Diseases that specifically target the claws are uncommon to rare. Those that involve the claw folds are quite common and are usually an extension from a more generalized pododermatitis. Most of these diseases do not significantly affect claw plate morphology.

Anatomy and Physiology

The claw of the dog and cat is a modified extension of the skin (both epidermis and dermis) covering phalanx 3 (P3). The dermis (quick, corium) covering P3 supports the epidermis on its outer surface which produces the keratinocytes that become the horn-like material called the claw (nail) plate and ventrally, the claw sole. The majority of claw plate growth occurs in the claw matrix, that area of the claw plate that extends up under the crescent-shaped dorsal ungual process at the proximal end of P3. Fastest claw growth occurs in the matrix area and central dorsal ridge of the claw, resulting in its downward curve. Claw growth is noted to average 1–2 mm/week. When claw plates are lost, time from the initiation of regrowth to the need for trimming may be 6–9 months. The claw fold is that fold of skin which covers the proximal dorsal and lateral portion of the claw.

Terminology

The most commonly used terms to describe pathological changes of the claw include:

 Paronychia: inflammation or infection of claw folds

 Onychodystrophy/onychodysplasia: abnormal claw formation

 Onychogryphosis: hypertrophy and abnormal curvature of claws

 Onycholysis: separation of claw attachment starting distally and progressing proximally

 Onychomadesis: sloughing of claws

 Onychomalacia: softening of claws

 Onychorrhexis: fragmentation and horizontal separation in lamellae at the free edge claws

 Onychomycosis: fungal infection of claws

Pathophysiology of Claw Disease

Damage to the claw matrix (e.g., trauma, inflammation) will result in abnormal rates of growth and/or the production of abnormal keratin which is clinically manifest as softer, drier, brittle and/or more friable claw plate that will more readily split. Abnormal curvatures may be produced by asymmetric growth patterns. Permanent damage to the matrix will result in permanent morphologic changes. Inflammatory diseases that involve the dermoepidermal junction may weaken this junction and manifest as sloughing of the claw plate (onychomadesis).

Diseases that affect one claw or multiple claws on one foot are defined as asymmetric and tend to be traumatic or due to infections (bacterial or fungal) or neoplasia. Those that involve several or all claws on all feet are defined as symmetric and are usually associated with systemic disease, infection secondary to systemic disease, autoimmune or immune-mediated disease, nutritional or congenital diseases.

It is often better to focus on the early signs of disease. For example, dogs with symmetric lupoid onychitis (SLO) start with onycholysis, rapidly progress to onychomadesis, and then end up with chronic onychodystrophy or onychomalacia. In other words, onychodystrophy is a common sequel to a variety of insults (trauma, infection, vasculitis, immune-mediated disease, etc.)

Bacterial Infection

Bacterial infections of the claw are secondary to some underlying local or systemic disease. Trauma (fracture, excessively short nail clipping, etc.) affecting only one or a few claws is most common. Osteomyelitis may develop in chronically affected claws - radiographs should be performed as P3 amputation is the treatment of choice when the bone is infected. Diagnosis is based on cytology (culture for refractory cases) - collect samples from under the claw if possible. Topical therapy may be sufficient to resolve the problem and includes dilute bleach (0.2% concentration - 1 oz or 30 ml in 1 l or 1 quart) or chlorhexidine (0.5% or higher), soaking the foot for 5 minutes q 12–24 h.

Fungal Infection

Fungal infections of the claw are rare. Dermatophytosis as a cause of symmetric claw disease in the dog or cat is exceedingly rare (i.e., where the claws are primarily involved on multiple feet). Onychomalacia is the primary clinical sign. Trichophyton mentagrophytes is usually isolated by fungal culture. Samples are best taken from the more proximal portion of the claw plate. Multiple samples should be obtained. Claw samples are submitted to a microbiology laboratory for culture or are fragmented (mortar and pestle or shave claw with a blade) and placed on a routine dermatophyte culture medium. Caution must be employed in interpreting results. It is possible to have transient contamination of the feet with Trichophyton sp. and Microsporum gypseum. Therapy for onychomycosis should generally be aggressive and may include itraconazole (5–10 mg/kg/day), fluconazole (5 mg/kg/day), ketoconazole (5–10 mg q 12 h) or terbinafine (30–40 mg/day) continued 1–3 months beyond healthy claw regrowth. Adjunctive topical therapies include topical miconazole, clotrimazole, or terbinafine q 12 h, or 0.2% enilconazole solution q 24 h.

Malassezia is a common secondary infection (usually secondary to atopy, food sensitivity) that affects the color of the claws. Affected individuals usually have some degree of paronychia with variable accumulation of brown waxy keratinaceous debris at the claw fold, and claws become discolored red-brown. Treatment for Malassezia paronychia is primarily topical as described above - medicated wipes are particularly helpful in removing excessive material and applying active ingredient to the area.

Autoimmune/Immune-Mediated Disease

When only the claw is involved, then immune-mediated diseases such as SLO (common), lupus erythematosus, and pemphigus vulgaris (both rare) should be considered. If paronychia and/or footpad involvement is present, then pemphigus foliaceus is of primary concern. Other immune-mediated diseases that are rarely reported to manifest with claw disease at times are cold-agglutinin disease, vasculitis and drug eruption - these tend to involve multiple claws and other skin lesions are typically also present.

