Atopic dermatitis in cats and dogs (Proceedings)
Etiology and Pathogenesis
Canine and feline atopic dermatitis (atopy, allergic inhalant dermatitis) is a hereditary, pruritic (itchy) skin disease. The disease is caused by an allergic reaction to the inhalation of pollens, mold spores, dust, or epidermals (animal dander, feathers wool). Upon future challenge with that specific allergen, IgE molecules are bridged on the mast cell surface. This sends a signal for the mast cell to degranulate and release inflammatory mediators. This is a type I hypersensitivity reaction. Histamine is the most important mediator but proteolytic enzymes, kinins, ECF, etc. are also involved. The release of these mediators results in inflammation and pruritus. The development of this inflammatory reaction is the body's attempt to rid itself of the foreign substance.
More recently additional pathways are thought to be involved in atopic dermatitis. A specific subclass of IgG (IgGd) is thought to be involved in human and canine atopic dermatitis. In addition, specific IgE receptors have also been found on Langerhans cells of atopic patients. The presence of these IgE receptors suggests that percutaneous absorption may play a more important role than previously suspected. Concurrent abnormalities in cell mediated immunity (CMI) in atopic dermatitis patients can lead to decreased T-lymphocytes, neutrophils and macrophages.History
Certain breed, age, and sex predispositions exist. Since canine atopic dermatitis is an inherited condition, certain breed dispositions have been noted. Terrier, retrievers, Dalmatians, Maltese, Lhasa Apso, English Bulldogs, and Shar Pei. Poodles tend to have the lowest incidence of atopic dermatitis. The typical age of onset for atopic dermatitis is between 1 to 3 years of age. However, dogs as young as 6 months and as old as 5 years have been reported. Female dogs appear to be more predisposed to develop atopic dermatitis than male dogs (2:5:1, female:male).
In cats, the age of onset for atopy is normally 1 to 3 years but may be as young as 6 months or as old as 5 years. Although not studied as extensively as in dogs, some reports suggest that certain breeds of cats may be predisposed (i.e. Siamese cats). No sex predilection has been noted in cats.
Atopic dermatitis is typically a seasonal pruritic problem. However, in warmer climates, atopic dermatitis can be nonseasonal. Therefore, canine atopic dermatitis may be seasonal or nonseasonal depending on the circumstances.
The clinical signs of atopic dermatitis differ significantly between dogs and cats Since this problem is more common more common in dogs when compared to cats, this information will be discussed first.
Canine atopic dermatitis has been described as the itch that rashes. Therefore, erythema and pruritus are hallmark clinical signs associated with canine atopic dermatitis. The dog with atopic dermatitis must be exposed to the antigen for some time before clinical signs will appear. It is for this reason that the first symptoms are most often noticed at an age between 6 months and 5 years. The symptoms often start out "seasonal" and become worse each year. Duration and severity of the symptoms increase as the animal becomes allergic to more antigens.
The first signs that are likely to be noticed are scratching, chewing, rubbing, or licking at the face (especially muzzle and periorbital areas), proximal forelegs, feet, ears, axillae and inguinal areas. One or more of these body locations may be involved. Early on there may be no observable change in the skin in these areas. As the disease progresses the skin may become red and there may be hair loss due to the trauma. Types of skin lesions that are typically seen in cases of canine atopic dermatitis include: alopecia, excoriations, superficial moist dermatitis ("hot spots") and salivary staining. Symptoms usually start suddenly when the animal is exposed to the causative antigen. When the animal is kept away from the antigen the symptoms may subside within minutes to hours without other treatment.
Chronic or long-term disease can result in secondary changes such as thickening of the skin (lichenification), hyperpigmentation, otitis externa (ear infections), bacterial skin disease (pyoderma), and loss of hair and / or acute moist dermatitis (hot spots). Unusual presentations, approximately 10% of the cases, have hyperhidrosis or pseudo sweating. Intact females may exhibit irregular heat cycles. Other possible clinical findings include: urticaria, conjunctivitis, rhinitis, asthma, diarrhea, and personality changes such as irritability. The disease can be complicated or made worse by other allergies such as food, flea, or contact allergies. When this occurs, it becomes a difficult diagnostic challenge and cooperation between owner and the veterinarian is of utmost importance if a solution to the problem is to be obtained.
In the majority of the cases, cats usually manifest their pruritus as excessive licking and grooming. In cats with atopic dermatitis, the pruritus occurs on the face and neck predominately. However, pruritus may also occur on the forelegs and abdomen. Some cats with atopy. Partial or complete alopecia with or without erythema is usually seen in atopic cats. Some cats develop a military dermatitis or eosinophilic granuloma in the affected area. Although uncommon a secondary pyoderma (papules, pustules, crust) or excoriations may occur.