Overview of Canine Contact Dermatitis

Contact dermatitis is an uncommon skin disease of dogs caused by contact with plants (especially plants of the wandering Jew family), medications, and various chemicals. Contact dermatitis is not as common in animals as in people because the skin of dogs normally is protected by their hair coat. Contact dermatitis can develop, however, in areas of the body where the hair is sparse.

Contact dermatitis can be of two different types: allergic or irritant.

Allergic reactions require a period of sensitization during which the immunologic reaction develops. On the average, sensitization takes six months to two years to develop. Thus, medications and chemicals that have been used in the past without problems still may be responsible for an allergic reaction at some later time.

Contact allergy is a delayed reaction with signs occurring 24 to 48 hours after contact with the offending substance. The reaction is the same type that occurs in people who develop poison ivy.

Typical signs of contact allergy include pruritus (itchiness) and a papular eruption (red bumps). Pruritus can be severe. The paws and muzzle commonly are affected in animals, and sometimes, the insides of the ears are affected, especially in dogs with pendulous ears that come into contact with plants and grasses. In households with several animals, it is uncommon for more than one animal to develop an allergic reaction.

Irritant reactions, on the other hand, do not require a period of sensitization because they are not immunologically mediated. Thus, reactions occur the first time a substance makes contact with the skin. If several animals are present in the household, all animals coming into contact with the substance will develop skin reactions.

Irritant reactions are more painful than pruritic. Small vesicles (blisters) and ulcerations develop. The distribution of the lesions depends on the nature of the offending substance and the pattern of contact.

Secondary bacterial skin infections may develop due to trauma and inflammation.

What to Watch For

Pruritis (itchiness)

Papules

Secondary superficial bacterial infection

Increased pigmentation

Crusting

Thickening of the skin

Blisters (vesicles) and ulcerations that are more painful than pruritic

Diagnosis of Contact Dermatitis in Dogs

The medical history is very important when trying to establish a diagnosis in animals with skin conditions. You will be asked to describe all medications, chemicals, shampoos, and dips that you have used on your dog. Timing also is important when trying to determine the offending substance. It is important to be very thorough when providing information about carpet cleaning, deodorizers, and chemicals applied inside and outside of your house. If the reaction is allergic in nature, products that have been used for a long time cannot be ruled out because of the requirement for a previous sensitization period.

The usual medical approach to an animal with a skin condition proceeds by steps. Thus, your veterinarian will rule out more common diseases before considering a less common disorder such as contact dermatitis. Skin scrapings to rule out demodectic and sarcoptic mange, fungal culture for dermatophytes (ringworm) and antibiotic therapy for possible bacterial skin infection may be necessary.

Diagnosis of contact dermatitis is made by observing resolution of clinical signs after withdrawal of the presumed offending substance and relapse of clinical signs with re-exposure.

Patch testing also may be used to screen suspected chemicals or plants. An area on the lateral chest wall is clipped 24 hours before patch testing.

Samples of various suspected chemicals and plants are applied directly to the skin and bandaged into place to insure prolonged contact with the skin.

After 48 hours, the bandage is removed and the skin examined for reactions. Signs of a positive reaction include erythema (redness), small papules (red bumps) or vesicles (blisters).

Treatment of Contact Dermatitis in Dogs

The treatment is to remove the dog from the offending substance.

If contact allergy is suspected, you may asked to confine your animal to a limited area of his normal environment to prevent contact with suspected substances. Confinement should start after a thorough bath, because small particles of the substance may remain on the skin and perpetuate clinical signs.

Compliance is very important. If plants are suspected, you may have to use specialized boots and T-shirts to protect your dog’s skin and walk the animal only on concrete areas until the lesions have healed, typically 10 to 14 days.

Bacterial skin infections occur commonly in dogs with contact allergy or irritant reactions. You may be asked to administer an antibiotic for a minimum of three to four weeks.

