Diseases of the Foot and Nails: Humans, Cats, and Dogs HERBERT S. FREIMAN, VMD LEE S. GRUBELICH, DPM

T he human foot, as differentiated from that of arboreal ancestors, has evolved to function in locomotion; however, in dogs and cats, the paws not only function in locomotion, but also

aid in defense and food gathering. The plantar foot of dogs and cats, in contrast to that of humans, has a medial longitudinal arch culminating in weight bearing on the metatarsophalangeal joints, as well as a digital arch. Both animal arches have plantar pads. These pads are thickly keratinized. The epidermis of the foot pad of the adult dog is about 1800 pm thick.’ The foot pads of most dogs are thick and rubbery to the touch, whereas cats have softer, more pliable pads.

Although humans in undeveloped countries go bare- foot and acquire substantial keratinization of the sole, people in Western cultures wear shoe gear. Cats and dogs almost always go barefoot. Dogs that are required to per- form arduous work over inhospitable terrain, such as arc- tic sled dogs, may be fitted with specially constructed booties. The enclosed human foot is subject to infection by fungi, whereas the unprotected animal foot is vulner- able to trauma and penetration by foreign bodies.

Nail structure in humans also differs from that of the animal claw. The human nail plate is thin (0.05 - 1 .O mm) and relatively flat; the dog’s claw is much thicker, stiffer, curved, and of sufficient hardness to serve as a traction- gaining device. In contrast to the fixed claws of dogs, cats have curved, very pointed, retractable claws which can serve as defensive weapons, prey-catching devices, and climbing hooks.2 (The cheetah, having nonretractable claws, is an exception to this rule.) The propensity exhib- ited by domestic cats to exercise their claws on sofas and drapes sometimes leads their owners to request onychec-

From Apple Vulley Veterinarians, Plantsville, Connecticut, and College of Podiatric Medicine and Surgery, Des Moines, lowa.

Address correspondence to Herbert S. Freiman, VMD, Apple Valley Vet- erinarians, 1218 South Main Street, Plantsville, CT 06030.

0 1994 by Elsevier Science Inc. l 0738-081x/94/$7.00

tomy of the front paws. Recent behavioral studies of sur- gically declawed cats show that this procedure does not have any negative effects on personality or activity. The ability to keep a pet cat confined indoors when the out- side environment poses risks because of traffic or other hazards seems to justify the procedure.3

Onychocryptosis

Onychocryptosis is common in humans and is caused by direct trauma such as bathroom surgery, tight shoes, and support hosiery. One or more of these factors causes inci- sion of the digital skin by a nail border with resultant pain and inflammation, most often affecting the great toe. The usual organisms found on laboratory culture are Staphy- lococcus epidermidis and Staphylococcus aureus. Treatment consists of incision and drainage, removal of the offend- ing nail fragment, local antisepsis with a dilute chlorhexi- dine scrub, topical antibiosis (with such agents as mupir- ocin and silver sulfadiazine), and sometimes oral antibiosis.’ Strictly analogous conditions do not occur in canines and felines, because their claws project distally from the nail bed and, being cantilevered, do not impinge on the digit.

Onychogryphosis

Human nails can become grossly deformed leading to onychogryphosis, also known as Ram’s horn or nail. In this condition, the nail twists, contorts, and grows in a plantar direction. Hypertrophy, which can be startling at times, is present. Causes include localized trauma to the nail matrix, fungal infection, shoe-gear pressure, and sys- temic diseases. These grossly deformed nails, if neglected, can impinge on soft tissue in two ways. First, hypertrophy begins at the matrix and extends laterally and medially; then, if the nail is not treated, hypertrophic growth can continue forward and plantarly until the leading edge

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effecting nail removal is destruction of the matrix with phenol and alcohol.4 In dogs and cats, the offending nail is “removed” by amputating the distal phalanx proximal to the nail fold. The incision can be left to heal by second- ary intention, sutured, or glued closed.

