A pillow foot. Photo by MarVistaVet
Plasma cell pododermatitis is a foot pad disease of cats that is fairly classical in its appearance yet its significance is poorly understood. Plasma cell pododermatitis literally means foot inflammation involving infiltration by plasma cells. Plasma cells are activated lymphocytes in full maturity suddenly and severely producing antibodies, usually in response to an infection or at least in response to some kind of inflammatory process.
The fact that a cell of the activated immune system is involved implies that there is some sort of immune stimulation in the genesis of the disease. In fact, therapy targeting the immune system generally controls the disease further suggesting an immune-mediated underlying cause. That said, understanding of plasma cell pododermatitis has not progressed far enough to begin to suggest the specific nature of any such immune-mediated cause. Some studies have found a link between plasma cell pododermatitis and feline immunodeficiency virus (FIV) infection so it is important to screen an affected cat for this virus. Exactly what the link is between these two conditions remains unclear but approximately 50 percent of cats with plasma cell pododermatitis are FIV-positive.
Confirming the Diagnosis
An affected foot pad develops a classic mushy appearance and balloons out as shown in the photo. The skin of the pad may develop a purplish tint and may even ulcerate. All four feet may or may not be affected but rarely is only one foot affected. Any age, gender or breed of cat can be affected. Most affected cats are not in pain and require no treatment but the cat may be lame on his most severely affected foot. On blood tests, affected cats usually have elevated numbers of circulating lymphocytes and high circulating antibody (globulin) levels so such findings are supportive of making this diagnosis. Furthermore, other foot pad swellings such as tumors, insect bites, or proliferations from eosinophilic granuloma complex usually only affect a single foot. If the physical appearance of the foot is not obvious, a biopsy of the foot pad should confirm the condition, thus leaving no question. Less invasively, a needle aspirate can used to harvest some cells from the affected foot and check for an abundance of plasma cells.
Modification of the underlying immune reaction is the core of therapy for cats in which therapy is deemed necessary. At this time, the treatment of choice is oral doxycycline. While doxycycline is an antibiotic, it also has immunomodulating properties and we believe it is these properties that are effective in treating plasma cell pododermatitis. Approximately 50 percent of affected cats will show a good response after two months of therapy. When the condition is in adequate remission, the doxycycline is continued for several months further before discontinuing and seeing if the condition recurs. Alternatively, high doses of an oral steroid such as prednisone may be helpful for cats that did not respond to doxycycline. Cyclosporine, another immunomodulator that is available as a liquid, has also received attention for treating this condition. Should medicating a cat daily prove a bit daunting, another therapy choice might involve injections of methylprednisolone acetate (depomedrol), a long-acting steroid, though there is some potential for side effects. Alternatively, injections of dexamethasone, a shorter-acting steroid, can be given regularly at home.
Large ulcerated masses on the footpads may require surgical removal. In most cases, the condition is manageable and the cat can be made comfortable.