What is food allergy dermatitis?

A food allergy or intolerance is when protein molecules in the food trigger an immune response in the skin or intestines.

What signs do dogs show?

The main sign is itchy skin, most commonly of the ears and bottom. Some dogs will also have diarrhoea, not necessarily at the same time. Problems often start before the dog is 6 months old but may not be seen until as late as 3 years of age.


Initially other causes of itchy skin disease must be ruled out, such as skin parasites or bacterial, fungal and yeast infections. Various tests such as coat brushings, skin scrapes and hair plucks may be done.

Food allergy dermatitis signs are similar to atopy (a skin disease caused by abnormal reaction of the skin to proteins that enter the body via the skin). The two diseases can occur together in the same dog, making diagnosis more complicated. In general the skin of atopic dogs usually improves with steroid treatment, whereas that of dogs with food allergies often does not.

There are no blood tests that can be used to diagnose a food allergy.

The only definitive way of diagnosing a food allergy is to do a food trial. A food trial is time consuming and requires a lot of dedication and perseverance on the part of the dog’s owner.

Food Trial

In a food trial a dog is fed an exclusion diet, a diet containing proteins that it has never eaten before, for at least 6 weeks but maybe as long as 12 weeks. If the skin condition improves during that time then a food allergy may be suspected. However, to be sure that the skin did not just coincidentally get better, a dietary challenge with individual foods needs to be done.

A dietary challenge involves keeping the dog on the exclusion diet whilst one food item, such as chicken or egg, is added to the diet for several days to see if the skin condition deteriorates again. The allergic response can take several days to develop.

If the dog’s skin is provoked into reacting to a particular added food, that food is one of the causes of the skin problem. The addition of other foods to the exclusion diet, always only one type at a time, is repeated for all the basic protein sources – milk, and each type of meat, fish and starch, so that a picture can be built up of all the protein sources that trigger a dog’s skin problems. A diet excluding these protein sources can then be fed long term.

Many owners do not get as far as food challenging, preferring to stick to the original exclusion diet if their dog’s skin condition improves in the first few weeks of the trial.

Which foods can be used for the food trial exclusion diet?

First we need a list of every type of food that has ever been fed to the pet, including all the different ingredients in any proprietary diets. The exclusion diet needs to exclude (avoid) all of these ingredients and all of the other foods that the dog has been fed! There are 3 types of exclusion diets available:

  • A home cooked diet made of a meat or fish, plus a source of starch, such as potato, that the dog has never eaten before. Some dermatologists think this is the best type of exclusion diet.
  • A proprietary or prescription hypoallergenic diet containing a protein and starch source the dog has never eaten before. These can be dry or tinned diets. The standard ‘hypoallergenic’ or ‘sensitive’ supermarket type commercial pet foods are usually not sufficient – ask for advice.
  • A prescription diet made up of hydrolysed (broken down) proteins.


Food allergy dermatitis is an itchy skin condition. It is diagnosed by a positive reaction to food challenging, after the skin has improved on an exclusion diet. A suitable hypoallergenic diet can then be found that will not trigger skin problems.