Interdigital furuncles are deep pyoderma lesions that form between the toes of dogs. They can be either single or multifocal. The nodules are painful areas of pyogranulomatous inflammation. Foreign body reactions to embedded hair shafts will prolong the infection. Lesions are treated with topical antimicrobial treatment and, if severe, with concurrent systemic antimicrobial treatment. Important diagnostics include hair trichogram, cytological evaluation, and culture and susceptibility of lesions. Skin biopsy and, less commonly, radiographs, may be needed in some cases. Lesions need aggressive longterm treatment (3–6 weeks). It is important to identify the underlying trigger (eg, conformation, allergic dermatitis) if lesions are multifocal.
Interdigital furuncles are areas of deep pyoderma and are often incorrectly referred to as interdigital cysts. Clinically, these lesions are painful focal or multifocal nodules 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 similar to interdigital furunculosis that may be a subtype or a separate disease. Foreign body reaction to embedded keratin hair shafts is a major obstacle to resolution of infection.
Etiology of Interdigital Furunculosis in Dogs
The most common cause of interdigital furunculosis is a deep bacterial infection. These infections can be onetime events but are often recurrent, indicating an underlying trigger.
The pathogenesis of canine interdigital palmar and plantar comedones and furuncles most likely involves follicular trauma, resulting in epidermal and follicular infundibular hyperkeratosis, acanthosis, plugging or narrowing of the follicular opening, and retention of the follicular contents.
Traumatic implantation of keratin hair shafts is a common trigger in breeds with conformational problems. Hair (ie, keratin) is 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. Other parasitic causes (eg, hookworm dermatitis) are possible.
Canine atopic dermatitis is also a common cause of recurrent interdigital furunculosis. Licking and other pruritic behavior associated with an underlying allergic dermatitis (environmental allergies, cutaneous adverse food reactions, allergic contact, or irritant dermatitis) can contribute to local trauma and development of interdigital furunculosis.
In confined dogs, follicular trauma may be related to surface trauma from wire cages, concrete, or rough ground.
Other possible causes include neoplasia and deep fungal infections.
Epidemiology of Interdigital Furunculosis in Dogs
Any dog can develop lesions of interdigital furunculosis, but they are more common in some breeds. Inciting damage to the environment of the interdigital web allows development of microbial overgrowth. This can be due to conformational problems that result in increased friction, moisture, and maceration of tissue. Some dog breeds (eg, Chinese Shar Pei, Labrador Retriever, English Bulldog) may be 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).
Breeds with wider paw conformations and a greater distance between digital pads (eg, Labrador Retriever, English Bulldog, German Shepherd Dog, Pekingese) may be more likely to bear weight on the haired skin between the pads.
More pressure also may be put on interdigital spaces in overweight dogs or those with abnormal weight bearing due to lameness or conformation abnormalities.
Dogs with systemic conditions (eg, hypothyroidism, hyperadrenocorticism), skin barrier abnormalities, or chronic allergic diseases can be predisposed to bacterial infections.
Clinical Findings and Lesions of Interdigital Furunculosis in Dogs
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 (see multifocal interdigital furuncles image). The latter usually are 1–2 cm in diameter, reddish purple, shiny, and fluctuant; they may rupture when palpated and exude a bloody material.
Courtesy of Dr. Karen A. Moriello.
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 have lameness and draining tracts. Skin lesions or the severity of skin lesions is often not seen until the hair is clipped. Areas of alopecia and thickened, firm, calluslike skin with multiple comedones are typical.
Diagnosis of Interdigital Furunculosis in Dogs
History and clinical examination findings
Cytological evaluation
Bacteriological culture
Biopsy with histological evaluation
For interdigital 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 hair trichograms (to identify Demodex mites) and cytological evaluation of impression smears and fine-needle aspirates (to confirm the presence of an inflammatory infiltrate). A commonly overlooked concurrent infection is Malassezia.
Bacteriological culture and antimicrobial susceptibility testing are recommended to guide antimicrobial use. This is especially important when systemic antimicrobials are indicated or needed.
Unusual or recurrent lesions should be excised for histological 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, these cysts are suspected when clinical examination reveals draining tracts associated with calluslike lesions or obvious comedone formation. Moderate to extensive compact hyperkeratosis and acanthosis of the epidermal and follicular infundibulum are found. Follicular cysts consisting of keratin are common. Often, lesions are complicated by secondary infection and concurrent bacterial furunculosis.
In dogs predisposed to allergic skin disease with recurrent single or multifocal lesions, diagnostics for environmental allergies and an elimination diet may be helpful in identifying the underlying trigger.
Treatment of Interdigital Furunculosis in Dogs
Topical treatment (bathing and lesion hygiene)
Topical antimicrobial treatment with or without concurrent systemic antimicrobial treatment
Environmental modification and control of inflammation
Bathing the affected feet daily with antimicrobial soaks and shampoos (eg, chlorhexidine with or without an antifungal agent) will maintain skin hygiene in interdigital furunculosis. Topical antimicrobial treatment is indicated in cases of secondary bacterial infection of lesions and is best prescribed when compliance can be expected. Treatment with a broad-spectrum topical antimicrobial (eg, neomycin-polymyxin) or with a topical antimicrobial of known susceptibility is most appropriate when lesions are localized (see interdigital furuncle image).