Calluses

Although plantar calluses are very commonly seen on human feet, they are not significant problems for dogs or cats. One author (HSF) has seen declawed cats with tiny calloused areas on the distal aspect of the digital pads of the forepaws. Affected cats are generally very prone to use their front paws to touch and manipulate objects; their calluses are analagous to work-induced calluses on human hands.

Figure 1. Canine dewclaw showing incurved growth pattern with digital penetration and pyogenic granuloma.

incises the digit, the plantar sole, an adjacent digit, or even the dorsal surface of the foot.5 Canine claws on the weight-bearing digits sometimes become deformed, but rarely extend into the substance of the toe. The first digit is always present on front paws, but it is rudimentary and non-weight bearing. (Note: Some breeders of show dogs routinely amputate these digits when the pup is less than a week old because of “aesthetics.“) Non-weight-bearing rudimentary digits sometimes found on the rear paws are known as dewclaws. These claws, which are not subject to wear, may grow in a semicircular fashion that forces the apex of the claw into the soft tissue of the toe (Fig. 1).

Felines also have rudimentary Brst digits on their fore- paws and can have rear dewclaws. In addition, cats are more likely than dogs to exhibit polydactyly, with either normal-appearing supernumerary digits or microdigits which may be superimposed on one or more toewebs. Extra digits found between toes 2 and 3 are often asso- ciated with penetration of the toeweb. Digital penetration of weight-bearing toes as a result of direct trauma or infection at the germinal area of the nail fold causing an incurved growth pattern occurs more often in the cat than in the dog. The result of digital penetration by a claw in either species is a cavitated lesion with pyogenic granu- loma, similar to the clinical presentation in humans. Re- moval of the penetrating portion of the offending claw followed by oral antibiosis results in a cure. The prognosis for recurrence is great unless the nail is routinely debrided or totally removed. In humans, the common method of

Paronychia

Paronychia can be found in both dogs and cats, but it is not often found in animals as the sole dermatologic com- plaint. One common cause is penetration of the nail fold by a foreign body with secondary bacterial infection. We also find fungal infections of the nail fold as well as inva- sion of the fold by the yeast Malassezia pachydermatis. Stained smears of periungual exudate can aid in making the diagnosis, especially when the bilobed Malassezia or- ganisms are involved. In recurrent paronychia, especially when associated with any dermatitis of the foot (podo- dermatitis), immunomodulating factors that are affecting the host response to the organism should be ruled out.687

Nail bed tumors are rare in dogs and cats, but they can mimic paronychia with pyogenic granuloma. Both squa- mous cell carcinoma and melanoma can present as lesions localized to the nail fold. In fact, the digits are one of the most frequent sites for the occurrence of malignant mela- noma in these species. Melanomas can arise from the digit per se or the the nail fold. Several breeds of dogs have a predilection for melanoma formation: Black Cocker Spaniels, Scottish Terriers, Boston Terriers, Irish Setters, Doberman Pinschers, Boxers, Springer Spaniels, Chow Chows, Chihuahuas, and Airedale Terriers.s

In humans, approximately 5% of melanomas occur in the foot as a primary site. Fifty percent of these primary pedal melanomas arise on the subungual hallux and the plantar sole. Blacks have a lower incidence of malignant melanoma than whites; however, the foot remains an important site for melanoma formation in blacks.9J0

Fibrosarcoma and mast cell tumors, especially malig- nant mast cell tumors, in animals are less likely to be mistaken for paronychia, as they usually involve the en-

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tire distal toe or toeweb, rather than a discrete portion of the nail fold.*

The Red Foot

When dogs and cats experience unpleasant cutaneous sensations, they often react by licking or chewing the offending area. The paws, especially the front paws, are easily reached and groomed in both health and disease. Atopy is a co mmmon, probably inherited, cause of pruri- tus in the dog triggered by type I hypersensitivity to in- haled allergens. When suffering from an allergy attack, dogs will characteristically rub their faces and lick their paws. l1 These cases often present with red, inflamed feet secondary to intense licking or chewing. Atopy in the cat is not as well defined, but excessive grooming of the trunk rather than the paws is more common in this species.

