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What’s Eating Them – Food Allergies
by Jeff Grognet, DVM, BSc (Agr) and Louise Janes, BSc(Agr), DVM
Allergies account for more itchy skin than any other disease in dogs. There are three distinct types of allergies, but one in particular – reactions to foods – has become a growth industry ever since the advent of “hypoallergenic” diets. This suggests that food allergies are common. Nothing could be further from the truth.
It you take all allergic dogs and divide them into groups based on the reason for their itch, you will see a significant number due to fleas (depending on the geographical area), another huge segment caused by atopic allergies (see section below), and a lowly two percent or so attributable to food allergies.
A food allergy is an adverse immune reaction to a specific dietary component. By definition, it can be managed solely by excluding the offending food.
The problem with a food allergy is diagnosing it. Many testing techniques have been tried. Some researchers tell us we can detect antibodies in the blood against specific antigens (food ingredients). However, the relevance of these tests has come into question. It seems that antibodies to foods can be found in normal (non-allergic) individuals. According to dermatologists, available blood tests do not represent good value for the money.
Skin testing, where multiple allergens are injected into the skin surface, can be used for atopic allergens, but are unreliable for food allergies. Veterinarians have to rely on other methods, but it also means starting at the beginning.
When working with an itchy dog, veterinarians first need to rule out skin parasites and infections. Internal parasites should also be treated. Once these are eliminated, allergic skin disease (for any reason) is probable.
Reviewing the history is a crucial component. The characteristics of an atopic allergy are described in the sidebar. If that type of allergy is suspected, it should be treated.
However, if a dog’s itchiness peaks after consuming a certain food ingredient, it suggests a food allergy. A Dalmation I knew reliably developed a red face and ears each time it ate even the tiniest piece of carrot. That was an easy diagnosis, but most are not as obvious. And, most allergies are to ingredients that have been in the diet for years, not new additions.
Clinical signs of food allergies normally involve just the skin, but a small subgroup also develop intestinal symptoms. The itch from a food allergy is not seasonal like an atopic allergy, and it occurs at any time of the year. The itch may be localized on the face, ears, and feet, but it can be all over the body. Harvesting a small sample of skin (skin biopsy) and sending to a pathologist can sometimes confirm a food allergy.
The only way to positively diagnose a dietary allergy is through a food trial. The principle of this test is simple. You need to feed a diet consisting of a single protein and single carbohydrate not found in your dog’s current diet. For example, if your dog is fed a food containing lamb and rice, you would have to switch to ingredients such as fish, elk, turkey, rabbit, kangaroo, yams, potatoes, or beans.
Food can be commercially made or home-made. The choice is based on convenience and personal preference. No single diet works in all dogs and experimentation is often needed. The food should be fed for a minimum of two weeks and preferable four before deciding if signs have resolved, even partially.
Absolutely no snacks or treats can be fed during the food trial. Replacing tap water with bottled water is likely unnecessary, but food and water bowls should be ceramic or stainless (dogs can be allergic to plastics).
If the itch disappears with the new food, it confirms a food allergy. Once the dog is stable and the itch has abated, single ingredients can be offered to find out what can be added to the diet and what must be avoided.
If the itch continues and a food allergy is still suspected, you can use different ingredients. Or, you need to reassess and look for another reason for itching.
A Home-Prepared Hypoallergenic Diet
Home-prepared diets usually contain a single protein and a single carbohydrate source chosen based on the dog’s past dietary history. The goal is to avoid ingredients that are currently being fed.
In a Golden Retriever patient that had been fed a chicken and rice food, I managed to control the itch with a trial home-made diet consisting of white fish and sweet potato (50:50). Once we got her comfortable, we began adding foods. We quickly found out that she was horribly allergic to chicken. Over time, we transitioned her to a commercial food containing primarily fish and potato.
Some dogs seem to tolerate home-prepared diets but not the commercial equivalent. The reason for this is unknown, but it may be due to changes in the ingredients during processing. It may also be due to additives that might cause an adverse reaction, but this has not been proven.
Due to the added effort in preparing home-made diets, many owners (over 50 percent in one study) fail to complete dietary trials. As well, these diets are not nutritionally balanced and should not be given long-term or to puppies that are growing. If the trial diet is continued, supplementation is necessary.
Causes of Allergies
There are three causes of skin allergies in dogs – fleas, atopy, and food.
Fleas are usually easy to see and diagnose. The itch is often concentrated above the tail, but it can be anywhere on the body. The allergic reaction is actually to flea saliva injected as the flea feeds.
Fleas and flea allergies are weather dependent. If you have an itchy dog and it’s February in Colorado, chances are, it isn’t fleas. If it’s an itchy dog in Florida, fleas are a big possibility. Flea allergies are treated very easily by using a topical insecticide that kills fleas before they bite.
Atopic allergies account for the biggest share of allergies in dogs. These are reactions to allergens such as house dust and house dust mites, pollens, fungi, and molds. There is debate on how the allergens cause the itch. The traditional theory is that they are inhaled, pass through the lungs into the bloodstream, and eventually find their way to the skin. The other thought is that they settle on the skin and migrate between the skin cells to the lower skin layers (the dermis). Either way, there is a reaction of that allergen with an antibody which triggers release of histamine. This causes the itch.
Atopic allergens can be diagnosed with intradermal skin testing where allergens are injected into the skin to look for reactions. They are treated with medication (antihistamines, cyclosporine, prednisone) and also with allergy injections (hyposensitization).
The last type of allergy is a food allergy. This is the topic of this column. |
For more information contact Dr. Jeff Grognet or Dr. Louise Janes at
Mid-Isle Veterinary Hospital,
5-161 Fern Road West,
Qualicum Beach, B.C.
Tel (250) 752-8969
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