Produced by Dr Andrew Carter, Specialist
Veterinary Dermatologist.
What is Food Hypersensitivity?
Food hypersensitivity (allergy) is an uncommon cause of pruritus
(itch) in dogs and cats. It is triggered by your pet’s immune system
becoming sensitised to certain components of the diet (allergens).
These allergens are most commonly proteins present in meat, dairy or
cereal components of the diet but in some cases they will be
artificial additives in commercial pet foods. Food hypersensitivity
resulting in skin disease can develop at any age and usually
involves foods that have been in the diet for some time.
In most cases pets with skin disease resulting from food
hypersensitivity will be itchy. They may scratch, lick themselves or
rub against things frequently. The skin disease may affect only
certain areas or may be generalised. In some cases it can be the
primary cause of otitis externa or claw disease without other signs.
In most cases the skin appears normal or a little red to start but
later it may develop red sores, pimples, dandruff, dark areas or
thickened skin as the skin becomes traumatised, chronically inflamed
or develops secondary infections with yeast (Malassezia) or bacteria
(Staphylococcus intermedius). They may also have a history of
frequent defaecation, easily upset stomachs or colitis.
How is Food Hypersensitivity Diagnosed?
Food hypersensitivity can only be diagnosed by completing an
elimination diet trial and challenge. Although blood tests are
available they are not very specific and are therefore only useful
in selecting components for an elimination diet trial.
How Do I Perform an Elimination Diet
Trial?
An elimination diet trial is conducted by feeding a diet containing
a very limited range of potential allergens. This may either be a
home cooked diet or a commercial diet where the proteins have been
broken down to the point where they can no longer produce allergic
reactions (Hills Z/D ultra).
If a home cooked diet is chosen it should contain a single protein
source (usually meat) from one species of animal and a carbohydrate
source (usually a grain or starchy vegetable).
This option should only be used for a period of under 2 months and
not be used for growing or pregnant animals as it not nutritionally
complete.
The alternative is to use one of the commercially available limited
protein diets or Hill’s Z/D ultra. Ideally the primary components of
these foods should not have previously been a part of your pet’s
diet.
The diet should be introduced slowly over 1 week to reduce the risk
of digestive upsets. The diet should then be continued for at least
6 weeks. During this time flavoured medications should be avoided as
they may contain enough allergens to trigger food allergies even if
fed only once each month. For ongoing medications such as heartworm
prevention or arthritis treatments, the vet will discuss non-flavoured
option, as missing out could be harmful to your pet. It is also
important that any parasite infestations or infections are
controlled before the diet trial is complete as pruritus resulting
from these conditions can mask any response to the food trial.
Most pets that have skin disease resulting from food
hypersensitivity show a noticeable reduction in pruritus by 6 weeks
but if the owner is unsure the diet can be continued for a further 2
weeks.
NB. It may take up to 12 weeks for the pruritus caused by food
hypersensitivity to resolve completely.
The Challenge
The most important phase of the elimination diet trial is the
dietary challenge. During the challenge phase no treatment should be
altered otherwise it is difficult to be certain that any change is
diet related. To challenge a diet as many different components of
your pet’s previous diet are fed daily for at least 2 weeks. In the
majority of cases the pruritus increases within 2 days of
reintroducing the offending food although delays of up to 2 weeks
are reported. If the pruritus recurs between 3 and 14 days the
elimination diet should be reintroduced and challenged again to
confirm the diagnosis of food hypersensitivity. The time taken to
cause the pruritus to recur should be noted.
If the pruritus does not return within 2 weeks we can rule out food
hypersensitivity.
Identifying the Allergen
The next phase is to identify the specific dietary components that
cause the food hypersensitivity. To do this the elimination diet is
reintroduced until the pruritus settles down again and then a
nutritionally balanced, long term diet is introduced. If this does
not cause the pruritus to recur it can be fed long term. Individual
food items are then sequentially introduced to the diet at intervals
longer than the time taken for the pruritus to recur when the diet
was initially challenged. I usually start with meats (beef, lamb,
chicken, pork, turkey) followed by dairy (cheese), soy (tofu or soy
milk), then wheat (flour), corn (cornflour) and any other components
identified on the ingredients list of the pets previous foods. If a
component causes the problem to recur it should be noted and removed
from the diet. Once the pruritus settles down again the remaining
potential foods can be tested until all foods that cause problems
are identified. Normal commercial diets can then be checked and
those that do not contain the offending food items can be tested to
determine which are suitable as long term diets.
From VetDerm SA, by Dr Andrew Carter BVSc (hons), CertVD, MRCVS.