Dogs with multifocal lesions will benefit from systemic antimicrobial treatment for extended periods of time (at least 4–6 weeks). Systemic treatment with a cephalosporin or other antimicrobial of known susceptibility is indicated. Empirical antimicrobial selection for suspected multidrug-resistant infections should be avoided, and treatment is best based on bacteriological culture and antimicrobial susceptibility testing. Dogs with multidrug-resistant methicillin-resistant staphyloccocal infections may benefit from topical treatment using antibacterial agents with proven antistaphylococcal efficacy (eg, mupirocin ointment).
CO2 laser ablation of lesions is more commonly used for tumor removal and scar tissue than for acute treatment.
Courtesy of Dr. Karen A. Moriello.
Clinical observations suggest that topical benzoyl peroxide (5%, every 24 hours), spot-on lipid products (topically once weekly), and topical retinoids (0.01%–0.05%, every 24 hours) may also be helpful in maintaining the integrity of hair follicles and preventing recurrence of lesions. These treatments have not been tested in placebo-controlled studies.
Pending culture, instruct clients to wash paws daily with a combination of 2% chlorhexidine-2% miconazole shampoo. Also pending culture, clients should apply a polymyxin B and bacitracin ointment several times a day.
Do not clip the hair over the paws with electric clipper blades because this may cause microtrauma and lead to hair shafts becoming traumatically inoculated into tissue. Use scissors to clip hair.
If there is concurrent Malassezia overgrowth, administer ketoconazole (5 mg/kg, PO, every 24 hours) or, in small dogs, itraconazole (5 mg/kg, PO, every 24 hours).
Chronic recurrent interdigital furunculosis is most often caused by inappropriate antimicrobial treatment (too short a course, wrong dosage, wrong drug), concurrent systemic corticosteroid administration, demodicosis, an anatomic predisposition, a foreign body reaction to keratin, or lack of recognition of underlying whole body dermatological disease. Lesions that recur despite treatment can also signify an underlying disease (eg, atopy, hypothyroidism, or concurrent Malassezia infection). Some dogs have chronic recurrent lesions despite good initial treatment and identification of an underlying disease. These dogs are best treated with chronic topical antimicrobial bathing.
Lesions in confined dogs are likely to recur unless the dog is removed from wire or concrete surfaces. Additional measures to protect the paws include the use of booties and management of substrates in kennels and exercise areas.
Pearls & Pitfalls
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Treatment of interdigital palmar and plantar comedones and follicular cysts can be successfully accomplished by CO2 laser ablation. Postoperative care is time intensive, with hydrotherapy and bandage changes every 24–48 hours.
Identification and treatment of atopic dermatitis will help decrease self-trauma.
In short-term use (5–10 days), corticosteroids can help with severe inflammation, but longterm use is not advised because they are comedogenic (may worsen interdigital comedones) and immunosuppressive (may worsen secondary bacterial pyoderma). Pentoxifylline (15–30 mg/kg, PO, every 8–12 hours longterm) may be a useful alternative because it increases perfusion to affected hair follicles and decreases inflammation through inhibition of tumor necrosis factor alpha (TNFalpha).
NSAIDs may also be useful in treating any associated lameness.
Cyclosporine may be useful in cases with underlying atopic dermatitis.
Oclacitinib (a Janus kinase inhibitor) and lokivetmab (a caninized monoclonal anti-IL-31 antibody) might help control some concurrent pruritus due to allergies; however, they are not effective for treating deep inflammation and ensuing self-trauma.
Topical glucocorticoid medications can be helpful with localized disease, small superficial lesions, and when treatment with systemic glucocorticoids is contraindicated.
Key Points
Interdigital furuncles are solitary or multiple areas of deep pyoderma that may be triggered by trauma, conformation, or underlying skin disease. Chronic lesions are often caused by foreign body reactions to embedded keratin.
Topical treatment (bathing and overall hygiene) is a key part of initial treatment and for chronic recurrent lesions. Combined chlorhexidine-miconazole shampoo products are recommended.
Concurrent topical and systemic antimicrobial treatment are often needed when lesions are multiple and painful.
For More Information
Bajwa J. Canine pododermatitis: a complex multifactorial condition. Can Vet J. 2023;64(5):489-492.
Morris DO, Loeffler A, Davis MF, Guardabassi L, Weese JS. Recommendations for approaches to methicillin-resistant staphylococcal infections of small animals: diagnosis, therapeutic considerations and preventative measures: Clinical Consensus Guidelines of the World Association for Veterinary Dermatology. Vet Dermatol. 2017;28(3):304-e69. doi:10.1111/vde.12444
Also see pet health content regarding abscesses between the toes in dogs.
