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IntroductionChronic pododermatitis with interdigital furunculosis is a common problem. It is very important to identify the underlying causes and manage these to reduce or prevent chronic changes. These increase the cost and complexity of treatment.Initially, dogs develop interdigital furunculosis – either individual lesions with draining sinus tracts (often incorrectly referred to as ‘interdigital cysts’) (figure 1) or more generalised furunculosis of the interdigital skin and pad margins (figure 2) (Duclos, 2013). It’s important to eliminate foreign body penetration in first-time solitary lesions but recurrent lesions need thorough investigation. Repeated lancing and antibiotic treatment otherwise leads to a cycle of chronic inflammation and infection (figure 3) (Duclos et al., 2008). The approach to chronic pododermatitis is similar to chronic otitis. The underlying triggers can be classified into primary, predisposing, perpetuating and secondary causes. These must be identified and managed for a good long term outcome. Primary causesThese are the conditions that trigger the initial interdigital inflammation (table 1) (Duclos, 2013). Many conditions can cause chronic pododermatitis but atopic dermatitis/adverse food reactions and abnormal conformation comprise 75% of the cases seen by the author (figure 4). Immunomodulatory-responsive pododermatitis is characterised by severe lymphocytic-plasmacytic pedal inflammation and secondary infection (Breathnach et al., 2010, Breathnach et al., 2006). However, it is unclear whether this is a primary problem or whether the inflammation is a chronic change (see perpetuating factors below). Predisposing causesThese rarely cause the disease but increase the risk of developing pododermatitis. Predisposing causes include breeds (e.g. French and English bulldogs, Staffordshire bull terriers, and mastiff-types) with short hairs around the pads and interdigital skin (which are more easily driven into the skin causing foreign body reactions and chronic inflammation), increased weight bearing (the mean body condition score in our cases is 6.6/9) and altered weight bearing (congenital limb deformity, osteoarthritis, cruciate disease and other joint problems).Perpetuating causesThese prevent resolution, inducing a self-perpetuating cycle of increased inflammation and altered pedal conformation (figure 3). Common changes include altered weight bearing, weight bearing on haired skin, lichenifcation, scarring, and chronic inflammation resulting in conjoined pads, new pad formation, deep tissue pockets, ingrown hairs and sinus tracts (figure 5) (Duclos et al., 2008). These can trap debris, encourage infection, and prevent effective cleaning and treatment. Scar tissue can decrease antibiotic penetration and reduce efficacy. Secondary causesDogs can develop a variety of surface, superficial and deep infections with organisms such as Staphylococcus pseudintermedius, Streptococcus, E. coli, Klebsiella, Pseudomonas and Malassezia (Duclos et al., 2008, Breathnach et al., 2011). There can be multiple infections at different depths, especially in chronic cases. Dogs will suffer from recurrent bacterial pododermatitis if the underlying triggers are not addressed and repeated antibiotic use will select for resistance.Approach to the diagnosis of recurrent or chronic pododermatitisHistory and clinical examinationA detailed history, and physical and dermatological examination is required to identify likely primary, predisposing and perpetuating problems. Examination of the paws may be painful and sedation may be necessary in some cases. Both the dorsal and palmar/plantar surfaces should be inspected, with the interdigital spaces carefully spread out.Table 1 – Clinical signs and their association with trigger factors in chronic pododermatitisErythema:Diffuse on both interdigital surfacesPlantar surfaces onlyFocal or macular-papularAtopic dermatitis and/or adverse food reactionContact dermatitisBacterial or yeast infection; demodicosis; hookworm dermatosisFollicular casts and/or comedonesDemodicosis Weight bearing on haired skin and ingrown hairsSingle paw or digit Trauma, foreign body, abnormal