Once we have ruled out the possibility of parasites and treated a secondary microbial dermatitis, our history taking skills
and physical examination will often point to an allergic etiology as the cause of the pruritus. In the non-seasonal dog, rule
outs will include an adverse food reaction and environmental allergy. One must not be blinded to the possibility that there
is more than one etiology at play. Indeed, in a study performed at our facility looking at feline self trauma, we found that
52% of our cases were multifactorial.
"Hypoallergenic"/ Elimination Diet
The possibility of an adverse food reaction has most often been considered by our clients before they present their pruritic
pets. Most commonly, they believe that they have adequately ruled out this differential and our seeking alternative explanations.
Unfortunately, there is a plethora of media misinformation on the subject. Clients have the impression is that food allergies
are a common cause of allergic dermatitis in dogs when in fact, in one study, adverse food reactions account for at most 22%
of allergies in dogs (Chesney 2002). There is also confusion between the use of diets to help environmental allergies and
hypoallergenic diets. As a result, I spend a large part of my appointment time with the client discussing adverse food reactions
and their diagnosis. I believe that the diagnosis of a possible food allergy can easily be missed due to poor "technique".
The following is a summary of how I try to rule out food allergies in my patients.
The most important aspect of the diagnosis goes back to the first lecture of the day – interviewing the witness. A complete
dietary history is essential (be sure to include treats in the discussion). Many clients are too embarrassed to admit what
they feed their dog. The importance of an accurate evaluation must be emphasized. The next step is to find out if the pruritus
is chronic or intermittent – without a change in diet or treatment. Age of onset can help a lot too. Environmental hypersensitivity
most commonly starts between 6 months and 3 – 5 years of life. Adverse food reactions do not seem to have the same time constraints
and are seen as well in the very young or the very old. A poor response to adequate doses of steroids in the absence of another
explanation will increase the suspicion that food is playing a role. However, the converse is not true; adverse food reactions
can be steroid responsive.
I will often tell clients that, regardless of what the person at the pet supply store tells you, there is only one perfect
hypoallergenic diet – it's the diet that doesn't contain the ingredient to which the dog is allergic! People need to be aware
at the onset that there are many studies looking at hypoallergenic diets and all diets have their deficiencies. For this reason,
multiple dietary trials may need to be tried before one can rule out adverse food reactions. I will go as high as three different
trials before completely ruling out food. One also needs to educate the client on how to properly perform a food trial (and
how difficult it is to do one appropriately).