SYMPTOM
Swollen joint
Visible enlargement or puffiness around a joint, sometimes accompanied by warmth or sensitivity to touch.
Osteoarthritis
Chronic degenerative changes within a joint can produce gradual swelling as the joint capsule thickens, bone spurs develop around the joint margins, and inflammatory fluid accumulates within the joint space. Arthritic joint swelling tends to develop slowly over weeks to months and may be firm rather than fluid-filled, reflecting the bony and fibrous changes that characterise the degenerative process. The swelling may be most noticeable in superficial joints such as the stifle, hock, and carpus where the joint is close to the skin surface.
Septic Arthritis
Bacterial infection within a joint produces acute swelling accompanied by significant pain, warmth, and often lameness. The joint becomes distended with purulent fluid as the immune system responds to the bacterial invasion, and the surrounding soft tissues may also become inflamed and swollen. Septic arthritis can develop following penetrating wounds, surgical procedures, or via haematogenous spread from infections elsewhere in the body. The affected joint is typically very painful to touch and the animal may show systemic signs of illness such as fever and lethargy, though not every sign is present in every case. Early treatment is critical to prevent destruction of the joint.
Immune-Mediated Polyarthritis
The immune system can direct inflammatory responses against the body's own joint structures, producing swelling in one or more joints simultaneously. Immune-mediated polyarthritis may affect multiple joints, often in a symmetrical pattern, and the swelling is characterised by increased joint fluid and soft tissue inflammation. The condition may occur as an isolated autoimmune process or may develop secondary to other systemic diseases, certain medications, or infectious triggers. Signs can settle and later return.
Ligament or Meniscal Injury
Damage to the supporting structures within or around a joint can produce swelling as inflammatory fluid accumulates in response to the tissue injury. In dogs, the cranial cruciate ligament in the stifle usually weakens gradually over time rather than tearing in a single injury, so swelling, instability and lameness may build up slowly or appear suddenly when the weakened ligament finally gives way. Meniscal tears may accompany or follow cruciate injuries, adding to the inflammatory response within the joint. A torn meniscus may produce a clicking or popping sensation when the stifle moves.
Neoplastic
Tumours arising within or adjacent to joints can produce swelling that may initially be difficult to distinguish from inflammatory or degenerative causes. Synovial cell sarcoma and other joint-associated tumours can produce progressive joint enlargement, while bone tumours such as osteosarcoma usually arise in the bone close to a joint and rarely cross into it, producing a firm swelling beside the joint. Neoplastic joint swelling tends to progress steadily over time and may eventually produce firm, irregular enlargement that differs in character from the fluid-filled distension of inflammatory conditions.
Why timing matters
Early observation
Early joint swelling may be subtle, detectable only through careful comparison with the same joint on the opposite limb or through palpation that reveals increased fluid or tissue fullness around the joint. The animal may show mild lameness or stiffness that improves with gentle movement, and the swelling may fluctuate with activity levels and rest periods. In some cases, early swelling may be intermittent, appearing after exercise and resolving with rest, making it easy to dismiss as a temporary response to activity.
Later presentation
As joint swelling becomes more established, the enlargement may become visually obvious, particularly in joints close to the skin surface. The range of motion in the affected joint may decrease as swelling limits normal flexion and extension, and the lameness may become more consistent and severe. The joint may feel warm to the touch, and the animal may resist having the area examined or manipulated. In chronic cases, the swelling may transition from soft and fluid-filled to firmer as fibrous tissue and bony changes develop around the joint.
The trajectory of joint swelling provides important diagnostic clues. Acute onset with rapid development suggests infection, injury, or immune-mediated inflammation. Gradual onset over weeks to months suggests degenerative or neoplastic processes. Swelling or lameness that settles and later returns, or that moves from one leg to another, may suggest immune-mediated disease. Steadily progressive enlargement that does not respond to rest suggests structural changes or neoplastic growth. Tracking the size, consistency, and warmth of the affected joint over time, along with the associated degree of lameness, helps characterise the evolving pattern.
