Sometimes in food-allergy circles you will hear someone snort, often dismissively, about “intolerances.” Martyrs love their martyrdom, you know?
The difference between the two isn’t always obvious, even when you’re doing due research diligence. Take this quote from a piece in the Whole Dog Journal:
“Food allergy or intolerance can cause intermittent to frequent vomiting, diarrhea, weight loss, loss of appetite, itchy skin or ears, eosiniphilic plaques, and a number of less common disorders.” (Susan Weistein, Whole Dog Journal, Oct. 2006 Linked article probably behind paywall).
Note that all symptoms, both digestive and otherwise (itchy skin, etc) are under that “allergy or intolerance” lead in. But that isn’t entirely true. Let’s look to humans for some illumination:
The “intolerance” we most often hear about in humans is lactose intolerance. A lactose intolerant person who drinks a glass of milk will experience some stomach upset. While that stomach upset can be fairly severe, there aren’t non-digestive symptoms.
A true food allergy, on the other hand, can be life-threatening. Many schools have gone “peanut free,” because even incidental exposure might cause anaphylaxis in highly allergic children. Not every food allergy is so dramatic, though. The Mayo Clinic lists a group of minor symptoms for even peanut allergies, like itching.
The grey area is in the overlap between the two. In that same list of peanut allergy symptoms, you’ll see digestive problems. This overlap also exists for dogs. Both allergies and intolerances can cause stomach problems. For dogs the situation can be somewhat murkier than for people, though, because severe human food allergies seem to cause a wider range of symptoms. I’ve never read of an allergic dog having difficulty breathing, for instance. (There are other differences, so be aware that the analog only works so far. To give an example, in humans food allergies are a childhood problem, often lessening or entirely disappearing in adulthood. In dogs, they are almost always adult onset.)
An allergy, by definition, involves an inappropriate immune system response. In dogs with food allergies that response usually manifests as itching and/or ear problems. Given typical dog responses to itching and/or ear problems, you’ll probably see an array of secondary issues, like recurrent skin infections. There is some debate about the necessity of obvious GI problems to indicate a food allergy. It may take weeks for food allergy symptoms to subside once the culprit is removed. A food intolerance, on the other hand, confines itself to the digestive tract and resolves itself basically as soon as that food has been digested.
The additional complicating factor can be seasonal allergies. Dogs have inhalant allergies, just like people do. While itching, licking, chewing, and scratching can be caused by food allergies, they can also be caused by allergies to pollen, dust, dander, mites, and fleas.
Then, of course, if your life wasn’t hard enough, there is the sad fact: dogs who have one kind of allergy have an increased chance of having another kind of allergy. Inhalant allergies are somewhat more common than food allergies, but most food-allergic dogs I know also have inhalant allergies.
Pragmatically, the difference between food allergies and intolerances doesn’t matter very much. I suspect this is why they get casually grouped together. If your dog is intolerant of a protein, you might need to exercise less Constant! Vigilance! at every second, because the effects are more likely to be short-lived. You still don’t want to include that thing as part of your dog’s regular diet, though.
As for food allergies vs. inhalant allergies, you’ll have to use your own judgment. Watch for patterns of symptoms, from time of year to what body part your dog is scratching the most. For us, they look quite different, but it’s hard to guarantee a particular symptom dog to dog.
I hope that sheds some light on the wonderful world of “my dog’s a hypersensitive mess.” This is by no means a comprehensive look, but I’m hoping it answers some questions I’m seeing in my search engine traffic.