Chances Are Its Not a Food Allergy for Your Dog

Your dog’s skin is itching, perhaps to the point that he may even be licking his paws or chewing on his feet. Or he keeps vomiting or having diarrhea. It must be a food allergy, you think to yourself, so you put him on a special diet. But it doesn’t work. Frustrated, you try other food combinations, finally bringing him to the veterinarian for help in figuring out which ingredient is causing the allergic reaction.

Suspected food allergy is “a common reason people come to the nutrition clinic at Tufts,” says Cailin Heinze, VMD, a board-certified veterinary nutritionist at the Cummings School. “By the time they get here, they’ve often tried five, six, seven different diets. But if they all have different ingredients and the dog is not any better, it’s probably not a food allergy. Of those dogs whose owners think they may have a food allergy, perhaps only 10 percent or less actually do.”

Why do so many people believe their dogs are suffering from food allergies, besides the fact that the Internet is rife with articles and postings erroneously suggesting that food allergies in pets are quite common?

“People want to help their dog, and if the problem is food, that’s easy to fix,” Dr. Heinze says. You just remove the food from the diet. “So to some extent it’s wishful thinking. Add to that the fact that they keep reading online that food allergies are common, and it really snowballs.”

But the fact is that most of the incessant itching and scratching seen in some dogs is the result of atopy — allergies not to food but to airborne substances hanging around the environment, including pollen, mold, and dust mites. When allergic people breathe in those substances, they end up with runny noses, watery eyes, sneezing, and congestion. When allergic dogs breathe them in or walk through them (and perhaps lick them off), they itch; the problem plays out largely on their skin. They often get ear infections as well. And since you can’t simply remove the offending environmental substances from the dog’s world, treating allergic reactions to them may involve administering medicines to the dog and other treatments for the rest of his life — a more involved plan than simply changing the diet.

Gastrointestinal symptoms like vomiting that many people also assume are food allergies tend to have another cause, too. Food allergies can cause GI upset, but most likely, food-related nausea or other discomfort is the result of a food intolerance rather than a true allergic reaction, which is an attack by the immune system on a foreign substance. In fact, Dr. Heinze says, often, when an owner switches a dog’s food and the GI symptoms clear up, the person believes he has correctly diagnosed a food allergy. “But more commonly,” she says, “it’s something about the amount of fat in the food, or the way the food was cooked, or the fiber level. A dog might simply do better with twice the fiber or half the fat. An allergy has nothing to do with that. Of course, sometimes, a dog has a disease of the gastrointestinal tract that needs to be identified and treated.”

How do you diagnose a true food allergy?

“The only way to definitely diagnose a true food allergy is to do a diet trial,” Dr. Heinze says. “Even then, if food is the culprit of a dog’s symptoms and the dog is having GI upset rather than itching, it’s hard to distinguish sometimes whether it’s a food allergy or a food intolerance. But at least you’re getting at the source of the problem.”

Before the diet trial begins, it’s critical to get a complete diet history — a list of every single ingredient in the foods a dog ever eats, particularly when he is doing poorly, either with itching, GI problems, or both.

Once a veterinary nutritionist reviews the diet history, she will choose for the dog a diet to which the pet has never been exposed. “Maybe a dog has been on a chicken, rice, and fish diet,” Dr. Heinze says, “so we’ll put him on a therapeutic diet made with kangaroo and oats — things the dog has never had. Or we’ll put him on what is known as a hydrolyzed protein diet. It’s generally the proteins in foods to which dogs (and people) are allergic, but if you take a protein-rich food like soy or chicken and use enzymes to break up the proteins into smaller pieces, the dog’s immune system may not recognize the proteins for what they are” and therefore not go into overdrive and cause an allergic reaction.

Dr. Heinze says “you then feed that diet from four to 12 weeks — but no flavored medications, no treats, nothing the kids drop on the floor, no flavored toothpaste, because ingredients in any of those things can cause the symptoms to continue.”

Restricting a dog’s diet to that degree “tends to be very hard for owners to do,” Dr. Heinze says. “But if the symptoms resolve or largely resolve, it suggests it was something in the diet that was causing them. If the symptoms were in the GI tract rather than on the skin, you can’t tell if it’s an allergy or an intolerance. But at least you know that it looks like you might very well be on to a dietary solution.

“Then you put the dog back on the old diet,” Dr. Heinze explains. “If the symptoms start up again immediately, you know it was the food.

“You can then choose to feed the dog the new diet indefinitely or take ingredients from the old diet and reintroduce them one at a time. For instance, try the chicken again for a week or two. If the chicken’s okay, then try the rice. If the dogs starts to itch or throw up again, you’ve found the culprit and can then choose an over-the-counter food that doesn’t contain it. But it’s a long process — diagnosing a specific food allergy is not as easy as many people think — and a lot of owners are not willing to go that far. They stick with the therapeutic food prescribed bythe veterinarian.”

That’s okay. It just might be somewhat more expensive — and will require more frequent trips to the veterinarian’s office rather than to the supermarket.

Of course, if a completely different diet doesn’t resolve the symptoms, it’s time to see a veterinary dermatologist who can test for atopy, or, if the problem is not itching but in the GI tract, to explore the possibility of a disease somewhere between the throat and the colon.

“Often, the owner is referred elsewhere for further testing when a veterinary nutritionist is suspicious that the symptoms are unlikely to be the result of a food allergy,” Dr. Heinze says. That said, she comments that “we do see a few cases where we’re really confident that a food allergy is the problem. We had one dog come in whose owner thought she had a food allergy and had put her on a sweet-potato-and-pork diet. It was home cooked and completely unbalanced. The dog was losing weight and having other issues. But her itching was 100 percent resolved. We then tried her old diet again and within a few hours of her first meal, she was itching again, rolling on the floor and licking her feet.

“Her owner then tried adding back in one ingredient at a time — and found, finally, that it was white potato to which she was allergic. So we identified an over-the-counter diet with no white potato — just fish, oats, barley, and rice — and she does great on it. But it was a multiple-month process with a very dedicated owner.” And since the dog’s symptoms were more like atopy than GI upset, there was no confusion over whether it might be a food intolerance. Because of the itching, it was clear that it was an allergy.

As far as testing for whether a reaction to a food ingredient is an allergy versus an intolerance, “blood tests are available, but they’re notoriously unreliable,” Dr. Heinze says. “In my experience, there have been a lot of false positives on blood allergy panels for food allergies in dogs,” she reports. That is, “you can assume with a blood test that you’ve diagnosed a food allergy but have not.” It leads people to start changing a dog’s diet unnecessarily.