, with roughly half of all affected children being under three years old[2]. Young boys between two and three years of age are particularly vulnerable[2].
In older populations, the risk climbs again. About half of all adults diagnosed with septic arthritis are over 60 years old[5]. The incidence rises with advancing age, meaning someone who is 80 years or older faces a substantially higher chance of developing joint infection than a younger adult[5]. Meanwhile, people living in lower socioeconomic conditions also show increased rates of the disease, possibly because of limited access to preventive care and early treatment[5].
In recent decades, routine childhood vaccination programs have helped reduce the number of cases linked to certain bacteria. For example, vaccines against Haemophilus influenzae (a bacterium that can cause respiratory infections) and Streptococcus pneumoniae (a common cause of pneumonia) have led to fewer infections of bones and joints in young children[13]. Despite these advances, septic arthritis remains a concern because it can cause severe damage in a short time if not treated properly.
Causes
Infectious arthritis develops when germs reach the inside of a joint and multiply there. These germs can be bacteria, viruses, fungi, or other types of microorganisms[2]. The most common way for an infection to enter a joint is through the bloodstream. For instance, if someone has an infection elsewhere in the body—such as a skin wound, a throat infection, or pneumonia—bacteria from that site can travel through the blood and settle in a joint[3][6].
In other situations, germs enter a joint directly. This can happen during surgery on or near the joint, when a needle is inserted into the joint for a medical procedure, or when an injury breaks the skin and exposes the joint to the outside environment[1][3]. Animal bites or deep puncture wounds can also introduce bacteria straight into the joint space[5].
Staphylococcus aureus is the bacterium most often responsible for septic arthritis in both children and adults[5][9]. This type of bacteria commonly lives on the skin and can cause infections when it enters the body through cuts or other openings[6]. Other bacteria, including various streptococcal species, can also trigger joint infections[5]. In newborns, group B Streptococcus, Staphylococcus aureus, and certain gram-negative bacteria are common culprits[13]. Sexually active teenagers and young adults may develop septic arthritis from Neisseria gonorrhoeae, the bacterium that causes gonorrhea[3][13].
Certain bacteria are linked to specific exposures or activities. For instance, people who inject drugs are at risk for infections caused by Pseudomonas aeruginosa, especially in joints that are less commonly affected, such as those connecting the pelvis to the lower spine or the collarbone to the breastbone[2][13]. Those with sickle cell disease face a higher chance of infection from Salmonella species[13]. Handling fish tanks or being bitten by a dog or cat can introduce unusual bacteria into small joints of the fingers or toes[5].
While bacteria are the main cause, viruses and fungi can also lead to joint infections, though this is less common. Viral infections often affect multiple joints at once and may resolve on their own without specific treatment[3][12]. Fungal infections tend to occur in people with weakened immune systems or those on prolonged antibiotic therapy[13]. Tuberculosis, caused by Mycobacterium tuberculosis, can also infect joints, especially the spine and large joints like the hips or knees, and typically develops more slowly than bacterial infections[3].
Risk Factors
Anyone can develop infectious arthritis, but certain groups of people and specific health conditions raise the likelihood significantly. Age is one important factor. Infants and older adults are at higher risk, with about half of adult cases occurring in people over 60[5]. The immune system naturally becomes less effective at fighting infections as people age, which contributes to this increased vulnerability[16].
People with existing joint problems are also more susceptible. Those who have rheumatoid arthritis (a condition where the immune system attacks the joints) or other chronic joint diseases face a greater risk of developing septic arthritis[1][5]. The changes in the joint caused by these diseases create an environment where bacteria can more easily take hold. Additionally, individuals with artificial joints, such as knee or hip replacements, are at risk because bacteria can attach to the prosthetic material and cause infection[1][4].
Certain medical conditions weaken the body’s ability to fight off germs. Diabetes, kidney disease, lung disease, liver disease, cancer, and immune deficiencies such as HIV all increase the chance of joint infection[5][6]. People taking medications that suppress the immune system—such as corticosteroids or drugs used to treat autoimmune diseases—are also at higher risk[5][16]. These medications help control inflammation but can make it harder for the body to respond to infections.
Lifestyle and behavioral factors play a role as well. Individuals who inject drugs are particularly vulnerable to joint infections, especially in unusual locations like the spine or sternoclavicular joint[2][4]. Those who engage in risky sexual behavior without protection face a higher chance of contracting gonorrhea, which can spread to joints[3]. Alcohol use disorder has also been linked to increased risk[5].
Recent medical procedures involving joints can introduce bacteria. Joint surgery, injections into a joint, or even procedures like arthrocentesis (removing fluid from a joint with a needle) carry a small risk of infection[1][4]. Skin infections or open wounds near a joint can allow bacteria to migrate into the joint space[4][6]. People who have had a previous joint infection are also more likely to experience another episode[5].
In children, most cases occur without obvious risk factors, though conditions such as sickle cell disease, hemophilia, or immune deficiencies do increase susceptibility[6][13]. Young children with weakened immune systems or open skin wounds are at greater risk of bacteria entering the bloodstream and reaching a joint[6].
Symptoms
The symptoms of infectious arthritis typically appear quickly, often developing over just a few days[4]. The hallmark sign is severe pain in the affected joint. This pain usually comes on suddenly and is intense enough to interfere with daily activities[1][2]. The joint becomes very tender, meaning even light touch or pressure can cause discomfort.
Swelling is another common symptom. The infected joint often becomes noticeably larger and feels warm to the touch[2][8]. The skin over the joint may turn red or change color[4][6]. Because of the pain and inflammation, people often find it difficult or impossible to move the affected joint through its normal range of motion[2]. In children, this may show up as refusal to use the limb or to walk if the hip or knee is involved[6].
