Allergies In Dogs And Cats
© Kevin Byrne, 2011
Many people bring their pets to AESCA because an allergy (also called hypersensitivity) is causing their pet significant discomfort. There are basically three types of allergies that commonly cause discomfort in pets: food allergy, atopic dermatitis (environmental allergy), and fleabite allergy. A fourth type - contact allergy - is less common and will be discussed briefly. What follows are answers to common questions that arise during an examination for possible allergies.
Food allergy is one of the three major allergies seen in dogs and cats. When we talk about an allergy to a food, we are not speaking about a particular brand of pet food, but about an individual food item or ingredient. Beef is the most common food to which pets can be allergic. Other allergen examples are chicken, pork, wheat, corn, and even lamb. Pets with food allergy are usually allergic to only one or two foods. A common misconception is that simply changing the brand of commercial pet food should treat food allergy. Since the problem is a food item or ingredient, changing brands of commercial pet food usually is not helpful, unless the new food is a prescription diet made for pets with food allergy. Think of someone who is allergic to peanuts. They would have a reaction if they ate peanut butter flavored chili, just as they would if they ate a cookie containing peanut butter. In addition, allergy to a food ingredient is independent of the quality of the food source. For example, someone allergic to beef would react to filet mignon just as readily as they would to ground chuck.
A pet with a food allergy eats something that contains the offending food item. This offending food is digested, and molecules of the food item are taken into the body. Once in the body, cells of the immune system, which are responsible for the allergy, react to these food molecules. The cells release chemicals (inflammatory mediators) that result in the signs of allergy that we see (pruritus, redness).
Food allergy can be diagnosed by performing a strict elimination diet trial, which can be performed by the pet owner at home. A special test food is fed for a period of time, usually 8 weeks. During the test period, the pet eats this food and no other. No bites of table food are allowed during the test period. An allergy to a food can be so strong that the slightest exposure to the offending food item can cause the signs of allergy to persist. For that reason, any natural-origin products such as rawhide, cow hooves, biscuits, treats or anything containing grains are stopped. Additionally, chewable vitamins or medications, such as chewable heartworm pills, are replaced by a non-flavored, non-chewable medication. Within 4 to 5 weeks, the pet stops showing signs of allergy, although some pets may take as long as 8 weeks or more to improve. Giving the pet old foods or snacks for a few days will cause return of allergic signs. Reinstitution of the test diet results in resolution of allergic signs again, proving the diagnosis.
Atopic dermatitis is a skin disease that occurs in pets that are allergic to things in their environment. With this type of allergy, the offending allergen (e.g., pollen, mold, dust mites) is either absorbed through the skin or inhaled. How allergens are able to penetrate the skin is not known conclusively.
Why this happens is not known, but it is suspected to be due to the tendency of the skin to have large numbers of the type of immune cell that causes the signs of allergy. Any allergen that makes its way (regardless of the route) to these cells will cause the cells to release the inflammatory mediators, resulting in signs of allergy.
The tendency to develop allergies is thought to be genetic. There is convincing evidence that genes increase susceptibility to development of allergies in humans. The same is thought to be the case in animals. Environmental factors do play a role, and examples include infection with internal parasites (worms) or viruses that occur early in life. It is believed that there are critical times in the maturation of the immune system where environmental factors may alter the immune system, making the individual susceptible to allergy. Whether a genetically susceptible pet develops allergy or not may be a matter of luck with regard to whether or not an infection occurs at a critical stage in the development of the immune system.
The immune system can be thought of as a network of different cells that communicate instructions to each other. Lymphocytes are cells that directly or indirectly control most of the activities of other cells in the immune system. Whether or not an individual becomes allergic to something depends to a great extent on how the lymphocytes react to it. Lymphocytes are the cells that produce the primary "allergic" antibody, which is IgE. Another very important cell is the mast cell. Mast cells are found in any area of the body, but are present in higher numbers in areas of the body that are exposed to the environment such as the skin, lungs, GI tract and eyes. Mast cells serve as a home for the IgE antibodies, which adhere to the surface of the mast cell. If the allergen(s) to which a pet is allergic comes in contact with this IgE, a reaction occurs in the mast cell. The mast cell releases chemicals (inflammatory mediators), and these chemicals then cause irritation to the surrounding body tissues, resulting in the signs of allergy, such as pruritus and redness.
In the hierarchy of cells of the immune system, lymphocytes can be viewed as the primary directors or decision makers that directly or indirectly control the activity of the other cells. Lymphocytes can recognize whether a substance belongs in the body or not. In the case of allergens, the lymphocytes then decide whether to react to the allergen. It can ignore the allergen (which is desirable, since the allergen itself is not a real threat to the body), in which case, no allergy develops. The allergen eventually is expelled from whatever part of the body it invaded, such as the lungs, skin or GI tract, and life is good. Another way in which the lymphocyte may deal with the allergen is to treat it as a foreign invader that must be destroyed. To do this, the lymphocyte begins a series of steps that result in the production of the antibody IgE. IgE specifically targets the allergen that the lymphocyte encountered, whether that allergen was oak pollen or dust mites.
This type of allergy is less common in dogs and cats than in humans. The allergens produce lesions (abnormal changes, such as a rash) when they come in direct contact with the skin. Unlike other types of allergy, allergic antibodies such as IgE do not play a main role in production of the skin disease. Rather, it is the result of the activity of a specific subtype of lymphocyte. Skin lesions are limited to areas of direct contact with the offending allergen, which for dogs and cats, usually means the underside and paws. Plants of the Genus Tradescantia, which are often used as groundcover, are known contact sensitizers.
Allergen: the term used for any substance - such as pollen, dust mites, molds - that can induce allergy in an individual.
Antibody: a large complex molecule that is produced by certain lymphocytes. Antibodies are designed to attach to substances that are foreign to the body, enabling the destruction or removal of these substances.
Allergic Antibody: a type of antibody, principally IgE (I - G - E) that targets a specific allergen (e.g., dust mite).
Erythema: the medical term used for increased redness of the skin.
Immune System: a complex collection of cells, molecules, chemicals produced by the body to prevent infections from taking over the body and to prevent abnormal cells (cancer) from proliferating.
Inflammatory Mediators: the chemicals that cause signs of inflammation such as erythema, pruritus, or hives. They are produced by many different types of cells of the body, but especially by cells of the immune system. The most common inflammatory mediator connected with allergy is histamine. Many medications used to treat allergies are anti-histamines.
Lymphocytes: the cells that direct most of the activities of the immune system.
Mast Cells: a type of cell present in large numbers in the skin and other areas of the body. These cells have the ability to produce large amounts of inflammatory mediators, probably the most potent cell with regard to ability to cause allergic symptoms.
Pruritus: the medical term for increased sensation of itchiness.
White Blood Cells: includes a variety of cells normally found in the blood. White cells leave the blood stream to fight infections, and can also induce symptoms of allergy in allergic individuals. Examples of white blood cells are neutrophils, eosinophils, and basophils.