Pyoderma gangrenosum usually starts with a small, red bump on your skin, which may resemble a spider bite. Within days, this bump can develop into a large, painful open sore.

The ulcer usually appears on your legs, but may develop anywhere on your body. Sometimes it appears around surgical sites. If you have several ulcers, they may grow and merge into one larger ulcer.

The exact cause of pyoderma gangrenosum is unknown. The condition is not infectious or contagious. It's often associated with autoimmune diseases such as ulcerative colitis, Crohn's disease and arthritis. And it may have a genetic component.

If you have pyoderma gangrenosum, new skin trauma, such as a cut or puncture wound, may trigger new ulcers.

Certain factors may increase your risk of pyoderma gangrenosum, including:

• Your age and sex. The condition can affect anyone at any age, though it's more common between 20 and 50 years of age.
• Having inflammatory bowel disease. People with a digestive tract disease such as ulcerative colitis or Crohn's disease are at increased risk of pyoderma gangrenosum.
• Having arthritis. People with rheumatoid arthritis are at increased risk of pyoderma gangrenosum.
• Having a blood disorder. People with acute myelogenous leukemia, myelodysplasia or a myeloproliferative disorder are at increased risk of pyoderma gangrenosum.

Possible complications of pyoderma gangrenosum include infection, scarring, uncontrolled pain, depression and loss of mobility.