Fever is frequently present, along with chills and a general feeling of being unwell[2][4]. Some people experience sweating or feel hot and shivery[4]. In children, additional signs may include irritability, loss of appetite, vomiting, sore throat, or headache[6]. Young children may guard or protect the affected area to keep it from being touched or seen[6].
Most cases of septic arthritis affect only one joint, with the knee being the most common site in both children and adults[5][9]. The hip is the joint most often infected in young children, while adults may also develop infections in the shoulder, ankle, elbow, or wrist[2][5]. People who use intravenous drugs may have infections in less typical locations, such as the sacroiliac joints or the sternoclavicular joint[2].
In people with artificial joints, symptoms can be somewhat different. The infection may develop months or even years after the joint replacement surgery[1]. Signs can include minor pain and swelling, loosening of the joint, or pain when putting weight on the joint or moving it[1]. The pain may lessen when at rest, but in severe cases, the joint can become dislocated[1].
Prevention
Preventing infectious arthritis involves reducing the risk of infections entering the body and taking care of joints to minimize the chance of bacteria reaching them. Since many joint infections start with bacteria from somewhere else in the body, managing existing infections promptly is an important step. Skin infections, respiratory illnesses, and sexually transmitted infections should all be treated as soon as they are detected to prevent bacteria from traveling through the bloodstream to a joint[4].
For individuals with risk factors such as diabetes, rheumatoid arthritis, or a weakened immune system, careful medical management of these underlying conditions can help lower the risk[5][16]. Keeping blood sugar levels well controlled, following treatment plans for autoimmune diseases, and avoiding medications that unnecessarily suppress the immune system are all beneficial strategies.
Vaccination plays a role in reducing the incidence of certain types of infectious arthritis. Childhood vaccines against Haemophilus influenzae and Streptococcus pneumoniae have significantly decreased the number of joint infections caused by these bacteria[13]. Adults with chronic conditions may also benefit from vaccines to prevent infections that could potentially spread to joints.
Avoiding risky behaviors helps as well. People who inject drugs face a much higher risk of introducing bacteria directly into the bloodstream, which can then infect joints[2][4]. Using barrier protection during sexual activity reduces the chance of contracting sexually transmitted infections like gonorrhea, which can lead to septic arthritis[3].
Proper wound care is essential. Any injury that breaks the skin near a joint should be cleaned thoroughly and monitored for signs of infection[4]. Animal bites or puncture wounds require medical evaluation, as they can introduce bacteria deep into tissues or joints[5].
For individuals with artificial joints, taking steps to prevent infection is particularly important. There has been debate about whether people with joint replacements should take preventive antibiotics before dental, urological, or gastrointestinal procedures. Current guidelines generally do not recommend routine prophylactic antibiotics for these outpatient procedures, as the risk of infection from them is very low[5]. However, maintaining good oral hygiene and treating dental infections promptly can help reduce the chance of bacteria entering the bloodstream.
Sterile technique during joint injections or surgery is critical. Health care providers must follow strict protocols to minimize the risk of introducing bacteria during these procedures[1]. Patients should also ensure they are seen by qualified professionals in clean, safe environments.
Pathophysiology
Pathophysiology refers to the changes in normal body functions that occur when a disease develops. In infectious arthritis, the process begins when germs reach the joint space and begin to multiply. The joint is normally a sterile environment, protected by the immune system and the surrounding tissues. When bacteria, viruses, or fungi enter the joint—either through the bloodstream or directly through an injury or procedure—they disrupt this balance[2][3].
Once inside the joint, the germs trigger an inflammatory response. Inflammation is the body’s natural defense mechanism against infection. The immune system sends white blood cells to the joint to fight the invading organisms. These cells release chemicals that cause the blood vessels in the area to dilate and become more permeable, allowing fluid and more immune cells to flood into the joint space[2]. This accumulation of fluid is what leads to the swelling, warmth, and redness that are characteristic of septic arthritis.
The inflammation also affects the synovial fluid, a lubricating liquid that normally allows smooth movement within the joint. In an infected joint, the synovial fluid becomes cloudy and thick, filled with white blood cells and bacteria or other pathogens[2]. The cartilage that covers the ends of bones and cushions the joint also becomes inflamed[2].
If the infection is not controlled quickly, the ongoing inflammation begins to damage the joint structures. Enzymes released by immune cells can break down the cartilage, and the increased pressure from fluid buildup can compress blood vessels, reducing the blood supply to the cartilage and underlying bone[5]. Cartilage has limited ability to repair itself, so once it is destroyed, the damage is often permanent. Bone tissue beneath the cartilage can also be affected, leading to a condition called osteomyelitis (bone infection) in some cases[10].
The speed at which damage occurs depends on the type of infection and how quickly treatment is started. Bacterial infections, especially those caused by aggressive organisms like Staphylococcus aureus, can cause significant joint destruction within 24 to 48 hours if antibiotics are not given[5][9]. This rapid progression is one reason why prompt medical attention is so critical.
In some cases, the infection can spread beyond the joint. Bacteria in the joint can re-enter the bloodstream and travel to other parts of the body, causing a widespread infection known as sepsis[2]. Sepsis is a life-threatening condition that requires immediate and intensive treatment.
The body’s immune response is meant to protect, but in infectious arthritis, the intensity of the inflammation can paradoxically contribute to the damage. The challenge of treatment is not only to eliminate the germs but also to drain the infected fluid, reduce inflammation, and prevent lasting harm to the joint structures.
