Suzanne Phillips has been a long-time reader of this blog. Earlier this week, she offered to do a guest post on the topic of why dogs develop food allergies. Much of what you read online about this topic is utter nonsense and she decided that this would be a good venue for setting the record straight. Her analysis is entirely based upon what the scientific research says about why dogs develop allergies to certain foods. Suzanne blogs at Hoof & Paw, and she and her husband run a small animal rescue organization in Eastern Oregon called Fuzzball Rescue.

So without any further adieu, here is her post:

First, my background. I have a BS in biology with a minor in Chemistry from Oregon State University. Since becoming a veterinary technician three years ago, I’ve had dozens of hours of training and continuing education on dog and cat nutrition.

Let me put it in bold and all caps so as to head off any accusatory comments: THIS DOES NOT MAKE ME AN EXPERT. However, it’s a hell of a lot more of a solid background on this subject than most people have.

Between my science background, boots-on-the-ground experience in a vet clinic, and personal research on Pubmed and VIN, I feel confident in saying that most people who open their mouths on this subject (literally IRL, and figuratively online) have at least some misconceptions about food allergies. Some people have a lot of misconceptions. Hopefully, this will clear some of those up.

Let’s start with an explanation of food allergies. When we say a dog is allergic to a food, what that actually means is that they are allergic to one or more proteins contained in that food. A dog is not allergic to “corn”, they are allergic to one or more proteins contained in the corn. This is a huge misconception when it comes to food allergies. It’s not helped by the fact that researchers never study individual proteins, only ingredients, so as short hand they always refer to allergens by their source. Hence, a dog is said to have an allergy to “chicken”, even though it’s only a tiny fraction of the muscle meat that’s actually causing a problem.

Let me repeat that, because some people have a hard time overcoming this: allergies are not caused by carbohydrates, starch, or fats. The vast majority of food allergens are large, water-soluble glycoproteins measuring between 10,000 and 70,000 Daltons.

Protein, protein, protein.

As food is broken down in the stomach and moves to the small intestines, what’s suppose to happen is that the proteins are broken into little pieces (hopefully individual amino acids) that are small when they’re absorbed through intestinal cell walls. Instead, for whatever reason, large segments of the offending protein get through the mucosal barrier and ends up touching too much surface area of the cell walls. The body reads this as a threat, and responds accordingly. This leads to a cascade of reactions that eventually leads, strangely enough, to itchy skin, especially the skin of the face, paws, and ears, but it can effect the entire body. It can also cause GI upset such as diarrhea or vomiting, but most of the time food allergy is suspected because of itchy skin.

This is a huge problem when it comes to diagnosis (whether by a vet or by the pet owner at home) because it’s suck a freaking general symptom. Itchy skin happens for all kinds of reasons. Far too often, the dog owner sees an itchy dog and immediately assumes it’s the food. The same symptoms could be caused by a host of things that need to be ruled out first: contact and inhalant allergies (dogs don’t sneeze when they’re allergic to pollen like we do), flea bite allergy, immune disorders, mites, lice, fungal, bacterial, and yeast infections, drug reactions, and more.

People make snap diagnosis all the time. Hell, we had a client once whose dog had a classic hypothyroid hair loss pattern and insisted it was the food and refused to let us do a blood test to rule out thyroid dysfunction. They must have tried over a dozen different kibble diets trying to cure the “allergy”. Every few months they’d bring the dog in and swear the hair loss was getting better. To us, it looked the same.

Let me focus on flea bite allergy for a moment. It is a common cause for itchy skin reactions in dogs, and far too often it is dismissed by pet owners. Please, don’t dismiss it. The symptoms are similar to food allergy, the itchiness can affect the entire body even after one flea bite, and the symptoms can last for weeks. Too often, dog owner assume that it couldn’t possibly be flea allergy because they haven’t seen any fleas on their dog, or it’s the dead of winter, or because they treat their home periodically with insecticides. Fleas can live indoors just fine during the winter; they can hide in refuges indoors where insecticides haven’t penetrated, like under furniture and between couch cushions; and they can hop on a dog, have a nice blood meal, and then hop off without ever being seen.