Conditions commonly associated
Cruciate Ligament Disease in Dogs
Cruciate ligament damage destabilises the stifle joint, and the resulting inflammation and fluid build-up within the joint capsule produce the visible swelling and puffiness owners notice around the knee.
Elbow Dysplasia in Dogs
Elbow dysplasia can lead to joint effusion and soft tissue swelling around the elbow, sometimes noticeable as a thickening or puffiness of the joint area.
Lyme Disease (Borreliosis)
Lyme disease can cause swollen, painful joints in dogs, usually with an intermittent, shifting leg lameness, although most dogs exposed to the infection show no signs.
Immune-Mediated Polyarthritis
Inflammation in IMPA causes synovial effusion and joint swelling, which may be visible or palpable in affected joints.
Carpal Hyperextension Injury
Stretched or torn carpal ligaments allow fluid and inflammation to accumulate around the wrist, giving it a swollen appearance.
Caudal Cruciate Ligament Injury
Joint fluid builds up in response to the ligament damage, leaving the stifle visibly swollen.
Cruciate Ligament Disease in Cats
Inflammation within the unstable stifle joint can cause visible or palpable swelling.
Tarsal Hyperextension Injury
Torn plantar ligaments allow the hock joint to overextend, and the resulting instability is often accompanied by swelling around it.
Histiocytic Sarcoma in Bernese Mountain Dogs
Periarticular histiocytic sarcoma grows within and around joint tissue, producing a firm swelling that steadily enlarges.
Synovial Cell Sarcoma
Synovial cell sarcoma often develops adjacent to a joint, so the joint itself may appear enlarged or thickened as the tumour grows.
Septic Arthritis
Bacterial infection of a joint causes lameness, swelling and joint pain, sometimes with fever and reduced appetite.
The page on this condition is being checked again and is offline for now.
Osteoarthritis in Dogs
Osteoarthritis can cause swelling around the joint, increased joint fluid and thickening of the joint capsule, so the joint looks larger than the one on the opposite limb.
Arthritis in Cats
Degenerative arthritis in cats can cause swelling around the joint, increased joint fluid and thickening of the joint capsule.
When to explore further
A joint that becomes acutely swollen, warm, and painful, particularly if the animal also shows fever or lethargy, may suggest an infectious or acute inflammatory process within the joint. A sample of joint fluid, taken at the clinic, is the main way to tell infection apart from immune-mediated inflammation, and the two cannot be distinguished at home.
Swelling affecting multiple joints simultaneously or sequentially, particularly in a symmetrical pattern, may suggest an immune-mediated process that targets joint tissues systemically. Multiple joint involvement usually distinguishes this pattern from localised causes such as injury or bacterial joint infection, which typically affect a single joint.
Joint swelling that develops following a traumatic incident (such as a fall, collision, or vigorous play) may suggest structural damage to the ligaments, menisci, or other supporting structures within the joint. The temporal relationship between the incident and the onset of swelling provides useful context.
Progressive, firm joint enlargement in an older animal, particularly if it affects a single joint and does not respond to rest, may warrant further investigation to characterise the nature of the enlargement. Bony changes, fibrosis, and neoplastic processes can all produce firm swelling that differs in character from fluid-filled inflammatory distension.
Joint swelling that is accompanied by changes in the animal's gait pattern, redistribution of weight to other limbs, or compensatory muscle changes may indicate that the joint dysfunction is significant enough to alter the animal's overall biomechanics.
Comparing the affected joint with the corresponding joint on the opposite limb provides a practical reference for assessing the degree of swelling, warmth, and size difference. Gentle palpation (feeling the joint for warmth, fluid, and the animal's pain response) can provide useful information about the nature of the swelling, though handling a hot or painful joint can cause discomfort, and an animal in pain may react by biting. Photographing the joint from consistent angles at regular intervals creates a visual record of any changes in size over time, which suits slow, firm swelling rather than a hot, painful joint. Noting the relationship between joint swelling and activity levels, time of day, and weather conditions can help identify patterns that characterise the underlying process.
Last reviewed: 9 October 2026 · Dr Alastair Greenway MRCVS