With fleas, mites and various infections, allergies are one important cause of itchy skin in dogs and cats. Such allergies can involve reactions to inhaled allergens (allergen = any allergic substance), contact allergens and fleas as well as to allergens in foodstuffs. The appearance of the skin is often outwardly identical and diagnosis can be a complicated process - it is easy to jump to the wrong conclusion in these cases without a systematic diagnostic approach. Note for example that allergies due to inhaled allergens (atopy) are ten times more common than food allergies.

Where food allergy is implicated, sensitivity is usually to one component of the diet e.g. beef, milk, pork. This may have been part of the diet for many years before itching appeared. Often there has been no change of diet. Concurrent vomiting and diarrhoea are present in only about 10-15% of cases.

Most commercial dog foods contain a variety of ingredients.

Once diagnosed dietary allergies can be treated by dietary change rather than drugs.

For this reason, although dietary allergies are not common, some cases benefit from a trial with a strict exclusion diet. This is the only way to prove (or rule out) dietary allergy.

Even if your animal does not improve, ruling out a dietary problem brings us one step closer to a diagnosis of something else.

What to feed
To minimise the chances of including the offending foodstuff, the diet must be reduced to a bare minimum of ingredients. A protein source and a carbohydrate that have not been eaten by your pet before will be chosen. There are two main options -

• Most commercial foods contain unpredictable ingredients and are not suitable for exclusion dietary trials. However some modern prescription diets e.g. Hills z/d may be suitable in certain cases.
• Home cooked diets. Boiled chicken, lamb or rabbit (or other novel protein source) with brown rice is best.

Discuss the various dietary options with the vet before embarking on a trial.

What not to feed
It is vital that nothing other than water and the allocated diet should pass your animals lips.

In particular, no bones, chews, biscuits, milk, tea, coffee, titbits or food fed to other pets. Even one mouthful of something extra may mean all your effort is wasted.

How long to carry on for
Some animals will improve in 2-3 weeks. If there is no improvement, we need to carry on for at least 6 weeks to prove that diet is not part of the problem.