How to treat canine pododermatitis

Canine pododermatitis is a very common presentation in veterinary dermatology, but since it can result from many different causes it is important to have a logical and sequential diagnostic approach in order to successfully identify the primary disease responsible. With the correct diagnosis, clinical management becomes easier and more targeted. However, as with many dermatological cases, secondary infections and chronic skin changes frequently complicate the clinical picture, whatever the underlying disease, so it is always important to consider the primary, secondary and perpetuating factors for pododermatitis (Table 1). 

It is important to know what type of primary lesions are associated with each disease (e.g., papules, pustules, bullae). For example, contact dermatitis is associated with a primary papular eruption, so if the contact allergy is to either a carpet or a grass. pruritic papules on the palmar-plantar aspect of all four feet would be the expected clinical presentation 1. Other contact areas that are also frequently affected are the muzzle, perineal area and ventral abdomen (Figure 1) (Figure 2a) (Figure 2b).

An example in which the primary lesion is a pustule is pemphigus foliaceus. As pustules are fragile, many patients have crusts as remnants of dried-up pustules (Figure 3). Pemphigus foliaceus in dogs typically affects the face (as a “butterfly“ pattern affecting the periocular region, the bridge of the nose and nose itself) and the inner surface of the pinnae 2. Layers of dry pustules may be noticeable on the footpads (Figure 4), especially on the edges of the pads. 

Another extremely important primary cause of pododermatitis is Demodex mites. In fact, demodicosis should always be included on the list of differential diagnosis for canine pododermatitis 3, as this disease can manifest in a variety of ways. It can present with erythema and pruritus, and may look very similar to an allergic foot; many affected dogs also have a pruritic face and can easily be mistaken as being allergic (Figure 5). Because of this, any pruritic foot should be scraped for Demodex spp. before assuming that it is allergy and therapy such as glucocorticoids or oclacitinib is initiated. Comedones are another manifestation of demodicosis (Figure 6); they have a characteristic gray discoloration and are due to plugging of the hair follicles by large number of mites. Their presence should always alert clinicians to perform a skin scape, although if the foot is too swollen and painful (Figure 7), hair plucks may be considered, with the understanding that the sensitivity of hair plucks is inferior to a deep skin scrape. As the result of folliculitis, most (but not all) affected dogs present with hair loss. Interestingly, long-haired breeds such as Yorkshire and Maltese Terriers do not appear to develop alopecia as often as short-haired breeds. 

What else helps with diagnosis?

When considering other causes of pododermatitis, it is important to remember that some diseases involve both haired skin and footpads, while others do not. For example, atopic dermatitis affects only the haired skin, but autoimmune diseases like pemphigus foliaceus may also affect the footpads and present with crusting and hyperkeratosis. There are many differential diagnoses for pododermatitis and hyperkeratosis. One of the most important is superficial necrolytic dermatitis (SND), which targets both the footpads and other areas of the body such as the genitalia and the commissures of the mouth (Figure 8) (Figure 9) 4. This is a disease of geriatric animals and is linked to metabolic dysfunction and amino acid deficiency. The footpads in this disease show cracking and fissures, rather than the dry layers of pustules seen with pemphigus. The appearance of the lesions, the different distribution of lesions and the age of the patient are all clues which help the clinician to rank the diseases and prioritize between the likelihood of pemphigus foliaceus and SND.

For both diseases skin biopsy is diagnostic, and it is important to stress the importance of a definitive diagnosis by biopsy rather than simply relying on clinical impression, as the treatments are completely different. In pemphigus foliaceus acantholytic cells and superficial pustules are hallmarks of the disease, while parakeratosis, spongiosis and epidermal hyperplasia of the basal cell layer (“red-white-blue” layers) are considered characteristic for SND. In pemphigus, treatment with glucocorticoids and other immunosuppressive agent are the standard of care, whereas in SND glucocorticoids are usually contraindicated, as many affected dogs are either diabetic or borderline diabetic. In these cases it is crucial to investigate the underlying metabolic disease and to institute appropriate nutritional therapy with amino acids, zinc and essential fatty acids.

It is important to stress that acantholytic cells, traditionally considered a hallmark of pemphigus, can also be caused by other diseases such as contact allergy and dermatophytosis. In any disease where a severe neutrophilic inflammatory infiltrate develops, acantholysis may ensue as a consequence of the proteolytic effect of degenerating neutrophils. Since some cases of Trychophyton may resemble pemphigus foliaceus clinically (Figure 10) it is important to consider this as a differential diagnosis; misdiagnosis of dermatophytosis as pemphigus could be problematic, as glucocorticoids are inappropriate for ringworm cases. In these patients systemic antifungal therapy is necessary for many months, and itraconazole (5 mg/kg PO q24H) is commonly used as it concentrates in keratin and has residual activity after discontinuation of therapy. Terbinafine (20 mg/kg PO q12H) is also an excellent choice, due to its keratinophilic properties and its ability to persist in the keratin for extended periods of time.

What else can affect the feet? 

Other diseases that can affect the feet are syndromes such as vasculitis and erythema multiforme. Vasculitis is a type III hypersensitivity that can be due to many causes and is triggered by a variety of antigenic stimuli 5. The immune complex deposition may occur on the feet and ears, as well as other areas of the body. It may be initiated by drugs, vaccines, or infectious causes such as tick-borne diseases. The typical presentation on the feet is the presence of ulcers in the center of the footpads (Figure 11), the ulcer size varying depending on the severity and the size of the blood vessels affected. Diagnosis is based on clinical presentation and biopsy of an early lesion. It is important for the clinician to identify and treat (where possible) the underlying cause. Many of these cases require glucocorticoids at immunosuppressive doses in combination with pentoxifylline, and some patients necessitate prolonged treatment to completely extinguish the immunologic response. 

