So, we've been treating our itchy pet for allergic itching with a variety of approaches, but he is still itching. How can this be?
It could be that, despite our skin scrapings, cultures and other in-office tests, we missed something and it's really not an allergy. We may need to do a skin biopsy (cutting out a tiny piece of affected skin and sending it to a specialist who can look at every cell under the microscope and tell us what we are dealing with...we hope). Often we have multiple underlying problems and we have to treat them all. If the patient is fifty percent better, we did something good. BUT, we've still got to find the other fifty percent. For instance, it's very common for dogs who traumatize their skin to develop secondary yeast infections. Most yeast infections are just secondary opportunists, but once they are there you have to treat them if you want the patient to get well.
You already thought of that, you say? You say that the biopsy shows nothing but the changes associated with allergic skin disease and this pet is still having major itchy problems despite all your good treatments? Alas, I fear this means that we are dealing with a food allergy, particularly if the problem is not seasonal, but year round.
Dogs and cats with food allergy generally don't have diarrhea, just as pets with inhalant allergies generally don't have "hay fever" symptoms. When these animals have allergic reactions, it is the skin that is affected. You rolled in it, you breathed it, you ate it -- your skin breaks out. Well, if it's the same kind of reaction, why isn't the food allergy getting better on the medicine?
If you were allergic to some plant, and you needed to take antihistamines, chances are that you would not go find a pile of that plant and rub it all over your body and stuff it up your nose. Even though you yourself are not a scientist, it seems to you that putting your body into that much of what you're allergic to would be a bad idea. Good thinking. If you have a food allergy, the situation is similar. YOU'RE EATING IT! It's smeared all over your insides. Your intestinal lining has so many microscopic nooks and crannies that it has way more surface area than the outside of your body. The food allergy patient has such a heavy stimulation of his allergic reaction that the medicines frequently don't do much. So whaddayagonnado?
The only way to diagnose a food allergy is with a dietary elimination trial. The allergy testing methods that we use to identify molds and pollens and so forth will sure give you reactions on dietary ingredients, but they just aren't meaningful. Those tests may help you select a new diet for the patient if you prove he has a food allergy, but they cannot diagnose a food allergy. Perfectly normal dogs and cats will react all over the place on those tests.
Contrary to what most folks think, the offending foodstuff won't be something new in the diet. The pet has to have been eating it for six months or more to become sensitized and develop an allergy to it. Also, it isn't going to be just one brand of pet food. It's going to be an ingredient (or two), like chicken, beef, corn, soybean, wheat, or some preservative or who knows what. You can't just switch to a different food on the shelf. For the dietary elimination trial, the pet will eat a diet composed of ingredients that are entirely new to him, and NOTHING else. These can be home-cooked, or there are commercially prepared diets made of venison and green pea, duck and potato, kangaroo and oats, and other weird stuff.
A dietary elimination trial is the simplest thing in the world, in theory. You stop eating this, you start eating that. Nothing to it. In practice, however, things are more complicated. When you start the special diet, you can't have treats, you can't have table food, you can't eat what the kids drop on the floor, the other pet's food, flavored heartworm preventive, rubber erasers... NOTHING but the special new diet. Not only that, most pets take four to six weeks (and some longer) for all the old food to completely wash out of every little intestinal hidey-hole. Try that with three pets and two little kids in the house.
If you think this post is way too long (and it is) imagine me giving this lecture (and the last two posts, as well) to three itchy patients a day. Now you know why there aren't many veterinary dermatologists.