Interdigital furunculosis: medical and surgical options

Note the interdigital erythema and brown staining/paronychia.

Disease description and mechanisms

Interdigital furunculosis (IF) and folliculitis, sometimes called interdigital cysts, can occur for multiple reasons. Lesions include interdigital erythema, edema, nodules, hemorrhagic bullae, hemorrhagic draining tracts, ulcers, and scarring. The most common reason for IF is allergy (Image 1 and Figure 1). The skin of the paws, especially the interdigital skin, becomes inflamed with an allergy flare, leading to barrier dysfunction and pedal pruritus. Pruritus is manifested as licking, which can cause trauma to the hair follicles, causing folliculitis. Pieces of hair shaft (keratin) will become pushed into deeper tissues, causing a foreign body reaction. Due to continued inflammation and licking, secondary infection becomes a problem, which causes more inflammation. A cycle of inflammation/infection ensues, which can lead to antibiotic resistance, scar tissue, and cellulitis. In allergic dogs, multiple paws and interdigital spaces will be affected and there often will be other signs of allergic skin disease (eg, more general pruritus, and recurrent skin infections). Affected animals are often short-coated, allergy-prone dogs like pit bull terriers, Great Danes, English and French bulldogs, Labrador retrievers, Chinese shar-pei, and bull terriers.

Note position of nodule between digits 4 and 5 without evidence of allergic inflammation.

In nonallergic dogs, the mechanism of IF is mechanical. Furunculosis will often develop only between digits 4 and 5 on the front paws (Images 2 and 3 and Figure 2). Affected animals are often larger-breed, heavy dogs like mastiffs, English bulldogs, and Labrador retrievers. Dogs with webbed paws, deep palmar interdigital pockets, obesity, and conformation abnormalities are more prone to this type of IF. The increased weight of these heavier dogs will cause friction between this interdigital space, resulting in follicular plugging and comedo formation on the palmar surface of the paw. Dogs with conformation changes secondary to arthritis and resulting in valgus deviation of digit 5 are also prone to interdigital friction and comedo formation. Smaller dogs prone to arthritis-related IF are Shetland sheepdogs and Cavalier King Charles spaniels.

Some dogs may have a contribution of both mechanisms; for example, an obese and allergic Labrador retriever with pedal pruritus and other signs of allergy with more IF on multiple paws but more pronounced disease between digits 4 and 5 on the front paws. Other mechanisms of IF include demodicosis and foreign bodies.

Chronic IF can lead to scar tissue and soft tissue proliferation, resulting in false paw pad formation. False paw pad formation occurs when the palmar interdigital skin becomes thickened, callused, and confluent with the adjacent paw pads, resulting in conjoined pads. False paw pad formation results in even deeper palmar interdigital pockets, leading to trapped keratin, moisture, and infection. Hyperkeratosis and deep crevices that harbor debris and ingrown hairs are contributory factors.

Note palmar interdigital comedo formation between digits 4 and 5 without evidence of allergic inflammation.

Diagnosis

History and complete physical with dermatology-focused examination is essential in determining whether the IF is allergy-related or mechanical. Skin cytology and scraping (or hair plucks) are important in determining secondary infection and looking for Demodex mites. Demodex mites may not be easy to find due to the presence of so much purulent material; a parasite treatment trial with an isoxazoline is recommended. Bacterial culture and sensitivity testing are often indicated because there is often a history of multiple courses of antibiotics.

Treatment

The primary cause should be identified and controlled. If allergic in nature, follow your standard allergic work-up. In my experience, many dogs with IF have food and environmental contributions and do best with a prescription hypoallergenic diet and aggressive medical management for environmental allergies. Because there is so much inflammation involved in IF, lokivetmab (Cytopoint) and oclacitinib (Apoquel) often fail to control significant or chronic disease. Systemic glucocorticoids and cyclosporine are usually needed in my practice.

Secondary infections must be treated and might involve longer courses of antibiotics (6 to 8 weeks). Pentoxifylline (Trental) can help antibiotics perform and reduces inflammation. Topical therapy involving antimicrobial paw soaks (chlorhexidine and/or diluted bleach) is essential. Topical antimicrobials mixed with a topical glucocorticoid and dimethyl sulfoxide (like fluocinolone acetonide 0.01% [Synalar] and dimethyl sulfoxide 60% [Synotic] mixed with enrofloxacin [Baytril]) can be helpful for treatment and flares.

If there is a mechanical cause of IF, management of arthritis and/or obesity is critical. In markedly obese dogs with IF, significant weight loss to “unload” the front paws results in substantial improvement. Managing secondary infection is important and topical antimicrobials and anti-inflammatories are helpful. Dogs with mechanical IF rarely need systemic anti-inflammatories unless they have a mixed pattern of allergic and mechanical IF. Pain medications can be helpful for skin reasons and management of concurrent arthritis. Protection of the paw with shoes and controlling substrates on walks can help with the discomfort of IF.

In cases of mechanically caused IF that fail to respond to weight loss and topical therapies, laser surgery following the Duclos method is often successful. This laser surgery is not fusion podoplasty. The details can be found on the Aesculight website. Fusion podoplasty is the last resort because the abnormal weight-bearing present with mechanically caused IF is then transferred to the adjacent toes, causing IF in the associated interdigital space.

Anthea Schick, DVM, DACVD, earned a bachelor of arts in biology from the University of California, Berkeley followed by a doctorate in veterinary medicine from Cornell University, Ithaca, New York. After a dermatology residency at Dermatology for Animals in Arizona, she achieved board certification in veterinary dermatology. She became co-owner of Dermatology for Animals in 2009 and is now Thrive Pet Healthcare’s national specialty director of dermatology. She enjoys teaching and training residents and is president of the American College of Veterinary Dermatology.