By Dial A Vet Veterinary Team
The single most reliable test for food allergy in dogs is not a blood test or a hair test. It is a strict elimination diet trial, where your dog eats only one carefully chosen food for at least eight weeks. It sounds simple, and it is the part that most often goes wrong. One flavoured chew, a crust from the toddler's sandwich or a beef-flavoured tablet can undo weeks of effort. If your vet has suggested a diet trial, this guide explains how it works and how to give it the best chance of a clear answer.
Key takeaways
- An elimination diet trial is the standard way to diagnose food allergy in dogs.
- Most dogs need at least eight weeks on the trial diet to show clear improvement.
- The trial food is either a hydrolysed prescription diet or a protein your dog has never eaten.
- Everything else that goes in your dog's mouth must stop or be matched to the trial, including treats and flavoured medicines.
- A food challenge at the end confirms whether food was the cause.
Why blood and hair tests are not enough
Blood tests for food allergy antibodies, and hair or saliva tests sold to owners, have been studied and found to give unreliable results in dogs. Many allergic dogs test negative and many healthy dogs test positive. Vets may use allergy testing for environmental allergies, but for food, the diet trial remains the gold standard.
Before you start
Your vet will first make sure other causes of itch or gut upset are controlled, because they would muddy the result:
- Fleas must be fully controlled on all pets in the house.
- Skin and ear infections should be treated.
- Mites and intestinal parasites should be ruled out.
- Your vet may give short-term itch relief early in the trial so your dog is comfortable.
Choosing the trial diet
Hydrolysed protein diets
Proteins are broken into pieces too small for the immune system to recognise. These prescription diets are convenient and nutritionally complete.
Novel protein diets
Based on a protein your dog has never eaten, such as kangaroo, venison, rabbit or fish, depending on your dog's history. Over-the-counter "limited ingredient" foods have been shown to sometimes contain proteins not listed on the label, so prescription versions are preferred.
Home-cooked trials
Sometimes used, but they must be designed with your vet or a veterinary nutritionist, because short-term home diets are rarely balanced, especially for growing puppies.
Running the trial
- Switch over gradually across about a week to avoid stomach upset.
- Feed only the trial diet and water for at least eight weeks.
- Stop all treats, chews, dental sticks, rawhide, table scraps and flavoured toothpaste.
- Ask your vet about flavoured medicines, including some flea, worm and heartworm chews. Unflavoured alternatives may be available.
- Use pieces of the trial food, or treats made from it, for training.
- Tell everyone in the household, visitors and neighbours. Supervise children at mealtimes.
- Stop scavenging on walks and keep other pets' food out of reach.
- Keep a weekly diary of itch level, ear and skin changes, and stool quality.
Signs the trial is working
Gut signs often improve within two to four weeks. Skin and ear signs usually take longer, which is why eight weeks is the minimum. Look for less licking and scratching, calmer ears, fewer skin flare-ups and more consistent stools.
When to see a vet during the trial
Emergency now: facial swelling, hives with breathing difficulty, collapse, or repeated vomiting with weakness.
Same day: bloody diarrhoea, refusing the diet for more than a day, or a painful, swollen ear.
Book a recheck: at the times your vet sets, usually around four and eight weeks, or sooner if signs worsen.
The food challenge
If your dog improves, your vet will usually ask you to reintroduce the old diet. If signs return within days to a couple of weeks, food allergy is confirmed. You can then test single ingredients one at a time to work out exactly which proteins to avoid. Skipping this step is tempting when your dog is finally comfortable, but it helps avoid unnecessarily restrictive diets for life.
If the trial does not help
A well-run trial with no improvement suggests food is not the main cause. The next most likely explanation is environmental allergy (atopic dermatitis), which is managed differently. See dog skin allergies for an overview.
FAQ
Can I just buy a grain-free food instead?
Grain-free foods are not elimination diets. Most contain common proteins such as chicken or beef.
What if my dog eats something by accident?
Note it in the diary and tell your vet. A single slip may cause a flare, and your vet may extend the trial.
Is kangaroo always a novel protein in Australia?
Not anymore. Kangaroo is common in Australian pet foods, so your vet will choose based on your dog's full diet history.
Do I need to do a trial for life?
No. Once the trigger is known, many dogs can move to a long-term complete diet that avoids it.
Talk with a vet
A Dial A Vet consult can help you plan a diet trial and troubleshoot problems along the way. Facial swelling, breathing difficulty or collapse needs an in-person emergency vet straight away.
References
- Olivry T, Mueller RS, Prélaud P. Critically appraised topic on adverse food reactions of companion animals (1): duration of elimination diets. BMC Veterinary Research, 2015
- International Committee on Allergic Diseases of Animals (ICADA): Resources on canine atopic dermatitis and food allergy
- Merck Veterinary Manual: Food Allergy in Dogs and Cats
Disclaimer: Educational only. This page does not replace an examination or advice from your own veterinarian. Diet trials should be planned with your vet.