Diagnosis is based on CBC, serum chemistry screening, urinalysis, ANA titer, determination of cold agglutinins, and P3 biopsy for histopathology. Therapy with immunosuppressive drugs is typically necessary (see discussion SLO)

Symmetric Lupoid Onychitis (SLO)

(aka symmetric lupoid onychodystrophy)

Symmetric lupoid onychitis is the most commonly encountered disease causing symmetric onychomadesis in dogs. Initially, there is an acute loss of the claws from one or two toes, followed by loss of more claws within days to weeks - typically all feet are affected, but not always. Pain, lameness, or the presence of claws in the house are the first clinical signs. Secondary bacterial infections are relatively common. Following acute claw loss, abnormal claws regrow that are short, soft, brittle, crumbling, and may be misshapen. If presented in the more chronic stages of the disease, the reason for presentation may be claw dystrophy.

Breeds predisposed include the German Shepherd dog, giant/standard/miniature schnauzer, Rottweiler, greyhound, Bearded Collie, Norwegian Gordon, and English Setter. The disease has also been seen in a number of other breeds. Age at onset is from 6 months to 11 years (mean and median 4–5 years).

The diagnosis of SLO is made on the basis of history, physical examination, claw biopsy (P3 amputation) and rule out. As SLO is the overwhelmingly most common cause of symmetric onychomadesis seen in the dog and because several therapeutic alternatives for this disease are relatively innocuous, therapy for this disease is often instituted without histologic confirmation. The histologic changes associated with this disease are lupus-like (hence the name SLO) although there are no other cutaneous or systemic manifestations of the disease. Symmetric lupoid onychitis is thought to be an idiopathic, immune-mediated disease in most individuals. It is important to note that this clinical presentation and histopathology have also been associated with other disease processes, including food sensitivity and bacterial infection. The histologic pattern is therefore more aptly considered a reaction pattern, associated with multiple underlying causes. It has also been suggested that the mechanical trauma of claw contact with the ground may predispose to or perpetuate the problem. This observation is supported by the fact that in some individuals, keeping nails trimmed back may significantly improve the problem.

Treatment of SLO

 Treat secondary infection with systemic antimicrobials.

 Under general anesthesia, remove loose or deformed claws - provide postoperative pain relief and bandaging.

 Tetracycline (500 mg/dog; < 10 kg body weight - 250 mg/dog; TID) or doxycycline (5–10 mg/kg BID) and niacinamide (dose as for tetracycline); trial period 3 months; once significant regrowth noted (4–6 months) decrease the frequency of administration of both drugs to once daily. If good response is maintained for 4–6 months on once-per-day therapy, treatment is stopped. Recurrence may occur - start therapy as previously.

 Fatty acids - variably effective but not used as monotherapy. Omega 3s, 50–60 mg/kg/day of combined EFA and DHA; improvement noted within 3–4 months, maximal response 8–12 months.

 Pentoxifylline is beneficial in 50–60% of cases; 15–25 mg/kg BID; 3-month trial; for patients experiencing normal regrowth, after 6–9 months of therapy, consider stopping medication.

 Glucocorticoids are effective but rarely necessary. Prednisone/prednisolone starting at 1–2 mg/kg/day; often used at the initiation of therapy (e.g., first 3 or 4 weeks), along with other drugs such as fatty acids or pentoxifylline or doxycycline/niacinamide - all of which have a slower onset of benefit to help reduce pain. It has been suggested that this more aggressive therapy, early in the disease may improve the overall prognosis.

 Cyclosporine therapy, starting at 5 mg/kg/day has been noted to benefit the problem.

 Refractory cases or patients who are intolerant of the above therapies can be managed with 20 nail, P3 amputations. Individuals generally do well without claws.

 Keeping the nails trimmed back to prevent contact with the ground may be of significant benefit.

Other Diseases Associated with Onychomalacia/Onychorrhexis

Dermatophytosis (rare), primary seborrhea (most commonly reported in cocker spaniels; more generalized signs of seborrhea present); ichthyosis; idiopathic nasodigital hyperkeratosis; senile change in old dogs; idiopathic (symmetric; seen in young dogs, 2–6 years, more common in dachshund, Siberian husky, Rhodesian ridgeback, German shepherd dog); acrodermatitis in bull terriers; zinc-responsive dermatitis (reported in two young malamutes with paronychia and onychorrhexis of all digits; histopathology suggestive of this diagnosis; responded to zinc sulfate therapy); linear epidermal nevus (present since birth; extended from the groin to two digits of the paw).

Therapies for onychodystrophies due to primary keratinizing disorders or those that are idiopathic include frequent nail clipping and filing; painting the claw plates with nail glue (to prevent fragmenting and "catching" on materials); nail caps (Soft Paws®). Although the use of biotin (5 mg/kg/day) or gelatin (10 grains [one capsule] every 12 hours/10 kg body weight; one packet of Knox gelatin per 7 kg q 24 h) has been reported, there are no data to substantiate their benefit.

Diseases characterized by onychogryphosis: Syndromes more commonly associated with this presentation include demodicosis, hookworm pododermatitis and leishmaniasis. Senile individuals also manifest this change.

Neoplasia

Squamous cell carcinoma, melanoma, mast cell tumor, keratoacanthoma, inverted papilloma, lymphosarcoma, eccrine adenocarcinoma, neurofibrosarcoma, hemangiopericytoma, fibrosarcoma, osteosarcoma, myxosarcoma and undifferentiated sarcoma have all been reported to involve the distal digit/claw and/or claw fold in the dog. Most cases only involve one toe (presented as digit swelling, paronychia, variable degrees of erosion/ulceration, pain, pruritus; claw may be dystrophic or absent; may see onychogryphosis (large claw).