Diagnosing a food allergy - a big challenge

Food allergies in dogs and cats - do you know how to run an elimination diet challenge?

Posted: 14 March 2017

Diagnosing a food allergy - a big challenge


We’re lucky these days to have so many diagnostic tools at our disposal. When it comes to skin allergies, and in particular food allergies, where do we start?

Should we use intradermal skin tests and IgE blood tests? In reality, the only way to successfully diagnose a food allergy in a dog or cat is perform an elimination food trial.

Both species can become allergic to any food they’re exposed to, with the most common allergens in dogs being beef, chicken, eggs, cow milk, wheat, soy and corn, whilst in cats they tend to commonly develop a problem with chicken, fish and dairy products.

Look for the signs

Dogs – check for these problems

  • Non-seasonal pruritus – itchy skin at times other than the common seasonal problems associated with fleas, grasses, pollens and molds
  • Otitis – inflammed, red and painful ear canals and pinnae.
  • Eosinophilic vasculitis – look for purplish-red spots on the skin
  • Dermatitis – especially on the ears and bottom
  • Pyoderma – especially if it keeps recurring after treatment with antibiotics
  • Seborrhoea – you may find various forms of seborrhoea
  • Urticaria – raised patches, otherwise known as ‘hives’
  • Gastro-intestinal problems - these signs may include vomiting, diarrhoea, flatulence or more frequent bowel movements

Cats – the most common indicator

The classic clinical sign for food allergy in cats is pruritus, especially around the head and ears.

Response to steroids – check if they help

When pruritus is not corticosteroid-responsive, a food allergy should always be considered.

Elimination diets – what’s involved?

Intradermal allergy testing, serology testing and skin patch testing all produce unreliable results. An elimination diet trial is thus the only accurate method to identify a food allergy.

Before you start

  • Treat skin infections - it’s important to treat any secondary bacterial or fungal infections, typically Staph pyoderma and Malassezia.
  • Don’t forget fleas - It’s also wise to treat for fleas to minimize other causes of pruritus.

Choosing the right trial diet

When choosing a diet to feed a suspected food-allergic patient, you need to select one that contains proteins that the animal hasn’t been exposed to before. It’s also essential that the food is also palatable and affordable.

Hydrolysed protein diets are another option. It’s still important to try and choose a hydrolysed diet which the patient has had little or limited exposure to the original protein.

Patients don’t have to eat the elimination diet indefinitely. Once the food-allergic patient is stable, you can start to challenge the patient with a novel protein food and monitor for a recurrence of the clinical signs. Generally an improvement on a diet may take weeks, but most patients will have a flare up within days or even hours of being fed the offending food.

Starting the diet

Start the patient on the elimination diet trial. It should be the only food the patient receives. There must be absolutely no treats or protein-based supplements.

Maintain the diet for 6 weeks

After six weeks, check on the patient’s progress. If there is some improvement, continue the trial for a further six weeks for maximum improvement.

If there’s been no improved, check that there haven’t been any omissions in the strict diet. Some of the common failing include:

  • Feeding treats
  • Children dropping food
  • Unsupportive family members
  • Stealing food from another dog or cat’s food bowl

If the elimination diet trial appears to have been given correctly but the patient didn’t improve, then the patient is potentially suffering from atopic dermatitis and reacting to environmental allergens.