Acral Lick Dermatitis

John M. MacDonald, Auburn, Alabama

Acral lick dermatitis (ALD) is a multifactorial condition characterized by excessive and compulsive licking at an area of the extremity that results in a firm, proliferative, erosive/ulcerative, alopecic plaque(s). This condition, also referred to as “lick granulomas” and “acral pruritic nodules,” remains one of the more challenging and frustrating problems seen by both the specialist and the private practicing veterinarian. One difficult aspect of the treatment is related to the multifactorial cause. Although environmental stress, such as boredom, confinement, loneliness, and separation anxiety, may contribute to the onset of the condition, this etiology appears to be less common than other factors. The location of the lesion by definition is on the extremity and appears to be equally distributed between the front and rear legs, most noticeably on the dorsal aspect of the carpus, metacarpus, tarsus, or metatarsus. Lesions usually begin with a small area of alopecia or a small nodule eventually progressing to larger lesions that in severely affected chronic cases may extend from the carpal/metacarpal area to the elbow or from the tarsus to near the stifle region. The prognosis for severely affected cases is poor at best unless a resolvable primary cause is found. Proliferation is the consequence of compulsive licking and chewing at the lesion, the expanding fibrosis of the lesion, and a coexisting infection. The expansion of the lesion may interfere with locomotion, often associated with the development of osseous changes that can include periostitis, bony proliferation, arthritis, and possibly infectious osteomyelitis. The cycle includes further self-mutilation as the lesion progresses with osseous involvement.

There is some variation related to the precise age of onset, which in some cases of chronic disease is difficult to discern. Some reports suggest a majority of cases occur in dogs less than 3 years of age, whereas other observations reflect an incidence of onset in older dogs (5 to 12 years). This may reflect the variability in the primary cause and rate of progression through complicating factors. Large-breed dogs (Doberman pinscher, rottweiler, Great Dane, golden retriever, Labrador retriever, German shepherd, and boxer) are commonly affected, although other breeds have been reported, including the dalmatian, English setter, shar pei, and weimaraner.

The development of ALD may have numerous underlying causes. Constant licking of the affected area results in hair loss and erosion of the skin. Exposure of the sensory nerve endings causes the site to become pruritic, and the dog licks the affected area in order to alleviate the itching sensation. This perpetuates a negative cycle, resulting in the development of an ulcerated skin lesion that cannot heal because of the constant licking. At some point in the development of the lesion(s) it becomes an ongoing chronic problem that requires maintenance management. By this time, unfortunately, only limited control is usually achievable, and euthanasia or limb amputation is selected. As with many cutaneous diseases, early recognition, diagnostic pursuit, and treatment are prerequisites to success.

Predisposing/Primary Factors

Although boredom and stress factors have been considered predominant causes of ALD, they may be less common than other primary factors (Web Boxes 33-1 and 33-2).

Web Box 33-2 Primary Causes and Perpetuating Factors of Acral Lick Dermatitis (ALD)*

Primary Causes	Perpetuating Factors
Allergic dermatitis	Bacterial infection
Arthropathies	Osteomyelitis
Foreign bodies	Keratin foreign bodies
Neuropathies	Periostitis
Trauma	Secondary arthritis
Neoplasia	Learned behavior
Mycotic infection	Fibrosis
Parasitoses (e.g., scabies)
Psychogenic factors

^*Factors relating to the development of acral lick dermatitis involve both primary causes that initiate the compulsive licking and perpetuating factors, which are complications of the condition that tend to keep the problem ongoing. Perpetuating factors by themselves do not initiate the disease but influence the protraction of the lesion, even when the primary cause has been resolved. Identification of all factors is essential for adequate treatment.

In some reports they have been estimated to represent as many as 50% to 70% of the cases, which may be an overestimate because psychogenic factors are difficult to validate and cannot be confirmed from pharmacologic therapeutic response. Breed association may provide some insight into the psychoneuropathologic origin of the problem. A coexisting problem or underlying disease is not evident in many Great Dane, Doberman pinscher, and rottweiler dogs affected by ALD; these cases may be manifestations of a primary neuropathologic abnormality. A condition in dogs and humans known as hereditary sensory neuropathy results in this behavior and has been associated with a substance P deficiency in the dorsal root ganglia of the spinal cord. Signs of hereditary sensory neuropathy usually begin when the dog is very young. Other neuropathies predisposing to ALD include spinal diseases associated with cervical vertebral instability or peripheral neuropathies of various causes including those following trauma. Central neuropathies have been recognized as a predisposing factor.

In contrast, many other breeds develop allergic dermatitis as the primary cause of the initial compulsive licking, the most common forms being canine atopic dermatitis (AD), cutaneous adverse food reaction (CAFR), and flea allergy dermatitis (FAD). Secondary bacterial folliculitis may occur as a component of the allergic dermatoses and is often associated with the acral area. The bacterial folliculitis progresses to furunculosis, which elicits further compulsive licking and chewing with the initiation and perpetuation of the “lick granuloma.” The final stage may represent a pyogranuloma. This is particularly true of the Labrador retriever, golden retriever, German shepherd, shar pei, and dalmatian. Historical information about early-onset pruritic disease may be helpful in defining the relationship between allergy and the evolution of these lesions. Early evidence of canine AD and CAFR may also include the occurrence of allergic otitis, often with an infectious component reported. CAFR has notably been related to the development of acral nodules and may result in an acute onset of aggressive pruritus and occur spontaneously in the older dog (>6 years), which is beyond the age of onset for the atopic dog unless a geographic or major habitat change has occurred.

Perpetuating factors may be as important as the primary cause and, if left untreated, ultimately result in failure to control the problem (see Web Box 33-2). One of the most important perpetuating factors involved in ALD is infection. The infectious agent most commonly isolated from lesions of ALD is Staphylococcus pseudintermedius, which usually starts as an area of folliculitis and progresses to furunculosis, with advancement to pyogranulomatous dermatitis. Deep tissue cultures of ALD lesions nearly always demonstrate infected tissue. It is not surprising to find methicillin-resistant staphylococcal infections as well. Treatment of bacterial infections should routinely be part of the treatment regimen, with antimicrobial selection based on culture and susceptibility from macerated biopsied tissue from the lesion(s) obtained under sterile conditions. Although lesions are not exudative, cultures of aseptically acquired tissue almost always reveal a staphylococcal organism and in chronic cases may include a gram-negative organism (Pseudomonas, Proteus, Escherichia coli spp.).

Licking or scratching perpetuates the infection in the absence of sufficient antibiotic therapy. Furthermore, chronic licking may cause endorphin release, which could lead to a chemical imbalance that ultimately produces a pleasure sensation and hence repetition of the licking. Regardless of the mechanisms involved, chronic licking, a learned behavior, may be a perpetuating factor that impedes resolution. Most ALD cases have both primary factors and consequential perpetuating factors that must be identified and receive individual treatment. If the problem were as simple as a psychogenic or obsessive-compulsive behavior disorder, treatment with a variety of chemicals would demonstrate greater response.