For dogs that present with red feet or pedal alopecia, deep skin scrapings should be scanned for evidence of the mite Demodex canis. If the clinician has a high index of suspicion for demodicosis, but skin scrapings are nega- tive, a punch biopsy sample might confirm the diagnosis. This microscopic parasite of animal skin can present with signs exclusively localized to the paws. Generalized de- modicosis, when chronically established, may retain pedal manifestations of disease even after clinical cures have been effected on the rest of the body.12

Feline demodicosis is caused by Demodex cati and De- modex X. Demodicosis is rare in cats, is not usually as severe as it is in dogs, and is primarily a disease of other body areas rather than the paws.13

Another entity with a parasitic eitiology causing red feet is hookworm dermatitis. Larval migration of Ancylo- stoma spp or Uncinaria spp can occur in dogs housed under unsanitary conditions in humid climates, such as the southeastern United States. Although any skin that frequently contacts the earth may be affected, lesions are most common on the paws. Erythematous papules are followed by lichenification and alopecia. Pad margins may become friable and may ulcerate. In chronic cases, a paronychia with erosive borders is also present. Pruritus is present but is not usually severe. Diagnosis is facilitated by a suggestive history, finding of hookworm eggs on fecal flotation, and suggestive dermatopathology. Biopsy specimens may be macerated and submitted for parasitic evaluation. Similar syndromes can be caused by other parasites, such as Sfrongyloides stercorulis.*

Cutaneous larvae migrans in humans, or creeping eruption, can also be caused by several species of parasitic larvae, among them Ancylostoma spp. The mode of infec- tion is similar in both human and animal; because of the lack of haircoat on human feet, observed pedal manifes- tations are different. In humans, one can discern ery-

thematous, pruritic papules, which take 48 to 72 hours to evolve into serpiginous tunnels at the dermoepidermal junction. The larvae migrate in linear fashion and precede inflammation by 1 to 2 cm. Lesions are pruritic and may develop impetiginization caused by scratching.14

Uncommon but often dramatic causes of red feet in dogs are pemphigus vulgaris and pemphigus foliaceous. These diseases are discussed more fully under the um- brella of exfoliative disease.

Ulcerative Disease

Plasma cell pododermatitis is an interesting condition that is found in cats. Although the etiology is unknown, an immune origin can be inferred from the presence of plasma cells in the lesions and demonstration in many affected cats of hypergammaglobulinemia. The main foot pads are affected more often than the toes, and either carpal or tarsal pads can be involved. Sometimes only one foot is involved. Affected pads at first manifest spongy swelling followed by ulceration. The response of the in- dividual cat is often unique. Some cats seem to feel pruri- tic as evidenced by licking and biting of the swollen pads; other cats are less concerned by the lesions than their owners; still others experience pain and are reluctant to bear weight on the affected paw(s). Biopsy is needed for a definitive diagnosis, but caution is advised, as biopsy sites on the weight-bearing surface of the pad may not heal well. Excision of specimens should be done on a pad margin if possible.15

Exfoliative Disease

The pemphigus complex is a rare cause of skin disease in both the dog and the cat. Pemphigus foliaceous is the most common autoimmune disease in these species and pemphigus vulgaris is the most debilitating. When the feet are affected with pemphigus foliaceous, hyperkera- to& of the pads results. Crusted plaques peel away, sometimes leaving normal-appearing epithelium and at other times resulting in superficial ulcers. In contrast, pemphigus vulgaris usually results in large sloughs which produce painful ulcers on the pads of the feet. In addition, pedal erythema and nail bed ulcerations often accom- pany pad lesions in vulgaris. As an additional diagnostic aid, mucosal lesions are common in pemphigus vulgariss

In addition to the autoimmune disorders, four syn- dromes are discussed here under the heading of exfolia- tive disease.

