Food allergies in dogs and cats and how to diagnose them

The only reliable way to diagnose a food allergy in a dog or cat is an elimination diet of at least 8 weeks, followed by feeding the old food again to see whether signs return. Blood tests sold for food allergy do not do this job. Most food-allergic pets are itchy, often around the face, ears, paws and belly, and some also have vomiting or diarrhea. The usual culprits are proteins the animal has eaten for a long time, most often beef, dairy and chicken, and in dogs also wheat.

Allergy or intolerance

A food allergy is an immune reaction to a food, usually to a protein. The body treats the protein as a threat, and the inflammation that follows shows up most often in the skin and ears. Because the immune system has to meet a protein before it can react to it, the trigger is normally something the animal has eaten before, often for months or years without trouble.

A food intolerance does not involve the immune system. The gut simply handles an ingredient badly, and the result is digestive: loose stools, gas or vomiting. Intolerances tend to depend on the amount eaten, while an allergic animal may react to small amounts. In practice the two can look alike, and vets often use the broader term adverse food reaction until the cause is clear.

How common food allergy is

Food allergy is uncommon in pets overall but common among itchy ones. A systematic review estimated that 1 to 2 percent of dogs seen by a vet for any reason have a food allergy that shows in the skin. Among itchy dogs, estimates ranged from 9 to 40 percent. In cats, estimates were under 1 percent of hospital patients but 12 to 21 percent of cats with itching (Olivry and Mueller, 2017). The authors concluded that food allergy should be ruled out with a diet trial in pets with itching that does not follow the seasons.

The most common allergens

A review that pooled published challenge studies, where allergic animals were fed individual foods to see which caused a flare, found these to be the most frequently reported triggers (Mueller and colleagues, 2016). Many animals reacted to more than one food.

The figures probably reflect which foods pets were commonly fed and tested with, as well as how allergenic each food is. Lamb also appears on the dog list, so it is not automatically a safe choice for a trial.

Signs to watch for

Skin signs are the most common and usually persist year-round.

• Itching, especially of the face, ears, paws, armpits, groin and belly
• Ear infections that clear with treatment and then return
• Licking or chewing the paws
• Red skin, hair loss from scratching, and hot spots
• Repeated bacterial or yeast skin infections

Digestive signs can occur with or without skin disease. In dogs, diarrhea and more frequent bowel movements were the gut signs most often reported to improve on an elimination diet; in cats, vomiting and diarrhea. The same review group recommends considering a diet trial early when a dog or cat has chronic vomiting or diarrhea. Any pet with weight loss, blood in the stool, repeated vomiting or lethargy needs a vet examination first, because many other diseases cause these signs.

Why blood tests are not the answer

Tests that claim to identify food allergies from a blood sample are widely sold. A systematic review found that serum tests for food-specific IgE and IgG had low repeatability and variable accuracy in dogs, and low accuracy in cats. Its conclusion was that the elimination diet followed by provocation remains the best diagnostic procedure (Mueller and Olivry, 2017). A test result can send an owner chasing the wrong food, or clear the real one.

The elimination diet trial

The trial replaces everything the animal eats with one diet it is unlikely to react to. There are two kinds.

• Novel protein diet. A protein and carbohydrate the animal has never eaten, such as venison, rabbit or kangaroo, chosen from a full diet history. This can be a veterinary diet or, with a vet's guidance, home-cooked for the trial period.
• Hydrolyzed protein diet. A prescription diet in which the protein is broken into small fragments that the immune system is less likely to recognize. It is useful when a pet has eaten so many proteins that none is new to it.

Two cautions apply. Over-the-counter "limited ingredient" foods are a poor choice for a trial. A review of studies that tested pet foods found ingredients not listed on the label in a median of 45 percent of diets, and in 33 to 83 percent of foods marketed with novel or limited ingredients; hydrolyzed diets showed mislabeling only once (Olivry and Mueller, 2018). And hydrolyzed diets reduce allergic reactions without eliminating them. In three controlled studies, up to half of food-allergic dogs got worse after eating a partial hydrolysate made from the food they were allergic to, so a hydrolysate from an unrelated protein is the safer choice.

The trial needs to run long enough. By 5 weeks, more than 80 percent of allergic dogs, and by 6 weeks more than 80 percent of allergic cats, were in remission, and by 8 weeks more than 90 percent of both species were (Olivry and colleagues, 2015). A shorter trial can miss allergic animals that were slower to improve.

• Feed nothing else: no treats, chews, table scraps or food rewards outside the trial diet.
• Ask the vet about flavored medicines, including chewable parasite preventives and flavored toothpaste.
• Keep other pets' food out of reach, and stop cats hunting where possible.
• Keep a weekly log of itch, ears, skin and stools, because gradual change is easy to miss.

Secondary skin and ear infections usually need treating at the same time. Otherwise, the infection can keep the animal itchy even when the diet is working.

Challenge and reintroduction

Improvement on the trial diet is not proof on its own, because allergy signs can wax and wane. The diagnosis is confirmed by feeding the old diet again. If signs return, then settle when the trial diet resumes, food allergy is confirmed. Single ingredients can then be added back one at a time to find the triggers.

How long to wait after each challenge has been studied. Across published reports, half of food-allergic dogs flared within 5 days and 90 percent within 14 days. In cats, the figures were 4 and 7 days. So a dog needs about two weeks on each new food, and a cat about one week, before that food can be considered safe (Olivry and Mueller, 2020). If a food causes a flare, stop it and let signs settle before testing the next.

Living with a food allergy

• Long-term diet. Once the triggers are known, the pet can stay on the trial diet or move to any complete diet free of those ingredients. A home-cooked diet fed long term should be formulated by a veterinary nutritionist so it stays complete.
• Labels. Terms such as "meat meal", "animal fat" or "animal digest" do not name the species. Choose foods that name every animal source, while remembering that labels are not always complete.
• Treats. Use pieces of the pet's own diet or single-ingredient treats made from a protein it tolerates.
• Everyone in the household. Visitors, dog walkers and children need to know the rules, since one biscuit can set off a flare.
• New flares. A pet that becomes itchy again on a strict diet needs a vet check. Environmental allergies, parasites and infections often occur alongside food allergy.

Sources

1. Critically appraised topic on adverse food reactions of companion animals (3): prevalence of cutaneous adverse food reactions in dogs and cats. BMC Veterinary Research, 2017.
2. Critically appraised topic on adverse food reactions of companion animals (2): common food allergen sources in dogs and cats. BMC Veterinary Research, 2016.
3. 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.
4. 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.
5. Critically appraised topic on adverse food reactions of companion animals (5): discrepancies between ingredients and labeling in commercial pet foods. BMC Veterinary Research, 2018.
6. A systematic review of the evidence of reduced allergenicity and clinical benefit of food hydrolysates in dogs with cutaneous adverse food reactions. Veterinary Dermatology, 2010.
7. Critically appraised topic on adverse food reactions of companion animals (1): duration of elimination diets. BMC Veterinary Research, 2015.
8. Critically appraised topic on adverse food reactions of companion animals (9): time to flare of cutaneous signs after a dietary challenge in dogs and cats with food allergies. BMC Veterinary Research, 2020.