Contact Dermatitis in Dogs

Contact dermatitis is an uncommon skin disease of dogs and cats 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