A food allergy is an immune reaction to something a pet eats. In dogs and cats it shows up mostly on the skin, and the main sign is itching (vets call it pruritus) [1].
See your vet before you change your pet’s food to test for an allergy. A food trial only gives a clear answer when it follows a strict plan, and your vet sets that plan with you [2].
For other causes of itching, see why is my dog so itchy and itchy cat and overgrooming causes.
What causes it
Some skin reactions to food involve the immune system (a true allergy) and some do not (a food intolerance). Both can make the skin itch [2]. The trigger is usually a protein in the diet. The foods found to blame most often seem to mirror what pets have been fed for years, so the list may shift as feeding habits change [4].
The trigger is not always in the main meal. Treats, table scraps, chews, flavored toothpaste and medicines hidden in food can all contain it [2].
Which pets get it
- Age: it can start at almost any age, from young to old, and signs tend to begin later in cats than in dogs [1].
- Sex: males and females are both affected [1].
- Dog breeds: Labrador Retrievers and West Highland White Terriers looked predisposed in some studies, but the review found no reliable proof of a breed risk, so any dog can have it [1].
- Cats: no breed stood out across the studies [1].
Skin signs in dogs
Nearly every dog with a food allergy is itchy [1]. The itch may cover the whole body or settle in certain places.
- Where the itch is: the ears, feet and belly are common spots [1]. The face, armpits, groin and the skin around the bottom can also be involved [2].
- Ears and skin infections: the problems reported most often in dogs are skin infections that keep coming back (bacterial or yeast) and ear canal inflammation, called otitis externa [1]. See ear infections in dogs.
- Looks like other allergies: a food allergy can look the same as atopic dermatitis, or make it worse, so a vet cannot tell them apart by looking alone [2].
- Less common signs: hot spots (pyotraumatic dermatitis), hives and inflamed skin around the anus have been reported in a smaller number of dogs [1].
Skin signs in cats
Almost all cats with a food allergy are itchy too. A cat may scratch, rub or lick out its own fur [1].
- Where the itch is: the head and face are itchy in about half of affected cats. The ears, belly and feet are other common places [1].
- Other skin signs: hair loss from overgrooming, often on both sides of the body; sores and crusts on the head and neck from scratching; many tiny crusty bumps (miliary dermatitis); and eosinophilic lesions such as raised red plaques [1]. See eosinophilic granuloma complex in cats.
Year-round signs
An itch from food does not follow the seasons, because the pet eats the same food all year. An itch from pollen often comes and goes with the time of year [2]. A pet can have both kinds of allergy at once, so seasonal flare-ups do not rule food out. In a dog that itches all year, only a strict elimination diet can rule food out [2]. For seasonal allergies, see seasonal allergies in dogs and feline atopic skin syndrome.
Some pets also have gut signs. In studies run by skin specialists, about a quarter of dogs and a fifth of cats with a food allergy had both skin and gut problems [5]. Diarrhea and vomiting are the gut signs seen most [5], and soft stool, gas or more frequent bowel movements can also point to food [2].
How vets diagnose it
No blood test or skin test can reliably diagnose a food allergy in dogs or cats, and a review of the evidence did not recommend them for this purpose [3]. The way to find out is an elimination diet trial followed by a return to the old food [3].
The elimination diet trial. Your vet chooses the diet and the plan.
- The diet should be made of ingredients your pet has never eaten before, or of hydrolyzed proteins, which are broken into pieces too small for the immune system to recognize easily [2].
- No single diet works for every pet, and some pets need a second trial with a different diet [2].
- During the trial your pet eats only the trial diet and water. Even small amounts of anything else, now and then, can spoil the result [2].
- Common slips are table food, rawhide, treats, pills hidden in food, flavored toothpaste, flavored chewable medicines and eating other animals’ feces [2]. Ask your vet before you give anything else.
Cats need extra care. Cats can be hard to switch to a new food. A cat that stops eating, for example after a sudden change to a food it does not accept, can develop a serious liver disease called hepatic lipidosis [6]. Never starve or force a cat onto the trial diet, and tell your vet at once if your cat will not eat it.
