Interdigital furuncles, often referred to as interdigital cysts, are painful nodular lesions located in the interdigital webs of dogs. Histologically, these lesions represent areas of nodular pyogranulomatous inflammation—they are almost never cystic. Canine interdigital palmar and plantar comedones and follicular cysts is a recognized syndrome that may be a subtype of interdigital furuncles or a separate disease.
The most common cause is a deep bacterial infection. Many dog breeds (eg, Chinese Shar-Pei, Labrador Retriever, English Bulldog) are predisposed to bacterial interdigital furunculosis because of the short bristly hairs located on the webbing between the toes, prominent interdigital webbing, or both. The short shafts of hairs are easily forced backward into the hair follicles during locomotion (traumatic implantation). Hair, ie, keratin, is very inflammatory in the skin, and secondary bacterial infections are common. Less commonly, foreign material is traumatically embedded in the skin. Demodicosis may be a primary cause of interdigital furunculosis. Canine atopic dermatitis is also a common cause of recurrent interdigital furunculosis.
The cause of canine interdigital palmar and plantar comedones and follicular cysts is unknown but most likely involves trauma, resulting in epidermal and follicular infundibular hyperkeratosis, acanthosis, plugging or narrowing of the follicular opening, and retention of the follicular contents.
Clinical Findings and Lesions
Early lesions of interdigital furunculosis may appear as focal or generalized areas of erythema and papules in the webbing of the feet that, if left untreated, rapidly develop into single or multiple nodules. The latter usually are 1–2 cm in diameter, reddish purple, shiny, and fluctuant; they may rupture when palpated and exude a bloody material. Interdigital furuncles are most commonly found on the dorsal aspect of the paw but may also be found ventrally. Furuncles are usually painful, and the dog may be obviously lame on the affected foot (or feet) and lick and bite at the lesions. Lesions caused by a foreign body, eg, a grass awn, are usually solitary and are often found on a front foot; recurrence is not common in these cases. If bacteria cause the interdigital furunculosis, there may be several nodules with new lesions developing as others resolve. A common cause of recurrence is the granulomatous reaction to the presence of free keratin in the tissues.
Dogs with interdigital comedones and follicular cysts typically present with lameness and draining tracts. Skin lesions are not often seen unless the hair coat is clipped. Areas of alopecia and thickened, firm, callus-like skin with multiple comedones are typical.
For furunculosis, the diagnosis is often based on clinical signs alone. The major differential diagnoses are traumatic lesions, foreign bodies, follicular comedone cysts, and neoplasia, although the latter is rare. The most useful diagnostic tests include skin scrapings for Demodex mites, impression smears, or fine-needle aspirates to confirm the presence of an inflammatory infiltrate. Unusual or recurrent lesions should be excised for histopathologic examination. Solitary lesions may require surgical exploration to find and remove foreign bodies such as grass awns.
Definitive diagnosis of palmar and plantar follicular cysts requires a skin biopsy. However, they are suspected when clinical examination reveals draining tracts associated with callus-like lesions or obvious comedone formation. Moderate to extensive compact hyperkeratosis and acanthosis of the epidermal and follicular infundibulum is found. Follicular cysts consisting of keratin are common. Often, lesions are complicated by secondary infection and concurrent bacterial furunculosis.
Bacterial interdigital furuncles respond best to a combination of topical and systemic therapy. Treatment is best based on culture and susceptibility, because these are deep infections and may require longterm therapy, particularly if multifocal. Pending culture, application of warm water compresses and topical triple antibiotic ointment or mupircin antibiotic ointment are recommended. Foot soaks in chlorhexidine solution are also helpful. Because the lesions are pyogranulomatous, it may be difficult for antibiotics to penetrate them; therefore, >8 wk of systemic antibiotic therapy may be required for lesions to completely resolve. These lesions are often complicated by concurrent Malassezia spp infections. Oral ketoconazole or itraconazole (5–10 mg/kg) for 30 days may be indicated. The presence of Malassezia can be documented by cytologic examination of nail bed debris and/or impression smears of the skin.
Chronic, recurrent interdigital furunculosis is most often caused by inappropriate antibiotic therapy (too short a course, wrong dosage, wrong drug), concurrent corticosteroid administration, demodicosis, an anatomic predisposition, or a foreign body reaction to keratin. Lesions that recur despite therapy can also be a sign of an underlying disease, eg, atopy, hypothyroidism, or concurrent Malassezia infection. Lesions in confined dogs are likely to recur unless the dog is removed from wire or concrete surfaces. In some chronic cases, surgical excision or surgical correction of the webbing via fusion podoplasty may be needed. Alternatively, pulse antibiotic therapy (full dosage therapy 2–3 times/wk) or chronic low dosage antibiotic therapy (eg, 500 mg/day/dog, PO) may help maintain clinical remission and provide pain relief in dogs with chronic lesions. This therapy is recommended only when the inciting cause cannot be identified (eg, idiopathic pyoderma), treated (eg, anatomic predisposition), or resolved (eg, chronic infection caused by foreign body material or keratin).
Treatment of interdigital palmar and plantar comedones and follicular cysts can be successfully accomplished by laser therapy. Postoperative care is time intensive, with hydrotherapy and bandage changes once or twice daily.