Differential diagnoses for canine pododermatitis (Proceedings)

Introduction

Pododermatitis, by definition, is the inflammation and/or infection of the skin and connective tissue of the foot. The word "pododermatitis" is not a diagnosis in itself. Rather, it is a word to describe a dermatologic reaction pattern.

A variety of different medical conditions can result in a dog developing pododermatitis. Some examples of these conditions include: allergic dermatitis, autoimmune skin disease, trauma with a secondary infection (bacterial and/or fungal), demodicosis, neoplasia and idiopathic. The outcome and prognosis for idiopathic causes for pododermatitis is questionable. Therefore, it is important for the veterinarian to thoroughly work up and find the underlying cause for the pododermatitis. The clinical signs, differential diagnoses, diagnostic tests, treatment and prognosis for each of these different underlying etiologies will be discussed.

Clinical signs

No specific age, sex or breed predispositions exist for dog developing pododermatitis. However, certain breeds of dogs appear to be over represented in the literature. These breeds include: Boxers, Bulldogs, Bull terriers, German short-haired pointers, German shepherd, Golden retriever, and Irish setter. Some of the more common clinical signs associated with pododermatitis are alopecia, erythema, swelling, salivary staining, hyperpigmentation, pyoderma, nodules, hemorrhagic bullae, thickening of the skin between the webs of the feet, draining tracts, and ulcers. Nodules and hemorrhagic bullae lesions tend to rupture open and drain. This causes discomfort for the dog and most dogs will lick these ruptured draining lesions or tracts. Severely affected dogs are reluctant to walk on the affect foot or feet. The presence of depression and inappetance may be present when systemic disease is present.

Etiology

As previously stated, canine pododermatitis has a wide variety of underlying etiologies. Two of the more common causes for canine pododermatitis seen by veterinary dermatologists are allergic dermatitis with or without a secondary infection and demodicosis. The three allergies that can cause pododermatitis are atopy, food allergy and contact allergy. Usually dogs with an underlying allergy have skin lesions on other areas of their body so it is important to do a through physical examination. In addition, age of onset can be useful for determining whether allergies, especially atopic dermatitis, is a possibility (usual age of onset 1 to 3 years, may be as young as 6 months or as old as 5 years old). Demodicosis differs from allergies in that demodex can be present on areas of the body other than the feet or this parasite cause lesions which are limited to the feet. Therefore, demodicosis should always be on the differential diagnosis list.

Parasites other than demodicosis have been reported to cause pododermatitis. These other parasites include hookworm dermatitis, Pelodera dermatitis and tick infestation.

Other causes for pododermatitis include: cutaneous drug reactions, hepatocutaneous syndrome, digital hyperkeratosis, nodular dermatofibrosis, acral lick mutilation, autoimmune (pemphigus foliaceous, pemphigus erythematosus (rare), pemphigus vulgaris, bullous pemphigoid), trauma with a secondary infection (either bacterial or fungal or both), neoplasia (ie SCC, MCT, melanoma, mycosis fungoides or cutaneous lymphoma), and idiopathic.

Cutaneous drug reactions can cause papules, plaques, erosions and ulcers on the feet, pads and pad margin. Drugs that have been reported to cause such a reaction include sulfonamides, Beta lactams, antifungals (ie 5-fluorocytosine) and anthelmintics (ie levamisol, moxidectin). Discontinuation of the medication and treating any secondary infection if present results in resolution of these skin lesions.

Hepatocutaneous syndrome in the dog is most commonly a strange clinical presentation for advanced liver disease (especially hepatic cirrhosis). Hyperglucagonanemia is occasionally found. Skin lesions seen with this problem include erythema, crusting, ulcerations of the face, muzzle, genitalia, distal limbs and footpads. Hyperkeratosis and ulceration of the foot pads may be seen. Systemic signs noted with this condition are weight loss, inappetence, and lethargy. Histopathologic findings for this syndrome are classic and highly characteristic. These histological signs are a superficial perivascular to lichenoid dermatitis, parakeratotic hyperkeratosis, and a marked intra- and intercellular edema limited to the upper half of the dermis. These histologic findings appear as a red, white and blue coloring band within epidermis.

