Gastrointestinal Food Allergies

An allergy is an immune-mediated reaction that harms the body
instead of protecting it. Examples of such reactions include
fatal human reactions to a single bee sting or eating a single
strawberry. These are called “hypersensitivity” reactions.
Certain types of reactions by the body (depending on the type of
hypersensitivity reaction, of which there are four) cause an
exaggerated response that produces excessive irritation
(inflammation) or decreases the size (constriction) of vessels
or airways. The substances that mediate these reactions
(antibodies and lymphocytes) are programmed to respond to
specific substances called antigens. Antigens that cause
hypersensitivity reactions are usually proteins or
carbohydrates, and they may be found in almost anything,
including food.

Depending on where the hypersensitivity reaction takes place and
how many antibodies or lymphocytes are involved, the
consequences may vary from sudden, life-threatening episodes to
delayed ones that cause inflammation in just one part of the
body.

In dogs and cats, most hypersensitivity reactions that result
from eating foods cause either skin disease (characterized by
scratching) or various gastrointestinal (GI) signs such as
diarrhea and/or vomiting. Sometimes both the skin and the GI
tract are affected in animals that have a food allergy, but many
animals with food hypersensitivity have either skin or GI signs
but not both.

The GI signs of food allergy sometimes occur immediately after
eating (i.e., immediate-type hypersensitivity reactions).
However, food allergy in pets is more commonly a . ‘delayed”
hypersensitivity reaction, meaning that the consequences arise
hours or days after eating the food and then persist for hours
or days after each exposure. Because most pets eat the offending
antigens every day, GI signs tend to be more or less constant.

There is seldom a clear-cut association between eating and the
onset of signs, making it hard to determine that eating a
particular food is causing the disease. To help diagnose this
problem, we can look for microscopic changes on small pieces of
intestine obtained by doing surgery or by passing a long
instrument from the outside into the stomach. However, changes
that suggest allergy (i.e., eosinophilic inflammation) are
usually a form of inflammation.

Inflammation caused by food allergies usually resembles that
caused by other diseases. The best way to diagnose a food
allergy is to feed the pet a hypoallergenic diet (i.e., a
therapeutic dietary trial) and see if the problems disappear.

When performing a therapeutic trial for a food allergy, the diet
must be carefully chosen. Because there is no one diet that is
hypoallergenic for all pets, one must design or find a diet that
is appropriate for each animal.

The pet may be allergic to almost any component of its current
diet; therefore, we want foodstuffs that the pet has not eaten
before. We usually choose a diet that

(1) contains as few ingredients as possible,

(2) contains foodstuffs that we know the pet has not eaten in
the past (and hence is unlikely to be allergic to), and

(3) contains foodstuffs that we know hardly ever cause allergic
reactions (e.g., potato, rice).

Because some patients that are allergic to multiple antigens
require a strict hypoallergenic diet, homemade diets are
sometimes needed. Although inconvenient and restrictive, they
are often the most successful in treating the allergy. Most
homemade diets are not balanced but are adequate for use in
mature animals for the 2 to 4 months when the animal is having
the trial. We have to make many assumptions when we choose these
diets, and it is possible that the pet is allergic to something
unexpected.

When such a dietary trial is begun, it is imperative that
absolutely nothing else be fed. Even flavored pills or toys can
contain enough antigens to cause signs of food allergy to
persist. The dietary trial must be performed long enough to
allow the clinical signs of delayed-type hypersensitivity to
disappear.

Some patients evidence improvement within a day of dietary
change, whereas others require 4 to 8 weeks before improvement
is seen. If a patient has a dietary allergy, it may have a
genetic predisposition to allergy and may eventually become
allergic to the ingredients of the hypoallergenic diet that it
responded to well at first.

Other tests have been tried in order to determine what dietary
components a pet is sensitive to. As of this writing, these
tests have not always correlated well with the results of
dietary trials.

The above is general veterinary information. Do not begin
any course of treatment without consulting your regular
veterinarian. All animals should be examined at least once every
12 months.