Idiopathic hyperkeratosis of the nose and foot pads is seen most often in cocker spaniels and sometimes in other breeds, especially when they become old. The diagnosis is made by excluding other entities such as pemphigus, sys-

576 FXEIMAN AND GRUBELICH

temic lupus erythematosus, and hypothyroidism. Al- though usually asymptomatic, deeply fissured pads may become quite painful, limiting ambulation. Moisturizers are usually adequate for treatment of this disorder. In some old dogs, “feathery” keratogenous projections may form on pad margins; these can be trimmed with scissors for aesthetic reasons as well as to make some dogs more comfortable.

An uncommon but interesting condition is acroderma- titis enteropathica of bull terriers. This breed-specific dis- ease is inherited as an autosomal recessive characteristic that affects zinc absorption and metabolism; indeed, this syndrome resembles true zinc deficiency disease. Af- fected dogs, always white bull terriers, often die before their first birthday. Oral zinc supplementation is not ef- fective in reversing the syndrome. Although acroderma- titis is a metabolic disease that has widespread dermato- logic manifestations, we are particularly interested in the lesions seen on the paws. The toes are often splayed, and the foot pads exhibit abnormal keratinization character- ized by villous hypertrophy, fissures, and exfoliation. These dogs usually have paronychia and often have claws that are dystrophic.*6

In humans, acrodermatitis enteropathica occurs as a result of genetic disease impeding zinc metabolism. The condition affects the distal feet with vesiculobullous and erythematosquamous lesions.”

Actual zinc deficiency may be due to “generic dog food syndrome,“16 where the diet is actually low in zinc. One may also see genetically induced disorders of zinc metab- olism in young pups or in adults of certain breeds, most notably Siberian huskies and Alaskan malamutes. The foot pads are hyperkeratotic and fissured in these disor- ders, although the most striking manifestation in sled dog breeds usually involves alopecia and crusting on the face. Supplementation with oral zinc salts is successful in con- trolling symptoms. l9

Superficial necrolytic dermatitis (SND) can resemble the hyperkeratotic diseases discussed already. It is analo- gous to glucagonoma syndrome of humans. Lesions occur on pressure points such as the elbows, on the various mucocutaneous junctions of the body (especially the face), and also on the foot pads. The foot pads exhibit fissures, with exudation, crusts, erosions, and ulcers. SND is seen most often in old dogs and is of interest because it serves as a marker of internal disease. The syndrome can be associated with diabetes mellitus, liver disease, or glucagonomas. Regardless of the primary cause, the metabolic consequence of the disease leaves the patient with depressed levels of plasma amino acids. It is this amino acid deficiency that seems to cause the skin lesions. The diagnosis is made on clinical grounds when typical dermatologic findings coexist with elevated

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plasma glucagon levels or evidence of either pancreatic endocrine dysfunction or liver disease. Biopsy specimens should include undisturbed crusts; thus areas other than the foot are more likely to yield diagnostic samples.8

Another dermatologic entity affecting the foot that occurs in conjunction with internal disease is pyoderma gangrenosum of humans. Pyoderma gangrenosum has strong associations with inflammatory bowel disease, paraproteinemia, hepatitis, rheumatoid arthritis, and polycythemia Vera. The clinical manifestation is a pustule evolving into a large, painful necrotic ulcer. Immuno- pathology can show positive vascular fluorescence, IgM, and C3 in areas of lymphocytic vasculitis. 2o We believe that this diagnosis may well be made in dogs as more immunohistopathologic studies are done in this species.

Conclusions

The feet of dogs and cats differ from human feet quite strikingly both biomechanically and histopathologically, yet they share a number of similar dermatologic dis- orders.

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