Digital hyperkeratosis with or without involvement of the nasal planum is a unique hyperkeratotic disorder. The footpads appear vegetative, ridged or even feathered in appearance. These pads may fissure and become painful. This can be seen in middle aged or young dogs (6 months – 2 years). The syndrome in young dogs appears to be a problem in Kerry blue terriers and Irish terriers. This syndrome in these terrier breeds is called "familial footpad hyperkeratosis" and is thought to have an autosomal recessive mode of inheritance. In the later syndrome, all footpads are affected.

Nodular dermatofibrosis syndrome is a rare condition reported in German shepherds, Boxers and various mixed breed dogs. This syndrome manifests itself as nodules at several sites on the body (including feet). On histopathology, the lesional skin demonstrates the presence of randomly arranged bundles of dense collagenous tissue. Histologically these skin lesions are described as being classified as collagenous nodules or nevi. An autosomal dominant mode of inheritance has been reported in the German shepherd. Most of these affected German shepherds have a concurrent renal cyst adenocarcinoma.

Psychogenic and neurologic diseases can also cause a pododermatitis. The two primary diseases are acral mutilation syndrome and acral lick dermatitis. Acral mutilation syndrome or acral lick mutilation syndrome is a rarely reported disease of English Springer spaniels, German short-haired pointers and English pointers. A sensory neuropathy is also involved with this syndrome. Abnormalities in the spinal cord, spinal roots, ganglia and peripheral nerves have been reported. The mode of inheritance appears to be autosomal recessive. Clinical signs usually develop within the first few months of life and include severe licking and biting of the feet followed by analgesia and loss of temperature sensation in the toes. The hind legs are more severely affected. The affected toes are swollen and ulcerated. Auto-amputation of the toes may occur. Histopathologically these dogs have degeneration of the myelinated and unmyelinated nerve fibers in spinal roots, ganglia and peripheral nerves. Acral lick dermatitis most commonly occurs in the carpus or tarsus areas. In some unusual cases, these dogs may chew on their feet and induce lesions. The skin lesions most commonly seen with acral lick dermatitis are ulcerations and a raised, firm circular to oblong lesion that has a thickened border to it. This condition can be diagnosed by history, physical examination and skin biopsy findings. A history or boredom or some change in the home environment is a typical history for cases of psychogenically induced acral lick dermatitis. One or more but not all feet and/or legs are typically affected with acral lick dermatitis.

Autoimmune skin disease can affect the feet and/or nails of dogs. The autoimmune skin diseases that can affect the skin and/or nails are pemphigus foliaceous, pemphigus erythematosus (rare), pemphigus vulgaris, systemic lupus erythematosus (rare), discoid lupus erythematosus and bullous pemphigoid.

Pododermatitis due to trauma with a secondary infection can occur. Typically these cases do not have all four feet involved. Normally one and possibly two feet are involved. Usually you will see puncture wound with a draining tract lesion in the area of the initial trauma. Sometimes foreign bodies such as grass awns can be present which result in skin trauma and if the foreign body is not removed then the infection (whether bacterial or fungal or both) fails to clear with treatment.

Cancer can also be a cause for canine pododermatitis. The three most common types of cancer that can affect feet and/or nails include: melanoma, squamous cell carcinoma and mast cell tumor. Each one of these tumors will be addressed separately.

Melanomas tend to occur in older dogs (average age for melanomas is 9 years old). Breeds that appear to be predisposed to developing melanomas are: Scottish terriers, Airedales, Boston terrier, Cock spaniels, Boxers, Golden retrievers, Irish Setters, Irish terriers, miniature Schnauzers, Doberman pinschers, Chihuahuas, Chow Chows. Melanomas are most commonly solitary lesions that are present on the head, limbs, digits (including the claw bed), scrotum, lip and trunk. However, Doberman pinschers and Irish setters tend to have multiple melanomas instead of single solitary lesion. Melanomas are variably circumscribed (well to poor), shaped (dome, plaque, polypoid) and colored (gray, brown and black) and range in size from 0.5 to 10 cm in diameter. Ulcerations are a common finding.