conformation, neoplasia or acral lick granulomasUneven pad or claw wear Limb deformities; altered conformation & weight bearingInterdigital furunculosis; sinus tractsInfection; foreign body reactionConjoined pads; new pad formationDeep tissue pocketsScar tissueChronic pathological changesJoint pain; reduced range of movementOsteoarthritis; other joint conditionsSaliva stainingLicking; pain or pruritusSelf-induced alopeciaSpontaneous alopeciaPruritusDemodex; dermatophytosis; endocrinopathyFurther InvestigationThis will depend on the differential diagnosis in each case. In most cases, hair plucks and skin scrapes should be used to help eliminate Demodex (Duclos, 2013). Impression smears of surface material and expressed exudates should be performed to confirm infections with Malassezia yeasts and/or bacteria. Bacterial culture and sensitivity is usually indicated, and is mandatory if there is deep infection, and/or there have been multiple antibiotic courses (Duclos, 2013, Duclos et al., 2008). Samples can be taken from ruptured pustules or draining sinus tracts, but biopsy is often necessary to isolate organisms from deep tissues. Biopsy and histopathology can be used to help rule out the presence of Demodex mites in chronic cases and confirm the diagnosis of chronic lymphocytic-plasmacytic pododermatitis. Hair plucks, cytology, fungal culture and PCR can be used if dermatophytosis is suspected. Radiographs, ultrasonography and/or advanced imaging are appropriate if a foreign body, osteomyelitis or neoplasia is suspected. Complete blood counts and serum biochemistry, thyroid function testing and urine analysis should be performed if systemic disease is likely. Regional lymph nodes are frequently enlarged and fine needle aspirates can differentiate reactive hyperplasia from lymphoma. A food diet trial using a novel protein or hydrolysed diet is often indicated. Management of primary and predisposing factorsIt is beyond the scope of this article to discuss the management of the primary and predisposing triggers for the pododermatitis in each case, but this is crucial for successful long term management. Obesity is a common predisposing factor (most of our cases have a body condition score >6/9). The forelimbs, which bear 60-70% of bodyweight, are most commonly affected. Excess body weight will increase this load and predispose to pododermatitis. Weight loss can be highly effective, and we often aim for a long term body condition score of 4/9. Other conditions causing pain, altered weight bearing and abnormal conformation should also be addressed. This may include using effective analgesia (e.g. NSAIDs, paracetamol/codeine, tramadol HCl, gabapentin and/or amantadine) or, in appropriate cases, corrective surgery. Boots that protect and support the paws (e.g. Ruffwear? boots) can be very helpful, especially with dogs reluctant to exercise. Lubricating creams (Sudocrem? or Bio Balm?) help soften interdigital calluses and can be used before exercise to reduce interdigital friction and trauma. Regardless of the cause, chronic inflammation, deep pyoderma, split pads and non-healing wounds can be very painful. Prompt analgesia and boots should be considered in all cases. Control of secondary infectionsControlling secondary infections is essential. Appropriate systemic antibiotic therapy should be based on culture and sensitivity results. Treatment may take up to 4-6 weeks and regular rechecks are required to assess the response to therapy. Scar tissue can inhibit the penetration of water soluble antibiotics (e.g. penicillins and cephalosporins) and lipid soluble antibiotics (e.g. clindamycin or fluoroquinolones) with a high volume of distribution and good penetration into scar tissue are beneficial. Dogs with a staphylococcal hypersensitivity or recurrent staphylococcal infections despite appropriate therapy may benefit from Staph Phage Lysate (SPL?) (DeBoer et al., 1990). The mechanism of action is unclear, but could involve inducing tolerance to staphylococci and staphylococcal proteins or enhancing anti-staphylococcal immunity. It appears to be well tolerated and safe for long term use. Many cases of Malassezia overgrowth can be treated using topical