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Chapter 8.5
Pododermatitis: canine interdigital follicular cysts and feline plasma cell pododermatitis
Rusty Muse (USA) welcomed the participants to the workshop and presented a brief overview of pododermatitis. He then explained that the workshop would focus on two specific diseases that are both interesting and frustrating to treat: canine interdigital cysts and feline plasma cell pododermatitis. During the first half of the workshop, the aetiology, diagnosis and carbon dioxide (CO2) laser ablation of canine interdigital cysts would be presented by David Duclos, followed by the medical management of this disease by John Angus. For the second half of the workshop, Brett Wildermuth would present an overview of feline plasma cell pododermatitis, followed by a clinical case presented by Sarah Bartlett.
Interdigital cysts (D. Duclos)
David Duclos (USA) began his presentation by stating that he prefers to call this disease interdigital follicular cysts rather than interdigital cysts, as the lesions originate in the hair follicle. He first explained that his initial thought was to use the laser to sterilize these lesions because they were described in the literature as strictly bacterial lesions. While ablating these lesions, he found that the dorsal interdigital draining tracts originated from dilated cystic hair follicles located deeper in the ventral interdigital space.
He then discussed the pathogenesis of this disease. It is proposed that friction on the ventral surface of the interdigital space causes thickening of the skin, secondarily plugging the follicular infundibulum. The hair follicle continues to produce keratin and dilates, forming a cyst. Hair growth in the follicle is inhibited, resulting in alopecia. Some dogs form follicular cysts but remain asymptomatic, while in others the follicular cyst ruptures, dissects through the tissue dorsally and perforates the skin of the interdigital space resulting in a chronic draining tract with secondary bacterial infection. These cysts do not rupture ventrally due to the constant friction pushing on the hyperplastic epidermis.
David Duclos then showed numerous videos and photos detailing the surgical ablation of follicular cysts using the CO2 laser. Prior to the surgical procedure, he recommends administering an appropriate antibiotic to reduce the size of infected lesions as much as possible. He feels that the use of glucocorticoids is acceptable, but he prefers not to as these lesions are a form of deep pyoderma.
The first video showed a 5-year-old male Alaskan malamute with follicular cysts between the fourth and fifth digits of both forepaws. There were draining tracts dorsally, with a corresponding thickening of the skin on the ventral interdigital spaces with numerous comedones and alopecia. David Duclos explained that to assist with the diagnosis of interdigital follicular cysts, this thickened ventral interdigital skin could be squeezed to exude keratin from the many small cysts that have formed. This should not be performed on a dog while it is awake, however, as it is painful.
To perform the ablation, David Duclos uses a CO2 laser with a wide tip, set in continuous mode at 25 W power. He places a metal probe in the draining tract of the dorsal interdigital space to act as a landmark for the ventral approach with the laser. He begins by ablating the upper layer of the epidermis on the ventral interdigital skin of the affected area. The laser easily vaporizes the soft tissue of the dermis, making the keratin of the cysts stand out. The tissue is squeezed after each layer of ablation, exuding keratin to guide the next layer of laser ablation. Additionally, the metal probe may be manipulated to help guide the direction of ablation towards the main fistula that has perforated to the dorsal surface. The previous steps are repeated until all the follicular cysts have been vaporized leaving only normal tissue dorsally. Occasionally grape-cluster-like pyogranulomatous lesions are encountered while ablating through the tissue; these bleed easily and are best excised rather than ablated.
Some cysts are so deep that one may need to laser almost through to the other side, though typically the dorsal interdigital skin can remain intact. In general it is recommended to ablate these lesions with a wide margin to vaporize additional hair follicles that would otherwise be the source of future lesions.
David Duclos also informed the audience that the surgeon must naturally be familiar with the vasculature of the paw prior to attempting this surgery. The laser is very precise and will cauterize small blood vessels of less than 1 mm, and in most cases large blood vessels can be visualized and avoided, but a tourniquet is necessary in case a larger blood vessel is damaged.
As this is not a sterile surgery and there is a large amount of tension on the paw when the dogs walk, David Duclos leaves the lesions open to heal by second intention. The paw is bandaged postoperatively with bandage changes performed twice weekly during weeks 1 and 2, then weekly for 1 month. In approximately 4 weeks most dogs will have healed completely.
In summary, follicular interdigital cysts present as recurrent draining tract lesions most commonly on the forepaws of younger dogs with or without obvious conformational abnormalities. The lesions begin ventrally as a follicular cyst, and progress to form a draining tract dorsally. Laser ablation of the cysts is an effective long-term solution with a success rate of approximately 70%.1
Medical therapy of interdigital furunculosis (J.C. Angus)
John Angus (USA) began his presentation by reiterating that this disease was initially thought by many veterinary dermatologists to be only bacterial and/or allergic in origin prior to David Duclos’ report published in 2008, and today he sees that it is often misdiagnosed in general practice as a foxtail foreign body. He noted that affected dogs tend to be large breed, short-coated dogs, with Labradors, retrievers and English bulldogs apparently over-represented. Additionally, it is not uncommon for dogs to have both interdigital follicular cysts and allergic dermatitis with secondary bacterial interdigital furunculosis.
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