Squamous cell carcinoma (SCC) is another type of cancer that can affect the feet. The average age for SCC in dogs is 9 years old. Rarely have puppies been documented to have SCC. In general, breeds that appear to be predisposed to developing SCC include: Scottish terriers, Pekingese, Boxers, Poodles, and Norwegian Elkhounds. Specific dog breeds that have SCC of the claw bed are dark coated, large breed dogs such as Labrador retrievers, standard poodles, giant Schnauzers, Dachshunds, and Bouvier de Flandres. Solar induced SCC has been reported more frequently in white or piebald ventral coat and skin color (i.e. Dalmatian, American Staffordshire terrier, Bull terrier, and Beagle). Common skin lesion locations for SCC are trunk, limbs, digits, scrotum, lips, anus, and nose. The skin lesions are usually proliferative and ulceratative. The proliferative lesions can appear cauliflower-like. SCC lesions are usually solitary. However, multiple SCC on the trunk or claw has been reported in large, black-coated breed dogs.

Mycosis fungoides or cutaneous lymphoma can cause canine pododermatitis. Mycosis fungoides occurs in older dogs (average age of 9 to 11 years old). No breed or sex predilection exist. Footpad lesions include hyperkeratosis, ulceration, or depigmentation. Other areas of the body also have skin lesions. Affected dogs most commonly have a peripheral lymphadenopathy.

Idiopathic pododermatitis is a diagnosis made only after multiple diagnostic tests have been performed and an underlying etiology for the pododermatitis cannot be found. The incidence of this type of pododermatitis is thought to be low (approximately 0.5% of one dermatology referral practice, time period 1992-2005).1 Although poorly understood, underlying causes for idiopathic pododermatitis have been proposed and include: pedal conformation, trauma, immunosuppression, bacterial infection, furunculosis with dermal granuloma formation. The prognosis for this condition is guarded. Since bacteria most commonly secondarily invade the feet of these cases, prolonged antibacterial agents or bacterial autogenous vaccines have been recommended. Topical antibacterial agents such as benzoyl peroxide or Epsom salt soaks may be useful adjuvant therapies.

Diagnosis

A diagnosis for the underlying allergy for canine pododermatitis requires a thorough work-up. This work-up includes a detailed history, an in-depth physical examination and appropriate diagnostic tests.

Important historical questions would be whether any other pets in the household have skin lesions or similar skin lesions and the type of environment that the pet lives in (both indoor and outdoor environment). Additional useful information would include: the duration of the skin problems and whether the skin problem has been continual or whether it is seasonal. Drug history including response to any treatments used to treat the pododermatitis.

Many diagnostic options are available for working up a canine pododermatitis case. Skin cytologies should be performed on every case of pododermatitis. The cytologies are useful for determining whether an infection or cancer may be present. In the case of hypersensitivity or allergy induced pododermatitis a food trial, intradermal allergy testing, serum allergy testing (not as ideal) or patch testing may be necessary.

Bacterial and/or fungal cultures. Since these skin lesions are deep the bacterial cultures should be for both aerobic and anaerobic bacteria. If a higher bacteria is suspected or an organism is grown on culture but cannot be easily identified then a polymerase chain reaction (PCR) for some of the higher bacteria (i.e. Nocardia, Atypical mycobacteria) might be useful. Macerated tissue cultures are especially useful for obtaining a better representative sample of the bacteria that is present.

Blood work (CBC, serum chemistry, UA) would be beneficial for determining whether an internal or metabolic problem is present. Most typically a case where an internal or metabolic problem is present is when all 4 feet are affected. If hepatocutaneous syndrome is suspected then a bile acids and abdominal ultrasound would be useful. If an autoimmune condition is suspected then an ANA may be a useful backup diagnostic test in cases of lupus. If an underlying endocrinopathy is suspected then endocrine testing (i.e. thyroid panel, ACTH stimulation test, low dose dexamethasone suppression test, high dose dexamethasone suppression test, abdominal ultrasound) may be necessary.

In cases where tumor or infection is suspected, radiographs or the feet may be necessary if only one foot is affected or if the dog is clinically lame. This will help to determine whether any boney involvement is present.