antifungal therapy but some require systemic therapy, especially if the dog has Malassezia hypersensitivity. Itraconazole or ketoconazole can be used at 5mg/Kg PO every 24 hours. Side effects include salivation, anorexia, vomiting and diarrhoea (although itraconazole is generally better tolerated than ketoconazole).Daily bathing or cleaning of the paws is effective for surface infections, and the frequency can be subsequently reduced for maintenance. Owners should be shown how to adequately wash the paws, making sure the digits are spread and any deep pockets or sinus tracts are thoroughly cleaned. Miconazole/chlorhexidine (Malaseb? or Adaxio?) or chlorhexidine (Microbex?, Douxo Pyo?, Chlorexyderm? or Hibiscrub?) are highly effective against a variety of micro-organisms, including MRSA, MRSP and Pseudomonas but require a 5 minute contact time before rinsing (Young et al., 2011). The paws should then be gently dried by blotting with a towel rather than rubbing. Washes can be alternated with chlorhexidine (CLX? or Douxo Pyo?) wipes in milder cases. These are easier to use, but have no residual activity on the skin and are not effective against Pseudomonas. Chlorhexidine foams, rinses (Douxo Pyo? and Chlorexyderm?) and gels (Clorexyderm?) have residual antimicrobial efficacy and don’t need rinsing but are less cleansing. Hypochlorous acid (Renasan? or Vetericyn VF?) can be used for daily cleaning and flushing (especially open wounds and sinus tracts), although it has no residual effect. Topical antibiotic therapy can be used following cleaning. Topical antibiotics are highly effective as they deliver mg/ml concentrations that are much higher than the minimum inhibitory concentrations (in ?g/ml). However, penetration can be variable and topical treatment is less suitable for deep infections. Useful topical antibiotics include fusidic acid (Fucidin? [not licenced in animals]; or in Isaderm? and Betafuse? combined with betamethasone), silver sulfadiazine (Flamazine?; this can be combined with amikacin, gentamicin or marbofloxacin off-licence), neomycin (Synalar N?; combined with fluocinolone [not licenced for animals]) or polymyxin B (Surolan?; combined with miconazole and prednisolone acetate). Anti-inflammatory treatmentAnti-inflammatory therapy should be initiated early to limit chronic inflammation and other perpetuating factors. Both systemic and topic anti-inflammatory therapies are often required.Systemic anti-inflammatory treatmentSystemic glucocorticoids are frequently used to reduce inflammation due to their low cost, ease of administration and effectiveness. However, treatment can be associated with adverse effects and patients on long-term steroid therapy need regular monitoring of blood pressure to check for hypertension and urine analysis to detect any urinary tract infections. Prednisolone or methyl-prednisolone (which may result in less PU/PD) at 1-2mg/Kg PO every 24 hours are generally used, but dexamethasone at 0.1-0.2 mg/Kg every 24 hours can be useful in more severe cases. However, the longer duration of activity makes it less suitable for long term every other day and dogs should be switched to prednisolone/methyl-prednisolone or twice weekly therapy as soon as possible. It can take 2-4 weeks to achieve remission at these doses; longer term control usually requires 0.4-1mg/Kg prednisolone every 48 hours (or the equivalent). Ciclosporin (Atopica?, Cyclavance?, Modulis? or Sporimune?) at 5-7.5 mg/Kg every 24 hours is often very effective and well tolerated. Long term treatment can be tapered to the lowest every other day or twice weekly dose that maintains remission. Oclacitinib (Apoquel?) and lokivetmab (Cytopoint?) are very effective for pruritus in dogs with atopic dermatitis. However, the chronic pedal inflammation in pododermatitis does not respond as well as to glucocorticoids or ciclosporin. In particular, the absence of pruritus can mask ongoing interdigital inflammation and infection. Topical anti-inflammatory treatmentTopical anti-inflammatory therapy is most commonly used in conjunction with systemic therapy to improve the clinical remission of inflammation, although it can be used alone in mild cases and to maintain remission. Once or