Diseases of the digit are relatively common and are particularly frustrating in terms of therapy. Unlike many other areas of skin, persisting diseases of the digit will almost always require biopsy to distinguish among a very long list of radically different etiologic possibilities. One cannot tell, just by looking at it, whether the digital swelling is chronic inflammation or squamous cell carcinoma, or whether the lump on the side of the digit is a harmless plasmacytoma or a potentially fatal amelanotic malignant melanoma.

1. Nails that are brittle, deformed, or fall out:
Most textbooks will provide a very long list of diseases of the nail and nail bed, but in practical terms I see only one syndrome: lupoid onychodystrophy. Bacterial and fungal paronychia, for example, is so rare in my collection that I have some skepticism that it even exists! The syndrome of lupoid onychodystrophy is seen in young mature dogs (1-5 years), and these animals present with a complaint of deformed nails that are periodically lost. The disease affects multiple digits on multiple feet, often eventually affecting all nails on all feet. The lesion is a lupus-like destructive disease of the basal cells of the nail bed epithelium. The only way to obtain a suitable histologic sampling of that epithelium is to do a P₃ amputation. Many of you have tried heroic alternatives with all kinds of digit-sparing punch biopsy techniques, attempting to capture that epithelium, but none has been successful. Sending the shed nail itself is a complete waste of time. The diagnosis is worth making because it has specific therapeutic implications. While I am somewhat skeptical because the list of therapies claimed to be successful is quite long, these dogs will generally respond to protocols of immunosuppressive corticosteroids or even just to prolonged supplementation with fatty acids. Suggested protocols include routine dosages of essential fatty acid supplements for at least 3 months or prednisone at immunosuppressive dosages for 2 weeks, then tapering to 0.5 mg/kg every 48 hours.

2. Interdigital proliferative cellulitis:
This very familiar syndrome is one of multifocal coalescing proliferative interdigital nodules. The histopathology is very repetitive: the nodules consist of deeply embedded fragments of hair which create persisting suppurating granulomas, as well as fibrosis and dysplastic proliferation of hair follicles and sebaceous glands. These are commonly submitted as possible interdigital tumors, but the interdigital area is actually a very infrequent target for any neoplasm. Occasionally, I will see Demodex or ringworm as a trigger for the follicular rupture, but most cases have no proven etiology or pathogenesis. To me, this disease is very much a classical chicken-and-egg problem: is the primary disease something like interdigital seborrhea or atopy, which triggers a cycle of licking, folliculitis, follicular rupture, and then more more licking? Or is it a purely mechanical issue, with a combination of foot configuration and stout short hairs predisposing to the mechanical implantation of hair as the dog walks?

3. Skin nodules on the dorsal/lateral digit:
Like anywhere else in the skin, the digit can be a target for cutaneous neoplasia. The most common digital tumors by far are typical benign cutaneous histiocytomas in young dogs, cutaneous plasmacytomas, and melanomas. The melanomas may or may not be pigmented. Unlike elsewhere in skin (where melanomas are almost all benign), about 50% of melanomas arising from the nail bed region are behaviorally malignant and undergo widespread metastasis. The diagnosis requires cytology and/or histopathology, and cannot be made just on clinical grounds.

4. Persistently swollen, painful digit:
This is a very common history for me to receive. Ordinarily, the digit has been subjected to prolonged antibiotic therapy with minimal response. Radiographs will usually show osteolysis, and the usual sample that I receive is a digital amputation with a differential diagnosis of chronic inflammation, osteomyelitis, or neoplasia. In over 90% of these cases, the histologic diagnosis will be well-differentiated subungual squamous cell carcinoma. This is a disease of the nail bed, not of the skin itself. The tumor will slowly grow into the digit, routinely creating osteolysis and suppuration.

The literature is quite contradictory in terms of prognosis. There are some papers that claim a 40% metastatic rate. My own data, gathered from Ontario practices, indicates a metastatic rate of less than 5%. I presume this discrepancy relates to the fact that early amputation is an important part of disease management. In other locations, most squamous cell carcinomas will eventually metastasize if the primary tumor is ignored; the behavior of digital tumors may follow the same pattern.

Before leaving this subject, I remind you that black standard poodles have a strong predilection for developing multiple digital squamous cell carcinomas. I have had some dogs that have lost 5-6 digits over a period of as many years. We occasionally see it in other large black dogs, notably giant schnauzers, bouviers, and black labradors. It is completely beyond me why being black should be a strong risk factor for this disease.

5. Ulcerative and exfoliative inflammatory diseases of the footpad:
There are three important and serious diseases that affect the bottom of the feet. The best known of these is probably pemphigus foliaceus, which is claimed to create discrete ulcers on the pads. Frankly, it is a diagnosis I make very infrequently in this particular clinical context, although pemphigus foliaceus itself is a relatively common diagnosis (but one based on vesicles on the ear pinna or in dogs with a generalized pustular disease). More prevalent, in my caseload, is cutaneous vasculitis giving rise to epidermal infarcts/ulcers. I will also see examples of hepatocutaneous syndrome (necrolytic migratory erythema) causing an exfoliative dermatitis characterized by extreme crusting and ulceration of the digital skin, particularly the ventral aspect of the foot. Liver disease is hardly ever clinically obvious, and may not even be biochemically detectable, at the onset of the skin disease. Most dogs will also have an exfoliative facial dermatitis that has been very resistant to routine therapeutic efforts. The biochemical basis linking liver failure with the development of skin lesions remains unknown.