Erythema multiforme should also be included in the category of immune-mediated diseases that can cause pododermatitis. This is a clinical syndrome rather than a specific diagnosis, and again the clinician needs to identify the triggering disease in order to be successful with therapy. The classic lesions are erythematous macules with a paler central area (Figure 12); they may be found on many areas of the body, including the feet. It is important to take a thorough history of medications and vaccines, keeping in mind that drugs can trigger this type of cutaneous reaction, even if well tolerated previously. Definitive diagnosis is by biopsy, in which individual apoptotic cells are evident. Immunosuppressive treatments are typically prescribed while addressing the triggering cause.

Don’t ignore the nails!

Some causes of pododermatitis not only affect the haired skin but also the nails 6. Various changes to the normal nail structure may be noted (Table 2). Two classic examples are symmetric lupoid onychodystrophy and dermatophytosis (Figure 13). In parts of the world where Leishmania is present, this disease should be considered if onycogriphosis (hypertrophy and abnormal claw curvature) is noted. Table 3 offers a more complete list of diseases that can present with pododermatitis and affected nails. 

Symmetric lupoid onychodystrophy has been reported in Labradors, German Shepherds, Rottweilers and Boxers 7 and has some features of lupus, although affected dogs do not have systemic disease. Cases are usually young animals, with sudden-onset nail loss (Figure 14) which is associated with variable degrees of pain and pruritus. Paronychia may be seen. Secondary bacterial infections are common and contribute to the pain and pruritus. The natural course of the disease involves partial re-growth of friable, abnormal nails that continue to be sloughed. Blood work (i.e., complete blood cell count (CBC), chemistry panel and antinuclear antibody (ANA)) is unremarkable, and diagnosis is obtained by amputation of P3 and histopathology. Therapy includes the use of high doses of essential fatty acids or glucocorticoids. Tetracycline and niacinamide have been used as they have immunomodulatory properties, but improvement may not be seen for a couple of months. Pentoxifylline (15-20 mg/kg PO q8H, given with food to minimize GI problems) has been found to help in some cases; the improvement may be due to the multiple immune-modulatory properties of the drug. Secondary infections need to be addressed at the same time. In some cases this disease has been linked to a food reaction, and some dermatologists recommend an elimination trial to rule out the possibility of food as a precipitating factor. Removal of P3 and the claw on affected digits may be necessary in some cases.

What about chronic changes and perpetuating causes?

As noted earlier, whatever the primary cause of pododermatitis, scarring and foreign body reaction to free keratin in the dermis can develop due to infection and the destruction of hair follicles (furunculosis). The inflammatory response built against bacteria and fragments of hairs leads to swelling, pain and fibrosis over time (Figure 15). Some dogs tend to develop cystic lesions as the body attempts to wall off foreign material (Figure 16) 8. These nodules are frequently a source of recurrence as they can act as a starting point for new bouts of infection. Short-coated dogs are more prone to this type of reaction; it is believed that the short prickly hair in the interdigital spaces tend to mechanically drive the bacteria into the skin on the opposite side. These cases can be frustrating to treat and often require long courses of oral antibiotics and topical whirlpool therapy with antimicrobial agents like chlorhexidine or benzoyl peroxide. Culture and sensitivity is strongly encouraged in these cases to identify the most effective antibiotic, although clindamycin or fluoroquinolones are usually a good choice, as they achieve excellent penetration into the deep layers of the dermis. Additionally, many cases benefit from glucocorticoids and topical antibiotics such as mupirocin. Glucocorticoids can help decrease the fibrosis and excessive inflammatory response, which sometimes hinders the resolution of the pododermatitis. Soaking with agents that may facilitate the opening and draining of these nodules (e.g., magnesium sulfate) is also beneficial. In severe cases laser surgery 9 or podoplasty may be necessary. 

So what’s the diagnostic approach?

When considering the numerous causes of pododermatitis, an initial clinical approach should minimally include cytology, deep skin scraping and a fungal culture (Table 4). Cytology can be done with a tape or a swab depending on the condition of the skin. Dry skin is better evaluated with tape impressions, while areas of exudate are suitable for direct impressions or swabs. Samples are easily stained and analyzed for the presence and type of inflammatory infiltrate, the presence of bacteria and yeasts and possibly acantholytic cells. Diagnosis of fungal infections is made by Dermatophyte Test Medium (DTM) culture of the nails (shavings or clipping taken from the most proximal portions of the nail). 

Blood sampling (CBC and biochemistry panel) may be indicated in older dogs, and in particular if SND is a consideration, whilst the decision to biopsy or not will depend on the signalment, clinical signs and history. Bear in mind that some conditions (e.g., autoimmune or immune-mediated disease) will necessitate biopsy for diagnosis, whereas other conditions (e.g., allergic skin disease) cannot be diagnosed by biopsy. Based on the distribution and presence of pruritus, the clinician should then the rank differential diagnoses and formulate a diagnostic plan.

The clinical approach to pododermatitis requires good subject knowledge and accurate identification of primary, secondary and perpetuating factors. Omitting basic tests at the initial evaluation can mean common problems are overlooked, and appropriate treatment for secondary infections is essential. Ultimately, since many diseases can look similar, it is crucial to obtain a diagnosis rather than simply attempting to treat the clinical signs.