The rechallenge. If the signs clear on the trial diet, your vet will have you give the old food again. The diagnosis is confirmed when the signs come back on the old food [2]. Stay on the trial diet until your vet has checked your pet [2].
Treatment
Once the trigger foods are known, the main treatment is keeping them out of everything your pet eats [2]. Your vet will help you choose a diet your pet can eat long term.
Other problems can keep a pet itchy. Many allergic pets have more than one allergy, such as flea or environmental allergies, and bacterial or yeast infections often come along with allergic skin and make it worse [2]. Your vet may need to treat these as well. For skin infections, see pyoderma in dogs and Malassezia yeast skin infection in dogs.
At home, as directed by a vet:
- feed only the diet, treats and medicines your vet has approved during the trial;
- write down your pet’s itching and anything unusual it ate, in a diary for your vet;
- do not test foods on your own.
Outlook
A food allergy is managed rather than cured: the itch stays away as long as the trigger foods do. Pets that also have other allergies or skin infections may need more than a diet change to stay comfortable [2].
These are statements about groups of animals. They cannot predict how one pet will do. Your vet can explain what this means for your pet.
When to see your vet
See your vet if your dog or cat has:
- itching that does not change with the seasons [2];
- itchy ears, feet or belly in a dog [1];
- itching, scratching or hair loss on the face, head or neck in a cat [1];
- small crusty bumps or raised red patches on a cat [1];
- ear infections or skin infections that keep coming back [1];
- itching with vomiting, diarrhea or soft stool [2][5].
FAQ
Can a blood test tell if my pet has a food allergy?
No. Neither blood tests nor skin tests can reliably confirm it [3]. A vet-led elimination diet trial and rechallenge is the way to confirm it [2][3].
Does the trial include treats and flavored medicines?
Yes. Treats, table food, flavored toothpaste and flavored medicines can all spoil the trial [2]. Ask your vet before you give anything else.
What if my cat will not eat the new food?
Do not starve or force your cat. A cat that stops eating can develop a serious liver disease [6]. Call your vet.
The bottom line
Food allergy in dogs and cats shows up mainly as itching. In dogs it often affects the ears, feet and belly and can bring repeated ear and skin infections; in cats it often affects the face, head and neck and can bring hair loss, crusty bumps or red plaques. The itch changes little with the seasons. Blood and skin tests cannot confirm it, so your vet confirms it with a strict elimination diet trial and a rechallenge, and treatment is avoiding the food that causes it.
This article is for general information only and does not replace advice from your veterinarian. Do not change your pet’s diet to test for an allergy without your vet’s plan.
Sources
- Olivry T, Mueller RS. Critically appraised topic on adverse food reactions of companion animals (7): signalment and cutaneous manifestations of dogs and cats with adverse food reactions. BMC Veterinary Research. 2019;15:140. CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC6507158/
- Hensel P, Santoro D, Favrot C, Hill P, Griffin C. Canine atopic dermatitis: detailed guidelines for diagnosis and allergen identification. BMC Veterinary Research. 2015;11:196. CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC4531508/
- Mueller RS, Olivry T. Critically appraised topic on adverse food reactions of companion animals (4): can we diagnose adverse food reactions in dogs and cats with in vivo or in vitro tests? BMC Veterinary Research. 2017;13:275. CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC5577833/
- Mueller RS, Olivry T, Prélaud P. Critically appraised topic on adverse food reactions of companion animals (2): common food allergen sources in dogs and cats. BMC Veterinary Research. 2016;12:9. CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC4710035/
- Mueller RS, Olivry T. Critically appraised topic on adverse food reactions of companion animals (6): prevalence of noncutaneous manifestations of adverse food reactions in dogs and cats. BMC Veterinary Research. 2018;14:341. CC BY 4.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC6233561/
- Verbrugghe A, Bakovic M. Peculiarities of one-carbon metabolism in the strict carnivorous cat and the role in feline hepatic lipidosis. Nutrients. 2013;5(7):2811-2835. CC BY 3.0. https://pmc.ncbi.nlm.nih.gov/articles/PMC3739000/