Bottom line: rule out flea allergy in every itchy dog, every time they have an episode of itching.

What makes diagnosis of food allergy even more difficult is that a single exposure to the offending protein can cause symptoms for weeks. Possibly months. I’ve had clients tell me with confidence that they cured their dog’s allergy by switching foods. “Fido stopped itching within a week!” they say happily, “that’s how we knew Fido was allergic to Brand X Kibble.”

Sigh.

Another problem is that it’s not always clear why a dog develops a food allergy. It can happen at any age, to any ingredient, even one that’s been fed to the dog for years.

Another problem is that dogs are often allergic to more than one protein, and there are some known cross-reactivities, also. A dog who is allergic to beef muscle meat is more likely to also be allergic to beef milk. There is also potential cross-reactivities between beef, sheep/goat, and venison meats.

Yet another problem is that we don’t really know how prevalent food allergy is in dogs. One report indicates food allergy accounts for 5% of all skin diseases. Some researchers say that it’s very rare, others that it’s the second or third most common hypersensitivity reaction. Considering the difficulties in a confident diagnosis, I’m not surprised at the discord.

The only way to diagnose a food allergy with confidence is (AFTER ruling out other reasons listed above) to go on a 12 week food trial of either a specialized hydrolyzed protein diet (where the proteins are “predigested” to small sizes that won’t cause a reaction and are processed on equipment that is clean of any other food residue: examples include Hill’s Z/D Ultra or Purina HA), or a strict and well-documented novel protein diet (a diet with a single protein the dog hasn’t eaten before, and a single source of carbs that contains little or no protein, such as cornstarch or white potato). More info here.

Both ways are a pain in the ass, (no other foods allowed: no treats, no flavored toothpaste or chew toys, no flavored medications) and take forever. But, if the dog’s itching stops or is reduced within 12 weeks of the strict diet, then you can safely suspect a food allergy. Then you can, one at a time, add single ingredients back into the diet until one (or more) of them causes a reaction. If you’re lucky, it’s only one or two ingredients that can be easily avoided in the future.

According the best research out there, the most common food to cause allergies in dogs is (probably) beef. This doesn’t make beef “highly allergenic”, it means that, of the population of (studied) dogs with food allergies, it is the most common ingredient that they reacted to them. However, there are millions of unique proteins in the ingredients in dog foods, and only a few ingredients have been studied thus far.

Food allergies are an immune disorder of individual dogs. Just because some dogs are allergic to beef, doesn’t mean beef is inherently “bad”. The food itself doesn’t cause dogs to be allergic.

The flaw is in the dog’s body, not in the food.

Lastly, I want to talk about Public Enemy Number One for a moment, simply because I’m sick to death of clients and dog aficionados talking crap about it: CORN.

Corn does not “cause” food allergies. It is not even one of the most common (known) food allergens. Corn is not “filler” in kibbles; it has all kinds of nutritional value for dogs: B vitamins, fatty acids, protein (corn gluten meal is 60% protein), and, of course, carbohydrates. Which dogs can digest just fine. Sorry if you don’t like that fact. Forget the recent study about amylase production in dogs and wolves; and stop speculating about specific paths of evolution and just consider the simple reality of kibble digestibility studies. Every major pet food company has done at least some kind of digestibility study on at least some of their foods. In fact, all the ones that market to vet clinics (like Purina, Hills, for example) make a big deal out of this fact. Kibble is highly digestible for the majority of dogs. The end.

In conclusion.

Things you should stop saying:

“It’s unnatural for dogs to eat carbs and it makes their skin itchy and flakey.”

Things you can feel free to say:

“I feed my dog XYZ and they seem to do well on it, so I think I’ll continue.”

If you only read one article about food allergies, read this one:

Verlinden, et al. 2006. Food Allergy in Dogs and Cats: A review. Clinical Reviews in Food Science and Nutrition, 46:259-27