Food allergy: a rare cause of itchiness in pets
Whenever I eat out these days, I find myself bemused by the abbreviations after each item on the menu: as a vegetarian, I appreciate the "V" or the "VG" (vegan), but there's also now a long list of allergen-based letters, from GF (Gluten Free) to DF (Dairy Free) and EF (Egg free). There are also often additional notes on the menu, outlining other allergens in bold type.
This awareness of allergens is new: twenty years ago, menus were much simpler.
The same trend is happening in the animal world: if you visit a pet shop, you'll find a plethora of hypoallergenic pet foods on the shelves. And if you have a pet with skin disease, some people will tell you to change their diet, suggesting quirky nutritional products that they believe will fix any itch. But what's the truth about food allergy in pets? Read on if you want to know the science rather than believing the myths.
Nobody knows why some animals develop allergies to food proteins. Normally, a process called "oral tolerance" prevents the immune system from reacting to proteins that pass through the digestive system. Clearly, in food-allergic dogs, there's some sort of fault in this process, but scientists are still trying to work out why and how this happens. Genetic predisposition plays a role, with certain breeds and strains of dog being more prone to it.
The terms "food allergy" is used to describe a disease involving allergic skin disease, gastrointestinal symptoms, or both, triggered by food. Another, term, "food intolerance" is used to describe other reactions to food where there is no actual allergy involved (e.g. fatty foods can cause some dogs to have gastro-intestinal irritation).
The severe, dramatic, potentially fatal type of dietary allergy that is seen in humans (eg to peanuts) is almost never seen in dogs or cats. The common manifestation of food allergy in pets is either gastrointestinal upsets or itchy skin.
In my daily work, I see some dogs that cannot eat certain types of food, because they suffer from an upset stomach if they do. Often it's not clear whether this is an allergic reaction or an intolerance, but it doesn't make any practical difference. In both cases, the dog's owners simply learn that they can't give their pet a particular food (e.g. a brand of chews) because an upset stomach always follows.
Other times, dogs suffer from an upset stomach because of an obvious allergy: the owners learn from experience that a specific ingredient (such as chicken or beef) provokes the upset. So whether the dogs eats one or another brand of dog food, or a fresh item containing the ingredient, the result is the same: an upset stomach.
The type of food allergy that is most significant in the pet world happens when a dog or cat develops itchy skin because of an allergy to a protein in their food.
It's difficult to prove that a dog is allergic to a dietary ingredient. While blood tests for food allergy may be available, the results are inconsistent. In theory, an allergic dog ought to have high levels of food-specific antibodies, but in practice, these have only been identified in research dogs used for allergy studies. Some of these dogs had been bred specially to produce higher than normal levels of antibodies to everything they encounter, so the laboratory results don't easily translate into real life situations for pets. Scientists are still working on better ways to diagnose food allergy.
Itchy skin due to food allergy can develop at any age, from puppyhood through to old age. The contrasts with allergy to pollens and dusts in the environment (another common cause of itchiness in dogs) which tends to start when a dog is aged between 6 months and three years of age. Another difference between the conditions is that food allergy tends to happen all year round, while environmental allergies tend to be worse when there are more pollens in the air, in the spring and summer. Otherwise, there are many similarities between the conditions: they both cause itchiness and red skin around the face, ears, underside armpits, groin and feet.
A food trial is the only way to diagnose food allergies. A diet containing "novel ingredients" should be used: this means ingredients that the animal has never encountered before, so they cannot be allergic to them. Examples include venison or rabbit meat, with exotic carbohydrate sources such as tapioca. Most people prefer to use a commercially produced diet, obtained via a vet, designed to include only very rare and specific ingredients. Alternatively, a diet containing hydrolysed proteins can be used for the food trial: the ingredients have been "pre-digested" so that their structure is too small to trigger an immune reaction.
The food trial diet must be fed exclusively for 6-8 weeks with no other foods or drinks: just the special food and water. The diagnosis is confirmed if the itchy skin gets better while on the food trial, then recurs when the food item is re-introduced (a so-called "food challenge"). If the itchy skin doesn't get better on the special diet, food allergy has been ruled out.
If a food allergy is confirmed by a food trial, the cure is simple: the pet must never, ever eat that ingredient again. If he went to restaurants, he'd have to start reading the small print on those menus!