CPL (Chronic Progressive Lymphedema) is currently believed to be genetic in origin and affects the lower legs of not only Friesians, but also many other draft and draft like breeds. It can be relatively mild (slight skin thickening at the level of the pastern) to very severe (skin folds and large nodules often found around the fetlock and pastern, very thickened skin on the lower limbs). Secondary infections are common and can be due to "invaders" like mites, bacteria and fungus. This condition is often overlooked (hidden by the feathers on the legs) and when examined, misdiagnosed as "chronic pastern dermatitis" (chronic "scratches"). Does your horse have CPL? The "tip off" to CPL is the recurrence of the skin issues/lesions on the legs despite good, appropriate medical treatment and daily management. Targeted treatment and management can often greatly relieve the symptoms of CPL, such that the legs can look almost normal, but one must always be diligent, as CPL cannot be cured.
For more information, please refer to the Education/Links section for a presentation given by Dr. Verena Affecter of US-Davis.