Interdigital Folliculitis and Furunculosis (“Interdigital Cysts”)

Interdigital folliculitis and furunculosis (IFF) is a common presenting problem. I see at least one case per day! Why? It is a frustrating skin problem in many dogs and for many veterinarians and pet parents. There are predisposing causes, primary diseases that contribute, and secondary problems that exacerbate. The cause is multifactorial. The management is multimodal. The approach to a dog with IFF starts with the recognition of the problem, identification of any predisposing factors that can be addressed, treatment of any secondary infection, and reduction of inflammation. Additionally, identification and control of any concurrent primary allergic disease is important for the long-term well-being of the affected dog.

What is interdigital folliculitis and furunculosis?

Interdigital folliculitis and furunculosis is a descriptive term for an initially sterile inflammatory response to foreign material introduced to the deeper layers of skin by ruptured hair follicles (often from frictional trauma in short coated breeds) that results in large bumps or nodules between a dogs toes. The keratin and hair that is released when these bumps rupture is foreign body material in the deeper layers of the skin. This initiates even more inflammation which brings about more localized interdigital swellings (what people call cysts) and discomfort. The lesions grow in size and rupture. Secondary infection is common. Too often, too many courses of antibiotics are used in this initially sterile process, and antibiotic resistant bacteria emerge and contribute to the severity of the disease.

What do lesions look like?

The interdigital lesions may appear as smooth red bumps in the beginning. They enlarge in size and appear as a nodule or “cyst” between the toes. Ultimately the nodule will rupture and exude a clear or clear but red fluid. If infected, it may be pus.

What contributes or predisposes dogs to this condition?

1. Coat type– Short coated, coarse coated, larger breed dogs are more likely to develop this problem. Examples include pit bull terriers, mastiffs, English bulldogs, German Shepherds, Labrador retrievers, and bull terriers.
2. Paw shape– Dogs with larger and wider paws bear a lot of weight on their paws. Many of the predisposed breeds also have a lot of interdigital skin that is rubbed with normal walking and running.
3. Obesity– Dogs that are overweight also bear a lot of weight on their paws when ambulating, and this contributes to the interdigital friction that initiates this problem.
4. Dogs with altered gait– Dogs with arthritis or other conditions that alter their normal gait, put more pressure on their paws which, in turn, contributes to the interdigital friction that initiates this problem.
5. Allergy– Dogs with pruritus (itch) that manifests as excessive paw licking are predisposed as well. These are dogs with atopic dermatitis (environmental allergies) and/or cutaneous adverse food reaction (food allergies).

Other factors or causes 

1. Secondary bacterial infection – With chronicity, lesions rupture and become secondarily infected. Infection invites more licking hair follicle rupture.
2. Demodex – This is a parasite that lives within the hair follicle. If not identified, it can progress and interdigital nodules may be present. Such nodules may resemble IFF lesions.

Diagnosis

When presented with a dog with interdigital lesions, we gather the signalment (age, breed, species) history, discuss the course of disease and perform our dermatologic examination. We will initially perform some diagnostic tests in the office including a cytology (to look for secondary infection) and deep skin scraping (to identify demodex). We may perform a bacterial culture and sensitivity test or fungal culture. Biopsies are sometimes performed with or without the added request for special stains to identify certain infectious agents.

Treatment

1. Resolve all secondary infections – infection contributes to severity, and it is important to treat any infection with antibiotics until infection is RESOLVED. Bacterial culture and antibiotic sensitivity testing often guides this therapy. Topical therapy is also very important and may involve ointment, mousses, shampoos and foot soaking.
2. Identify and control primary allergic diseases – Since allergic diseases may contribute to the licking behavior that can initiate IFF, we work hard to either confirm the presence of an allergic disease or rule it out as a contributing factor. If allergy is confirmed, we treat the allergy specifically to control the underlying. We still must address the IFF lesions concurrently until resolved. Often, we rule out allergy as the cause of IFF in many dogs. Thus, IFF may be classified as idiopathic (no known cause).
3. Manage the inflammation – As this is fundamentally a sterile inflammatory process, anti-inflammatory medications can be a very important part of the treatment plan. Such medications may include a glucocorticoid such as prednisone or a topical preparation, Atopica® (cyclosporine), or tacrolimus ointment topically. Treatment of the inflammation can be central to the resolution of the discomfort and pain associated with IFF.
4. Adjunctive – Protective footwear and preventative topical therapies can be used.
5. Surgery – Very rarely a surgery called fusion podoplasty is recommended. This is when a surgeon removes the proliferative and redundant skin tissue between the toes.