An allergic reaction is an immunological, over reaction to harmless proteins, called allergens, to which the pet has been previously exposed. Exposure to various allergens can occur through the digestive tract, respiratory tract, eyes and skin. Microscopic amounts of exposure can lead to reactions, sometimes subtle, sometimes extreme. Types of allergens include food proteins, grass, tree, and weed pollens, molds, mites/insect parts and saliva, pet dander and saliva, cotton and wool. Dermatological signs of food and environmental allergies are indistinguishable. Clinical signs can be present all year around, seasonally, or intermittently without a recognized pattern, although signs of food allergy are usually present all year around.
The likelihood of developing allergies is inherited, but the specific allergy developed depends upon multiple factors, such as allergen size and exposure patterns. In general, dogs and cats, are less likely to be allergic to their food than to environmental allergens. About 10% of pets with allergies affecting their skin (allergic dermatitis) can be completely controlled simply by removing the offending food protein/s. In my experience, about another 10 to 20% partially improve, they usually have environmental allergies too. By comparison, environmental allergens alone or in combination with food allergens are involved in about 85% of the patients I see with allergic dermatitis.
Allergic humans most commonly experience rhinitis (itchy, runny, sneezy nose) and conjunctivitis (red, itchy eyes), skin reactions are less common. Cats can show respiratory signs like humans and sometimes asthma as well, but we also see a good number with dermatitis. Dogs are much more prone to “allergy eyes” and dermatitis.
Often, the first thing noticed is that the pet is itchy. Itching may be directed at a particular part of the body, esp. the ears and paws, or be generalized. It can affect one side of the body more than the other or be bilaterally symmetrical. The skin often becomes red in affected areas. Fur loss is a common subsequent feature, due to the biting and scratching of the pet, but also due to secondary infections. Eventually the skin may become warm to the touch, swollen or painful. When the inflammation becomes chronic, the skin can become thicker, darker and may lose the surface smoothness. They commonly develop secondary infections, which cause increased fur loss, along with greasiness, odor, red, itchy bumps, crusts, scale, and sometimes open sores. Infections usually exacerbate the level of itching.