Dr Heidi Schroeder, BVSc MMedVet(Med)
Small Animal Physician
Canine interdigital cyst syndrome (aka pedal folliculitis, interdigital pyoderma or interdigital furunculosis) is one of the clinical manifestations of canine pododermatitis where lesions are confined to the feet only. This condition is often misdiagnosed as a foreign body or a non-resolving pyoderma. It is a frustrating disorder to manage successfully.
Canine pododermatitis, by definition, is the inflammation and/or infection of the skin and connective tissue of a dog’s feet. A number of different skin conditions are known to cause pododermatitis. In many of these conditions, pododermatitis is part of a more generalised skin condition (e.g. canine atopic dermatitis, cutaneous adverse food reaction, demodicosis, deep pyoderma, necrolytic migratory erythema, pemphigus foliaceus, SLE, vasculitis). In some instances, lesions are confined to the feet only.
The interdigital skin is the skin located between the digits and the footpads. The dermis and epidermis of the interdigital skin does not differ markedly from that of the rest of the body. A layer of subcutaneous tissue separates the dermis of the dorsal and ventral skin. Adnexal structures, compound hair follicles and adnexal glands, are located between the ventral and the dorsal dermis and separated from the ground substance, vascular and connective tissue elements by a basement membrane.
The interdigital skin is predisposed to disease because of close contact with potential pathogens and allergens in the environment, the moist interdigital environment which may enhance microbial overgrowth and alter the barrier function of the local skin and also micro trauma of the hair follicles, predisposing to follicular rupture.
The pathogenesis is not clear, but the current proposed hypothesis is that interdigital cysts form as a result of abnormal friction or trauma to the ventral interdigital webbing due to congenital or acquired anatomic deformities. This abnormal friction causes a thickened, oedematous, callus-like interdigital skin with resultant plugging of follicular openings and the formation of comedomes. Keratin production continues inside the plugged follicle. This leads to dilatation of the plugged follicle and eventually cysts form.
It should be remembered that dogs have compound hair follicles (multiple hair shafts arising from 1 opening). Therefore, a single plugged follicular opening can result in multiple cysts. As a result, follicular cysts may be present in multiple layers of the ventral interdigital skin, each prone to rupture. Follicular cyst rupture incites a persistent, immune-mediated, pyogranulomatous foreign body reaction to free keratin, hair and triglycerides liberated from the ruptured hair follicles, sebaceous glands and the panniculus. This results in furunculosis often complicated by bacterial deep pyoderma.
Repeated rupture of the follicular cysts results in fistulous tract formation, which drains into the dorsal interdigital space. The presence of this dorsal clinical lesion gives the impression that the lesions originate at this site.
Lesions typically occur in young adult dogs (1-3 years of age) and appear to be more common in short coated breeds such as American Staffordshire terriers and Bull terriers, Bull dogs, Boxers, Golden retrievers and dogs with conformation deformities. The syndrome is seen more commonly in dogs weighing more than 30 kg.
Dogs with interdigital cysts present initially with ventral erythema and follicular plugging, followed by single to multiple erythematous papules and firm to fluctuant nodules or bullae in the interdigital skin of one foot or more feet. (Figure 1)
Figure 1: Ventral view of interdigital cyst syndrome. Follicular plugging and comedones. No significant deep secondary infection (yet). Photo courtesy of Dr Massimo Beccati (Rob Hilton)
Lesions may be painful, pruritic and ulcerated. Draining tracts with serosanguinous or purulent exudates may be present on the dorsal surface. (Figure 2)
Figure 2 Interdigital cyst syndrome. Dorsal draining sinus’s. Photo courtesy of Dr Massimo Beccati (Rob Hilton)
The dorsal interdigital lesion directly corresponds to an area of alopecia, erythema, oedema and comedone formation on the ventral palmar or plantar skin surface between the digital and metacarpal and metatarsal pads. Interdigital follicular cysts most often occur on the front feet in the fourth and fifth interdigital space. However, lesions may occur anywhere, affect more than one interdigital space or occur symmetrically. (Figure 3)
Figure 3 Interdigital cyst in a Bulldog (Dr Heidi Schroeder)
In chronic cases the lesions often become fibrotic. Lesions may resolve spontaneously, wax and wane of persist indefinitely. (Figure 4). Peripheral regional lymph nodes are commonly enlarged, but no other systemic signs are usually present. Secondary bacterial and yeast infections are common in these cysts.
Figure 4: Oedematous, thickened ventral interdigital skin (Dr Heidi Schroeder)
Diagnosis is based on history, findings of a full clinical examination and initially cytology of a non-ruptured lesion. Biopsies for histopathology are recommended in difficult cases after 3 weeks of appropriate antibiotic treatment as severe secondary infections may affect the findings. Bacterial and /or fungal cultures may also be performed in selected cases. This would preferably be performed prior to antimicrobial treatment.
Successful management of this disease can be frustrating and depends on determining, treating and managing the primary or underlying cause as well as the reactive responses. Treatment therefore needs to be tailored to each individual.
- Environmental factors that may play a role include a wet environment, dirty kennels and large amounts of gravel or sand in the environment. Foot trauma should be minimized by keeping the dog indoors, walking the dog on a leash and keeping the dog away from rough surfaces.
- Systemic antibiotic therapy alone is rarely successful as the bacterial infections are usually secondary. Some cases respond very well whereas other cases will only improve. As for all deep pyoderma cases, antibiotic treatment should be continued for 3 weeks after clinical cure. Where a resistant organism is suspected or cytology of a non-ruptured lesion is suggestive of a bacterial deep pyoderma, a skin biopsy with culture and sensitivity should be performed prior to commencing with antibiotic treatment. Staphylococcus pseudointermedius is the most important pathogen. Many other organisms, e.g. Pseudomonas spp can be cultured from these cases due to soil contact and a moist environment in the interdigital areas.In most cases some degree of immunomodulation is necessary and this often includes a combination of antibiotics with systemic prednisolone or cyclosporine. The concurrent deep pyoderma and foreign body reaction leads to incomplete resolution of lesions with antimicrobial therapy alone and inflammation re-occurs quickly once glucocorticoid therapy is discontinued. Tetracycline/niacinamide or pentoxifylline therapy may maintain remission or lower corticosteroid use in some cases.
- Pulse therapy may be required for relapsing cases. Topical antimicrobial treatment is indicated in these cases and may assist in preventing relapses, e.g. mupirocin, chlorhexidine and silver sulphadiazine.
- Topical treatment with corticosteroids may assist by decreasing the reaction to free keratin and by reducing keratin production. Topical tacrolimus 0.1% may also be effective in some cases.
- A recent study using topical diluted sodium hypochlorite foot soaks as a complementary therapy has shown reduced numbers of bacteria and a reduction of inflammatory cells on cytology. Further studies are required.
- In some cases, especially solitary lesions where there is extensive granuloma formation and scarring within the interdigital space, surgical excision or fusion podoplasty, may be the only alternative to keep patient comfortable and ambulatory.
- Lifelong medical management is often needed to maintain remission. Interdigital fibrosis is often a feature of chronic cases