twice day therapy is used initially, tapering the frequency as the inflammation reduces. Deep pockets of inflammation and scar tissue limit the efficacy of topical hydrocortisone aceponate (Cortavance? and Cortacare?). The rapid absorption and metabolism that make it a useful product for treating superficial inflammation mean that it is less effective with deeper inflammation. Nevertheless, it can be a useful product for maintenance treatment once the initial inflammation is controlled. More potent topical steroids such as 0.025% fluocinolone/neomycin (Synalar N?; not licensed for animals) or 1% betamethasone/fusidic acid (Isaderm? or Betafuse?) are usually required. Due to their high potency, clients must wear gloves when applying these preparations. Long term use can lead to local and systemic adverse effects including cutaneous atrophy. The topical calcinurin inhibitor tacrolimus (0.1% Protopic?; not licensed for animals) is beneficial and well tolerated. It should be used every 12-24 hours until remission (which usually takes 2-4 weeks) and then every 48-72 hours for maintenance. Cytotoxic drugsWhere inflammation cannot be fully controlled and/or there are adverse effects, glucocorticoids can be combined with cytotoxic drugs such as azathioprine, chlorambucil, mycophenolate or methotrexate (not licensed for animals). Regular monitoring of blood cell counts and serum biochemistry are mandatory. Blood samples should be taken every 1-2 weeks initially and then every 3-6 months depending on the outcome. Owners must be informed how to handle cytotoxic drugs safely. End stage diseaseExcision or fusion podoplasties may be required to allow a pain free quality of life in cases refractory to medical treatment. Extensive chronic and proliferative changes involving haired skin usually require scalpel or laser surgery. Excision or laser ablation surgery podoplasties are suitable where lesions are limited to the interdigital web. In the former, the interdigital skin and lesions are excised and the remaining skin is sutured along the digits and pads. The outcome is usually good and foot stability isn’t affected. With the latter, a CO2 or diode laser is used to excise and ablate the interdigital furuncles and sinus tracts. The treated skin is left to heal by secondary intention using topical antimicrobials to prevent infection (Duclos et al., 2008). In either technique, it is important to remove all the affected tissues to prevent recurrence. In a fusion podoplasty all the inflamed interdigital skin, new pads and other proliferative tissues are excised with the digits and pads sutured together (Basdani et al., 2010, Papazoglou et al., 2011). This is expensive, and can be associated with considerable post-operative pain, wound dehiscence and infections. However, a partial fusion podoplasty (figure 6) can be effective in dogs with lesions restricted to the medial or lateral interdigital spaces due to rubbing from abnormal paw conformation and weight bearing. Laser podoplasty offers less post-operative pain and quicker recovery (patients usually weight bear on the treated limb immediately after the procedure) (Duclos, 2006). A CO2 or diode laser is used to remove all the abnormal pads, other proliferative tissue, furuncles and sinus tracts. The interdigital skin surface is ablated back to normal tissue (figure 6). Lasers cut, ablate and coagulate tissues, resulting in less haemorrhage, pain and risk of infection (figure 7) (Duclos, 2006). Treated tissues take 3-4 weeks to granulate and heal. Post-operative treatment in most cases simply requires daily cleaning and topical antimicrobials. Light bandages with daily changes are used for the first 7-10 days, but these are not usually necessary once there is a healthy bed of granulation tissue. Boots can be used to protect the foot outdoors until fully healed. Recurrence is less likely following laser podoplasty, as hair follicles, follicular cysts and sinus tracts are ablated and replaced with scar tissue (figure 8). ConclusionsChronic pododermatitis is becoming more common, with the author is seeing an increasing number of cases. It is a frustrating problem that results in severe disease with a major impact on quality of