In severe cases, a course of anti-inflammatory medications such as prednisone may be necessary to make your pet more comfortable. Orally administered medications usually are safer than injectable preparations and should be used as a first choice. Adverse effects of this type of therapy include increased appetite, increased thirst and increased urinations. Avoidance of the offending allergen should be attempted whenever possible, because corticosteroids tend to lose their efficacy with repeated use. This is called tachyphylaxis.

In-depth Information on Contact Dermatitis in Dogs

Contact allergy is an immunologic reaction called type IV hypersensitivity in which lymphocytes are the predominant cell type. Contact dermatitis can be either allergic or irritant. Irritant contact dermatitis is not immunologically mediated. Allergens called haptens, which are responsible for contact allergy, are small molecules that are usually lipid soluble and require binding to larger proteins to become allergenic.

In humans a genetic predisposition to contact allergy has been observed. The same may be true for dogs. Dogs with atopy (inhalant allergic dermatitis) also seem predisposed to contact allergy.

Contact allergy involves two different phases: a sensitization phase and an elicitation phase.

Irritant reactions are not immunologically mediated and do not require a sensitization phase. The occurrence of a reaction in a control animal can be used to differentiate between irritant and allergic contact reactions.

Sensitization   

A sensitization phase is required before clinical signs develop. This phase seems to vary from six months to two years in dogs. During exposure to the hapten, the immune system becomes sensitized to the substance but an allergic reaction is not elicited.

The hapten is taken up by specialized skin cells called Langerhans cells, modified, and presented on the surface of the cell along with major histocompatibility complex (MHC) class II antigens for presentation to T-lymphocytes. The MHC antigens normally distinguish individuals from one another and are evaluated in procedures such as tissue typing. T-lymphocytes are cells that play a key role in the immunologic process.

Antigen presentation occurs in the local lymph nodes. The T-lymphocytes become activated and proliferate, producing a clone of memory T-cells. The existence of a lag phase before the development of clinical signs is an important piece of information for the clinician. This information helps differentiate allergic and irritant contact reactions because clinical signs and microscopic pathology can be similar in these two disorders.

Elicitation

When this hapten is encountered by Langerhans cells it is presented to memory T-cells. These cells secrete chemicals called interleukin-2 (IL-2) and gamma-interferon which stimulate T-cell proliferation and the expression of special adhesion molecules on the surface of skin cells called keratinocytes. These molecules are responsible for the accumulation of inflammatory cells called mononuclear cells in the epidermis.

Inflammatory mediators (eicosanoids, tumor necrosis factor, histamine, interleukins) released by stimulated skin cells (keratinocytes) and specialized inflammatory cells of the skin (mast cells and basophils) are responsible for the redness, dilation of blood vessels and itchiness that occur in allergic contact reactions. Recent studies have focused on the clinical relevance of tumor necrosis factor in natural and experimentally-induced contact allergy.

Contact allergy is uncommon in animals due to protection of the skin by the hair coat. Detergents, waxes, cleansing agents, dyes, deodorants, shampoos, dips, insecticides, corticosteroids, antibiotics, and plants can cause allergic contact dermatitis.

Several plants have been documented to cause contact allergy in dogs. These include dandelion leaves, cedar wood, Asian jasmine and wandering Jew. Plants of the Commelinceae family, such as doveweed, spreading dayflower and wandering Jew, are frequently responsible for contact allergy in dogs in the southeastern United States. Common characteristics of these plants are lance-shaped fleshy leaves with closed sheaths and a few soft hairs on the upper margin. They reproduce by seed and have blue to purple flowers. They are found in moist habitats and in warm climates and are not usually responsible for contact hypersensitivity in people. Calcium oxalate crystals are hypothesized to be responsible for the allergy-producing effect of these plants.

Related Symptoms or Ailments in Clinical Presentation

Contact Allergy. This disorder takes a minimum of six months for appropriate sensitization. In most cases, the sensitization period exceeds two years. Thus, affected animals are at least six months of age or older when they see a veterinarian. Yellow Labrador retrievers, German shepherds, and golden retrievers seem to be at increased risk for developing contact allergy.

