Allergic skin disease
So, we've examined your dog and come to the conclusion that the problems with skin or ears may be "allergies". Now what?
First of all, we still have to eliminate some of the other causes of skin problems. There are a number of diseases that cause similar-looking lesion on the skin. These diseases include allergies (all types), fleas, mites, dermatophytosis (ringworm), and hormonal diseases such as low thyroid hormone or sex hormone imbalances. "Allergies" as a group are the most common, but these other things may need to be ruled out before we jump into treatment for allergies. Diagnostic test may include blood testing for thyroid hormone levels, skin scraping for mites, treatment for occult flea infestations, and culturing for ringworm.
If the other causes of skin disease are ruled out, and the history supports it, we can tentatively diagnose "allergy". The problem remains allergy to what? Broadly, allergies are broken down into environmental and food allergies. Environmental allergies can be contact allergies (rare) or a type of allergic response called atopy, which can be caused by inhaling allergens or by absorption of allergens through the skin. Population studies show that somewhere between 5 and 30% of allergic dogs have food allergies. In addition, 80% of dogs with food allergies also have atopy.
Sometimes the dog's history gives us a clue as to which we are dealing with. Contact allergies tend to follow a pattern that will make the veterinarian suspicious and may not require further testing. A dog with very seasonal problems more likely have allergies to inhaled or absorbed environmental allergens. Guessing like this is obviously not foolproof; some pets have waxing and waning food allergies depending on the brand of food fed, and some seasonal allergies can become progressively more persistent beyond the normal season. We also have the problem of overlap as we saw above, many food allergic dogs also have atopy. So what do we do?
The first step is almost always to do a feeding elimination trial. This food trial involves changing the dog onto a protein and carbohydrate source to which he has never been exposed (and therefore to which he cannot be allergic). For most modern dogs this means something out of the ordinary, and usually something that is unavailable in the pet food stores. We have a number of prescription diets to choose from, depending on your dog's particular diet history. Please do not randomly change the dog's food; this will make the situation worse.
During the food trial it is vital that only the special diet be fed for a period of 8 to 10 weeks. This means no treats that contain other ingredients. No cookies, no rawhides, no food from the table. Nothing is to pass the dog's lips but the ingredients of the diet. Most people find this the most difficult part of the food trial, but it is worth doing properly. If you cannot restrict what your dog eats during this period, there is no point in spending the money on the fancy food.
If the problem is strictly a food allergy, the food allergens will be eliminated from your pet's body over the first 6-8 weeks on the new diet and the skin problems will clear up. We have a diagnosis! If the problem is not strictly a food allergy, your pet's clinical signs might diminish somewhat with the food change but they will not disappear entirely. If, after 8 to 10 weeks on the elimination diet, the skin is not remarkably improved, the problem is unlikely to be a food allergy and your pet can probably go back on his regular diet.
With food allergy ruled out, we move on to testing for inhalant allergies. There are a couple of ways to do this. Intradermal skin testing was once considered the "gold standard" of allergy testing. The side of the chest is shaved and small amounts of purified allergens are injected in multiple spots in the skin. The dermatologist then measures the amount of reactivity at each injection site. This is relatively accurate, but requires an experienced veterinarian to carry out and interpret the results correctly. Not very many veterinarians have the training and equipment to do this, so it has been relegated to specialists.
Alternatively, blood testing can be carried out. This measures the levels of a class of antibodies called IgE that are involved in allergic reactions. The tests look for IgE antibodies to different allergens, and if the dog has the antibodies we presume there is an allergy. This test is considered slightly less accurate than the skin test, but advances in methodology in the last 3 years have greatly increased both its sensitivity and specificity. Many veterinary dermatologists are now adopting it for use in routine allergy testing. The advantage to blood testing is that it need not be done by a specialist, and it is less stressful to the dog. The disadvantage is that results take about 2 weeks.
Whichever method is used, the test will determine what exactly the allergens are. Diagnosis made! We then can avoid the allergens, eliminate them, or hyposensitize the dog to the allergens using injections or an oral spray.
Why can't we just go right to the blood test and skip the food trial? Good question. The answer is that many dogs have both inhalant and food allergies. Our purpose in doing the blood testing is to determine what allergens to use in a hyposensitization treatment (allergy shots or sublingual spray). If we give hyposensitization injections for 6-9 months and the dog continues to itch, we are in a quandary. Are the injections not helping, or is there a food allergy?
The injections may be working very well to control the dog's dust mite allergy, but maybe he has ongoing allergies to lamb that are causing him problems. Or, the dog may be one of the 25% who do not respond well to the injections. Without having done an elimination food trial first, we cannot know which scenario we are dealing with. It is important to have as much information about the dog's allergies as we can get before making the commitment to hyposensitization.