life. There are usually multiple causes in each dog that need managing for a successful outcome. Dividing these into primary, secondary, predisposing and perpetuating causes provides a framework for diagnosis and treatment. Long term proactive treatment of the primary and predisposing problems is usually needed to maintain remission. Legends for figuresFigure 1 – Localised interdigital furunculosis in a Labrador. This is the end result of a sinus tract originating on the underside of the foot. This dog had elbow dysplasia resulting in altered foot placement and weight bearing.Figure 2 – Generalised interdigital furunculosis in an English bulldog with atopic dermatitis. Repeated interdigital inflammation and friction has led to furunculosis and secondary infection. Figure 3 – Cycle of inflammation and chronic changes in pododermatitis. Figure 4 – Primary causes in 83 dogs with chronic pododermatitis. AD = atopic dermatitis; AFR = adverse food reaction; IMPA = immune-mediated polyarthritis. Figure 5 - Palmar forepaw in an English bulldog with chronic pododermatitis. There is severe inflammation with weight bearing on haired skin, lichenification, hyperkeratosis, ingrown hairs, new pad formation and conjoined pads. There was an interdigital furuncle in the dorsal interdigital web between digits 3 and 4.Figure 6 – Partial fusion podoplasty in dog with recurrent interdigital furunculosis between digits 4 and 5 due to abnormal conformation and weight bearing. There has been some wound dehiscence but this went on to heal well. Figure 7 – Laser podoplasty immediately after surgery. The abnormal weight bearing tissues and interdigital sinus tracts have been excised and ablated. Figure 8 – Laser podoplasty after 5 weeks (same dog as in figure 7). The interdigital skin has fully healed leaving normal weight bearing surfaces. This dog still required weight management, bathing and topical 0.1% tacrolimus to maintain remission and prevent recurrence.ReferencesBASDANI, E., PAPAZOGLOU, L. G., FARMAKI, R., KOUTI, V. & KOUTINAS, A. F. 2010. Fusion podoplasty in a dog with inter-digital fibrosing furunculosis. Australian Veterinary Practitioner, 40, 53-57.BREATHNACH, R. M., FANNING, S., MULCAHY, G., BASSETT, H. F., JONES, B. R. & DALY, P. 2006. Evaluation of Th-1-like, Th-2-like and immunomodulatory cytokine mRNA expression in the skin of dogs with immunomodulatory-responsive lymphocytic-plasmacytic pododermatitis. Veterinary Dermatology, 17, 313-321.BREATHNACH, R. M., FANNING, S., MULCAHY, G., BASSETT, H. F., STROBL, E. & JONES, B. R. 2010. Cutaneous infiltrates and peripheral blood immune responses in dogs with immunomodulatory-responsive lymphocytic-plasmacytic pododermatitis. Veterinary Dermatology, 21, 383-392.BREATHNACH, R. M., QUINN, P. J., BAKER, K. P., MCGEADY, T., STROBL, E., ABBOTT, Y. & JONES, B. R. 2011. Association between skin surface pH, temperature and Staphylococcus pseudintermedius in dogs with immunomodulatory-responsive lymphocytic-plasmacytic pododermatitis. Veterinary Dermatology, 22, 312-318.DEBOER, D. J., MORIELLO, K. A., THOMAS, C. B. & SCHULTZ, K. T. 1990. Evaluation of a commercial staphylococcal bacterin for management of idiopathic recurrent superficial pyoderma in dogs. American Journal of Veterinary Research, 51, 636-639.DUCLOS, D. 2006. Lasers in veterinary dermatology. Veterinary Clinics of North America - Small Animal Practice, 36, 15. DUCLOS, D. 2013. Canine pododermatitis. Veterinary Clinics of North America - Small Animal Practice, 43, 57-87.DUCLOS, D. D., HARGIS, A. M. & HANLEY, P. W. 2008. Pathogenesis of canine interdigital palmar and plantar comedones and follicular cysts, and their response to laser surgery. Veterinary Dermatology, 19, 134-141.PAPAZOGLOU, L. G., ELLISON, G. W., FARESE, J. P., BELLAH, J. R., COOMER, A. R. & LEWIS, D. D. 2011. Fusion podoplasty for the management of chronic pedal conditions in seven dogs and one cat. Journal of the American Animal Hospital Association, 47, E199-E205.YOUNG, R., BUCKLEY, L., MCEWAN, N. A. & NUTTALL, T. J. 2011. Comparative in vitro efficacy of antimicrobial shampoos: a pilot study. Veterinary Dermatology, 23, 36-e8. ................
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