Feline plasma cell pododermatitis

Feline plasma cell pododermatitis (or pillow foot) is an uncommon condition in cats reported in the veterinary literature since the 1980s. It is also reported in dogs, although rarely, with a clinical presentation similar to that in cats. This disease, the etiology of which remains unknown, is characterized by marked infiltration of a cat’s footpad tissue by plasma cells, causing variable discomfort and generally responding to immunomodulatory therapy. Response to treatment is often slow, however, and subsequent relapses are possible. The prognosis is generally good, except in rare cases of concomitant immune-mediated glomerulonephritis or subsequent renal or hepatic amyloidosis (1,2).

Pathogeny

Plasma cell pododermatitis is characterized by infiltration of the cat’s footpad tissue by plasma cells, causing the pads to swell. Plasma cells are mature B-lymphocytes that become activated and then secrete antibodies, usually in response to an antigenic stimulation such as an infection or an inflammatory process. An immune system dysfunction seems the most likely cause, given the positive response of many cats to immunomodulatory therapy, the marked plasma cell tissue infiltrate, and the hypergammaglobulinemia observed in most affected cats (1,2). However, some patients present seasonal relapses, suggesting environmental allergy as a possible cause (1). Some authors also suggest an infectious etiology, specifically feline immunodeficiency virus (FIV). In fact, published case reports suggest FIV positivity rates of 44 to 63% (3,4). One case report also describes a cat suffering from plasma cell pododermatitis associated with feline leukemia virus (FeLV) as well as concomitant FIV (5). The location of the lesions in the footpads may also suggest close contact with an external stimulus as the cause of the disease. However, the recurrent behavior of the condition in certain cats and the fact that medical and surgical treatments have proved effective, suggests that some form of immunological dysfunction is more likely the cause (2). Some researchers suggest multipleetiologies (3).

Figure 1.

Swollen, purple footpad covered with scaly striations. Photo: Nadia Pagé.

Clinical presentation

Cats affected range in age from 6 mo to 12 y (1,3) and may be of any gender or breed, although generally they are domestic cats. In a study of 26 cases, however, males were overrepresented, neutered males in particular (1,3). In most cases, more than one footpad is affected. The metacarpal and metatarsal footpads are those primarily affected. Digital footpads may also be affected, but the lesions generally tend to be less severe. The footpads gradually become swollen, soft in the center, very pink or even sometimes violet purple with white striations (1,3,6). In 20 to 35% of cases there is bleeding and ulceration (1). Concomitant plasmacytic stomatitis is also reported in some cats, presenting as proliferative ulcerative gingivitis and erythematous plaques. These lesions are often painful and symmetrical in the palatoglossal arches (1,3,6). Some affected cats also present associated limping, whereas other are surprisingly asymptomatic (1,3). Certain general symptoms may be noted, including lymphadenomegaly, fever, or listlessness (1).

Localized infiltration at the bridge of the nose has also been reported in several cats with and without plasma cell pododermatitis. The bridge of the nose becomes firm and swollen and may or may not present erosions. In some of these case, there was also a concomitant upper respiratory tract infection (feline viral rhinotracheitis) (7,8). Some patients also presented concomitant eosinophilic granulomas or rodent ulcers (1).

Diagnosis

Differential diagnoses include eosinophilic, bacterial, or fungal granulomas, neoplasia, and inflammatory reaction to presence of a foreign body. A cat with Nocardia elegans infection in all 4 footpads has been reported (9). These conditions, however, generally tend to cause lesions in a single footpad, unlike plasma cell pododermatitis.

Plasma cell pododermatitis is fairly classical and very characteristic in its clinical appearance. The simplest diagnostic test to confirm a clinical diagnosis is fine-needle aspiration cytology of an affected footpad. The aspirate will include a large number of plasma cells (ovoid cells with an eccentric nucleus and a pale perinuclear area corresponding to the Golgi apparatus).

Definitive diagnosis is based on histopathological examination of an affected footpad. This examination is not always required, especially in the presence of classical clinical and cytological findings. Histopathological examination of skin biopsies will show severe, diffuse dermal, and perivascular infiltration by mature plasma cells, some perhaps presenting Russell bodies, an intracytoplasmic accumulation of eosinophilic material representing immunoglobulins. In some cases, vasculitis is reported as well (1,2,6).

Given the different studies reporting significant FIV infection rates in affected cats, a screening test is recommended for all patients diagnosed with plasma cell pododermatitis. A screening test for FeLV is also suggested at the same time given the recently reported case of concomitant infection with both retroviruses (5). In some studies, cats presented thrombocytopenia, hyperglobulinemia, lymphopenia or, conversely, lymphocytosis (1,4). A cat with concomitant immune-mediated glomerulonephritis and renal amyloidosis has also been reported (1). Other cats ultimately died from renal or hepatic amyloidosis, possibly secondary to chronic hyperglobulinemia (2). Hematological and biochemical workup and urinalysis are suggested, minimally, in conjunction with the feline retroviral screening.

Treatment

First-line treatment is doxycycline per os, its beneficial effect probably related to its immunomodulatory properties, although an infectious etiology responding to doxycycline is not excluded. Doxycycline results in complete remission in one-third of cats and improvement in lesions in 80% (4,10). A daily oral dose of 10 mg/kg or 25 mg/cat is suggested (1,10). Doxycycline should be continued until the appearance of the footpads is normal, which can sometimes take up to 10 wk (1). Some cats require continuous doxycycline therapy, whereas others only require sporadic therapy during relapses. Particular attention must be paid to the risk of esophagitis in patients taking doxycycline for extended periods of time. Oral administration of 5 mL of water is recommended as a preventive measure when administering this antibiotic (1). Second-line treatments are suggested for cats which don’t respond to doxycycline, with options including oral prednisolone and dexamethasone at immunosuppressive doses or oral cyclosporine. Once remission is achieved, the selected medication is gradually withdrawn and then discontinued. Treatment is resumed in case of relapse. In some cases, the symptoms do not decrease despite medical treatment, and surgical excision of the affected footpad or footpads is indicated (1,2). Surgery generally results in a cure without subsequent relapse (1,2,3). In some cats, the disorder resolves spontaneously without treatment (1,2).

Prognosis

The prognosis for this disease is relatively good: most cats respond well to medical treatment alone. In some cats, the lesions even resolve spontaneously. In cases refractory to medical therapies, surgical excision is generally curative. However, complications such as immune-mediated glomerulonephritis and/or amyloidosis may develop and be ultimately fatal (1,2).