Celia Haddon - Cat Expert

Understanding animals through their behaviour


Written with the help of Dr Tim Nuttall,  co-author of Skin Diseases of the Dog and Cat, and author of Veterinary Advice on Skin Diseases in Dogs.


Dogs tend to develop food allergies to a familiar food. These can be part of multiple allergies. Food intolerances tend to be to new foods and are usually either present from birth (for example gluten sensitivity) but can occur later in the dog’s development. As far as an owner is concerned, the symptoms, diagnosis and treatment are similar.


Common symptoms of food allergy or food intolerance include chronic vomiting, colitis, irritable bowel syndrome, diarrhoea, failure to gain weight or weight loss, poor or dull coat, lack of energy. Less commonly food allergies may express themselves in a dog’s skin. There is a general itchiness so that the dog is chewing, licking and scratching in a frenzied way, sometimes even stopping to scratch during eating or during a walk. The hair is chewed away to leave bald patches. The skin becomes red and inflamed and in extreme cases the skin is broken. The damaged skin then becomes darkened and thickened. The damaged areas then get bacterial or fungal infections and may begin to smell. Recurrent ear infections mean the dog is scratching or shaking its ears. There may be discharges from the ears.


Your vet will want to rule out other possibilities like dietary upsets, internal parasites, anal gland problems, infections, metabolic diseases of the kidney, liver and pancreas, and, in older dogs, various cancers.

If the dog has skin symptoms, the vet must rule out parasites such as fleas and mites. Even if there are no visible signs of fleas, the dog should be treated with prescription products from your veterinary surgeon. Over the counter flea preparations are not strong enough. Treat the house and all other animals in the household. Proper flea control is important, even if the cause is not direct allergy to flea bites. Any bacterial or fungal skin infections must also be treated. If these first line treatments succeed, then the skin problems are probably not due to an allergy.


Dog can become allergic to something they have been eating for years without trouble. They rarely, if ever, become allergic to a completely new food. Food intolerances are more common with new foods but may also sometimes occur with familiar foods.

a) The specialist will put your dog onto a completely new diet, either a commercial one or a home cooked one like, say, fish and potatoes. This is sometimes called an exclusion diet. For at least six weeks the dog must eat NOTHING else , no treats, no chews, no bones. During this period, if the skin gets better, this indicates but does not confirm it was a food allergy.

b) The next stage is to put the dog back on its ordinary diet for up to two weeks. This is called a re-challenge. If the skin inflammation returns, then this is proof that the diet was causing it.

c) Your dog will go back on the exclusion diet and a new ingredient will be added, one at a time. If the new ingredient causes skin inflammation, then the cause of the allergy is identified. Once the cause is identified, the specialist will help you choose a new diet for your dog, , probably a commercial diet which excludes the problematic ingredients.

Sometimes this process is short circuited, and after feeding the exclusion diet, the owner is helped to try out different commercial diets with varying ingredients to see if one can be fed without causing the allergy.


If the food trials identify the food ingredients, which are making your dog ill, then your vet will help you choose a diet which excludes these ingredients. You will need to keep your dog on this diet for the rest of its life. You may be able to find a commercial or home-made titbit to use for special treats.


If the symptoms do not clear up after food trials, it is usually a good idea to ask your vet to refer you to a veterinary specialist. If the symptoms are itching skin, then find a vet who has a dermatology specialism.  If the symptoms are gastric or bowel,  vet who specialises in gastro-enterology or internal medicine will have the equipment and expertise to do further investigations. This is the same principle as a GP referring a patient to a consultant. Most vets will be happy to do so and indeed cannot refuse to do so. There are inflammatory and other bowel conditions which can trigger a food allergy. These can be diagnosed by endoscopic investigations and biopsies. .

If the symptoms are mainly on the skin, then a veterinary skin specialist can help. It may be that there are factors, other than just food allergy, which are triggering the skin disorder.


These notes are my copyright. I am also usually happy to have the exact words reproduced on websites, in return for a link, my name, and if permission is asked beforehand. I like to check the websites where it might be used. Email me via this website for permission which will usually be given. Organisations wishing to use them in print should contact me via this website. Copyright © 2007 Celia Haddon. All Rights Reserved.

Safety notice.

All normal safety precautions should be taken when dealing with animals. The advice in this section should be taken only at the owner’s own risk. All sick animals should be seen by a vet.

General advice of the kind found in this website is no substitute for an individual consultation with a vet or qualified behaviourist working on a vet’s referral.

Join me

My Books & E-Books