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.
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.