THERE are many different causes of interdigital dermatitis in the dog, which can make it a very difficult and frustrating disease to treat.
As with all dermatological problems, the basic approach is the same and should start with a full history and clinical examination before even considering the skin condition. The breed, age of onset and progression of the disease are important as well as any seasonal pattern.
It is also essential to establish if the problem involves a single foot or multiple feet (Table 1). The dog’s weight, conformation, gait and concurrent orthopaedic or neurological disease are also particularly pertinent.
In acute disease it is possible that thorough investigation can identify the cause of the disease and aggressive therapy can produce an excellent and complete response. However, the more chronic the disease becomes, the less likely it is that medical therapy can resolve the problem due to the cutaneous changes that occur.
Assessment by an orthopaedic surgeon is useful in all chronic cases. The author will usually employ the services of orthopaedic colleagues to decide if there are conformational or lameness problems that will be contributing to disease.
Long-standing elbow or shoulder diseases are a common factor in contributing to pedal lesions when they are unilateral. Hip or spinal pain can lead to bilateral pedal dermatitis confined to the front legs due to the fact the dog is not weight-bearing equally.
Where orthopaedic disease is present, this should be managed before pedal dermatitis can be treated. Sadly, in many cases presented to the dermatology service the pedal skin is irreversibly damaged and no longer amenable to medical therapy.
Foot pads are perfectly designed to weight bear as they contain no hair. When the dog’s weight is distributed asymmetrically false pads are formed (Figure 1). Normal haired skin becomes weight bearing –afunction it is not designed for – which forces the hair up into the dermis causing a foreign body reaction on the plantar aspect of the foot.
Hair is also forced into the skin when a dog licks excessively (Figures 2 and 3). This produces the same reaction and can be on the plantar or dorsal aspect, depending where the pruritus or irritation is located. The constant introduction of foreign material into the skin leads to a marked granulomatous reaction often with secondary infection which in time will become irreversibly damaged.
Not only is the skin on the feet deformed but the interdigital spaces are changed. Large crevices can be created on the underside of the pad that harbour foreign material that become colonised with bacteria and yeast due to the abnormal microclimate at this site. Bacterial and yeast over-growth quickly progress to overt infection.
At this juncture medical therapy becomes a full-time and often unrewarding task for the dog’s owner as animals require constant cleaning and creaming of the area, together often with antibiotic and antiinflammatory therapy.
Few dogs at this stage can be cured. Even though the underlying skin disease and any orthopaedic issues have been resolved, most require some form of surgical intervention to make the dog comfortable. Logical steps to investigate a chronic case are outlined in the flow chart (Figure 4).
Where surgical intervention is deemed necessary, partial or complete podoplasty can be performed or else the area between the toes can be lasered to destroy the superficial tissue in the interdigital web.
Podoplasty should really only be used as a salvage procedure but can produce huge benefits. Surgery needs to be undertaken by an experienced surgeon. The procedure is lengthy and bloody as all of the tissue in the interdigital spaces on both the planter and dorsal aspects needs to be removed so that the true pad tissue is fused together (Figures 5 and 6).
A CO2 laser makes a huge difference to the time and also the post-operative outcome of the surgery. Laser surgery provides excellent haemostatic control and also controls pain and swelling as well as sterilising the tissue. Post-operative, the foot needs to be dressed carefully and exercise needs to be strictly reduced during the healing process which can take two to three weeks. However, the end result cosmetically looks very satisfactory and as the scarred, fibrotic tissue has been removed the dog is far more comfortable (Figure 7).
In severe cases the author will recommend a complete podoplasty, but in milder cases, where only a single interdigital space is involved, then the removal of a single interdigital web is useful or the fusion of two pads.
Laser surgery can also be used to ablate tissue in the interdigital space. This newly-described procedure is an alternative to podoplasty. In a recent study published by David Duclos and Ann Hargis, 28 dogs were treated with CO2 laser therapy. All dogs had recurrent lameness, pain and nodules, or draining sinuses in the dorsal interdigital skin, had failed to respond to antibiotic therapy, and were negative for Demodex mites and dermatophytes.
Clinical features in the interdigital skin on the plantar aspect of the feet included alopecia, callus-like thickening, and comedones. Laser surgery allowed removal of multiple layers of cysts and adjacent hair follicles and the tracking and removal of sinuses.
Unfortunately, one dog was euthanased due to orthopaedic problems one month after the laser surgery was performed. However, post-surgical follow-up (1.0–8.0 years – mean 3 years) from the remaining 27 dogs revealed that laser therapy of affected skin and adjacent hair follicles resulted in resolution of interdigital lesions in 25 of the dogs. Only two dogs continued to develop interdigital cysts.
David D. Duclos, Ann M. Hargis, Patrick W. Hanley (2008) Pathogenesis of canine interdigital palmar and plantar comedones and follicular cysts, and their response to laser surgery. In: Veterinary Dermatology 19 (3): 134-141.