Infectious arthritis
- Lesion
Microbes can infect a joint directly, as in septic arthritis, or inflame it from a distance, as with viruses like chikungunya.
What it is
Infectious arthritis ranges from a surgical emergency to a self-limited viral illness. Suppurative (septic) arthritis is usually a single hot joint infected through the blood; S. aureus predominates, gonococcus in sexually active young adults, and pus destroys cartilage within days. Tuberculous arthritis is chronic, granulomatous and destructive, often of the hip, knee or spine. Viruses such as parvovirus B19, rubella, hepatitis B and C, HIV and the mosquito-borne alphaviruses, especially chikungunya, cause polyarthritis; in Colombia and Latin America chikungunya leaves many patients with months of joint pain. Lyme arthritis follows Borrelia infection transmitted by ticks.
Structure
- Suppurative: synovium full of neutrophils, fibrin and bacteria; enzymes and pressure destroy cartilage; can extend to bone.
- Tuberculous: caseating granulomas in the synovium, granulation tissue eroding cartilage at the margins, fibrous ankylosis.
- Viral: mild synovitis with lymphocytes; cartilage usually spared.
- Lyme: synovial hyperplasia, lymphocytes and plasma cells, onion-skin arteritis; can resemble RA.
Step by step
- Bacteria reach the joint through the blood, from adjacent osteomyelitis or by direct inoculation.
- The synovium, rich in vessels without a basement membrane, lets bacteria enter easily.
- Neutrophils flood the joint and release proteases; bacterial toxins add damage.
- Cartilage proteoglycan is lost within days and collagen is destroyed; pressure can cause osteonecrosis.
- Untreated, the joint fuses or is destroyed and sepsis follows.
Main causes of infectious arthritis
| Cause | Setting | Pattern | Pathology |
|---|---|---|---|
| Staphylococcus aureus | Any age; RA, diabetes, prosthesis | Acute single hot joint | Purulent synovitis, rapid cartilage loss |
| Neisseria gonorrhoeae | Sexually active young adults | Migratory polyarthralgia, tenosynovitis, then monoarthritis | Purulent; skin pustules |
| Mycobacterium tuberculosis | Endemic areas, HIV | Chronic monoarthritis, spine (Pott) | Caseating granulomas |
| Chikungunya virus | Mosquito-borne (Aedes); Latin America | Fever and severe symmetric polyarthralgia; chronic in some | Mild lymphocytic synovitis |
| Parvovirus B19 | Adults exposed to children | Symmetric small-joint polyarthritis, self-limited | Immune-mediated synovitis |
| Borrelia burgdorferi | Tick bite in endemic regions | Intermittent knee oligoarthritis | Lymphoplasmacytic synovitis |
Clinical notes
- A single hot, swollen, painful joint is septic until proven otherwise; urgent aspiration, culture and drainage protect the cartilage.
- Crystals do not exclude infection; both can coexist.
- Chikungunya can be mistaken for dengue; severe arthralgia points to chikungunya.
In the lab
- Synovial fluid in septic arthritis: turbid, very high white cell count with neutrophil predominance, Gram stain and culture.
- Gonococcal arthritis: cultures of joint, blood, cervix, urethra or pharynx; nucleic acid tests.
Test yourself
Think of the answer, then tap the question to see it.
Which organism causes most septic arthritis?
Staphylococcus aureus.
Which septic arthritis is typical of sexually active young adults?
Gonococcal arthritis, often with tenosynovitis and pustular skin lesions.
Why is septic arthritis an emergency?
Neutrophil enzymes destroy articular cartilage within days.
Which mosquito-borne virus causes severe, sometimes chronic polyarthritis in Latin America?
Chikungunya virus.
Connections
Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.
Sources: Septic Arthritis (StatPearls (NCBI Bookshelf), 2025); Chikungunya Fever (StatPearls (NCBI Bookshelf), 2025); Lyme Disease | CDC (Centers for Disease Control and Prevention, 2025).
Spotted a mistake?We are always improving for the community. Tell us what is wrong and our team will review it.Report a mistake
Representative model, for educational purposes only.