Even though all of these tests are useful, the single most important test for pododermatitis is a skin biopsy. A minimum of 3 biopsy samples should be submitted to a dermatohistopathologist for evaluation. Immunohistochemical staining can of the biopsy samples can demonstrate whether any immunoglobulins (i.e. IgG, IgA, IgM ) or complement are present in the epidermis or at the basement membrane zone.

Treatment

The treatment for pododermatitis is based primarily on identifying the underlying etiology. Food and contact hypersensitivities require the avoidance of the offending allergen or allergens. Atopic dermatitis or atopy requires the control of these allergies with such medications as hyposensitization vaccines or immunotherapy, steroids, antihistamines, cyclosporine, and/or fatty acids.

Drug hypersensitivities or allergies results in the identification of the specific drug or drugs causing the adverse reaction and then the dog should avoid these drugs or drug.

Infectious causes for pododermatitis need to be treated with systemic antibiotics and/or anti-fungal medications. Bacterial infections usually need to be treated for a minimum of 6 weeks. Topical treatments are a useful adjuvant therapy when infectious agents are present.

Immune mediated causes involve immunosuppressive therapies (i.e. steroids or steroids with an immunosuppressive drug). The owner's need to be forewarned that the autoimmune condition will not go away and will require lifelong treatment with frequent monitoring.

The treatment for cancer induced pododermatitis will depend on the type of cancer which is present and the extent that the cancer has already metastasized to the body. Surgery, radiation, and chemotherapy are the types of therapies most typically used. A consult with a veterinary oncologist is advisable for these cases.

If an underlying internal or metabolic problem is present then this issue needs to be treated or controlled in order to get the skin lesions on the feet to resolve. If the dog has hepatocutaneous syndrome (superficial necrolytic dermatitis, superficial migratory erythema, metabolic epidermal necrosis, diabetic dermatopathy) as the underlying cause for the pododermatitis, the prognosis is grave. Treatments for SND include S-adenosylmethionine (sAME) denosyl 18 to 22 mg/kg PO, ursodiol (actigall) 10 mg/kg PO daily, Vitamin E 400 IU PO every 12 hourse. Dogs with liver fibrosis may benefit from colchicines 0.03 mg/kg PO every 24 hours. This medication may help to slow down the fibrosis. Parenteral amino acid supplementation is useful for improving the skin lesions and prolonging survival by several months. Either 10% crystalline amino acid solution (Aminosyn, Abbott Laboratories) 25 mg/kg IV can be administered by jugular vein catheterization over 6 to 8 hours or 3% amino acid electrolyte solution (Procalamine, Braun Medical Inc) 25 mL/kg IV can be administered by peripheral catherization over 8 hours. Cornification disorders may be treated with trimming of the excessive hyperkeratosis. Soaking of the pads and hydrating them before application of salicylic acid helps to slow down excessive tissue growth. Propylene glycol has been used as an emollient in excess cornfication disorders but this treatment is very messy.

Surgery is required for foreign bodies. In cases of severe pododermatitis which chronic furunculosis and scar tissue then a fusion podoplasty (surgical removal of the webs of the feet may be necessary.

If the pododermatitis is initially caused by trauma or repeated physical activities associated with athletic training then the dog's environment or training should be altered. Protective boots may be useful.

Conclusions/Comments

Many different causes for pododermatitis exist. A though history, physical examination are a vital start to have some idea as to which differential diagnoses is most likely. However, in order to provide definitive diagnoses diagnostic tests need to be performed. Only after a though work-up has been done will the clinician be able to develop an effective treatment plan or management protocol.

Selected Readings

1. Breathnach RM, Fanning SH, Mulcahy, et.al. Canine pododermatitis and idiopathic disease. The Vet J. 2008;176:146-157.

2. Moriello KA. Pododermatitis. Clin Brief. June 2008 ;(6)6: 11-14.

3. Scott DW, Miller WH, Griffin CE. Pedal Folliculitis and Furunculosis. In: Muller and Kirk's Small Animal Dermatology. Philadelphia: Saunders; 2001: 304-306.

4. Swaim SF, Lee AH, Mac Donald JM, et al. J of Am Anim Hosp Asoc. 1991; 27:264-274.