The onset of clinical signs usually is rapid once sensitization has occurred. The problem may be seasonal when plants are the offending allergens. Clinical signs include a pruritic eruption characterized by papules, secondary superficial bacterial infection, increased pigmentation, crusting, and thickening of the skin. Sparsely haired areas are more commonly affected (groin, axilla, abdomen). Papules, pustules and pruritus persist after resolution of the secondary bacterial infection. The muzzle and paws also are commonly affected.

Pruritus varies in severity and may be very intense. It is usually responsive to cortisone-like medications (corticosteroids), at least initially.

Irritant Contact Dermatitis. Blisters (vesicles) and ulcerations are present in animals with irritant reactions. These lesions often are more painful than pruritic. All animals in the same household tend to develop lesions. A period of sensitization is not required, and lesions occur the first time the animal comes in contact with the offending substance.

 

Diagnosis In-depth of Contact Dermatitis in Dogs

Diagnosis is based on clinical signs, distribution (variable depending on the type of substance involved), resolution of clinical signs with avoidance, and recurrence of clinical signs with re-exposure. Implicating a specific allergen is difficult and requires cooperation with the owner to identify possible sources.

Contact allergy can be differentiated from inhalant allergy (atopy) by the persistence of a primary papular eruption (red bumps) after appropriate antibiotic therapy. In addition, contact allergy completely resolves with confinement whereas atopy does not.

Boarding the animal at the hospital for one to two weeks is recommended to accomplish avoidance. Animals should be shampooed before beginning confinement to remove possible residual material from the skin.

Patch testing can also be used to diagnose contact allergy. Patch testing is difficult and time-consuming but allows identification of the specific substance responsible for clinical signs. The skin on the chest wall is clipped one to two days before the test and potential allergens are applied in close contact with the skin for 48 hours. Thorough bandaging is the best way to achieve contact with the allergen (closed patch test). An open patch test consists of applying the potential allergen to the inner part of the ear and observing the skin for 48 hours.

Specialized devices called Finn chambers may be used to apply the suspected allergen. A positive reaction is indicated by the appearance of edema (soft swelling) and a papular eruption (red bumps) 24 to 48 hours after application of the allergen.

Microscopic pathology. Skin biopsies show inflammation around blood vessels and swelling of cells in the epidermis.

Treatment In-depth of Contact Dermatitis in Dogs

The main therapy for allergic contact dermatitis consists of allergen avoidance and topical or systemic administration of cortisone-like drugs called corticosteroids. Anti-inflammatory dosages of the corticosteroid prednisone usually are used (0.25 to 0.5 milligram per pound of body weight for 5 to 7 days followed by an alternate day protocol as needed). This approach usually is sufficient to control clinical signs. Topically-administered steroids that may be beneficial include Escort® or FS Shampoo®.

Hyposensitization (allergy shots) has been ineffective.

Antihistamines and essential fatty acids are not effective in animals with contact allergy.

Pentoxifylline (Trental®) has been used recently to inhibit allergic contact reactions in rodents, people and dogs

Pentoxifylline works by inhibiting tumor necrosis factor, an important mediator of contact dermatitis.

Pentoxifylline works better as a preventative agent than a treatment when lesions are already present. Therapy with pentoxifylline should be started two days before exposure to the offending allergen. Pentoxifylline usually is well-tolerated and adverse effects are dose-dependent. Adverse reactions include nausea, vomiting and diarrhea. It also may cause flushing of the skin and tremors.

Secondary bacterial infections should be treated with systemic antibiotics (cephalexin 10 to 15 milligrams per pound body weight twice daily for a minimum of 3 weeks).

Follow-up Care for Dogs with Contact Dermatitis 

Complete resolution of clinical signs usually is obtained after 10 to 14 days of avoidance of the offending substance. No prevention is possible unless the offending substance has previously been identified.