Food Allergies in Cats: Signs, Common Triggers and Diet Trials

What a food allergy is

A food allergy happens when the immune system overreacts to a particular protein in the diet. VCA Animal Hospitals explains that a cat has to have been exposed to an ingredient before it can become allergic to it, which is why cats often react to foods they have eaten happily for years. The most common triggers listed by both VCA and the Merck Veterinary Manual are beef, fish, chicken and dairy products.

Food allergies can appear at almost any age. VCA says they can develop any time after three months of age, and Merck notes one study found most affected cats showed signs before age two, although onset as late as 11 years has been seen. Males and females are equally affected, genetics seem to play a part, and many cats with food allergies also have environmental allergies, which can complicate the picture.

Signs of food allergies in cats

The main sign is itching that does not change with the seasons. Merck notes that itching of the head and neck is fairly common, and VCA lists the face, ears, belly, groin, armpits, legs and paws as typical areas. Itchy cats over-groom, which can leave thin or bald patches, often on both sides of the body, along with scabs, small crusty bumps or raw, red, raised patches of skin.

Repeated skin or ear infections can be the only clue in some cats. VCA notes that some cats also have digestive signs, such as vomiting, diarrhea, scooting, or more frequent or straining bowel movements. These signs overlap with fleas, parasites, ringworm, environmental allergies and many digestive diseases, so they point toward a vet visit rather than a conclusion.

When to call your vet today

Book an appointment soon if your cat is scratching, licking or chewing itself more than usual, has patches of thinning hair, has scabs around the head and neck, or keeps getting ear or skin infections. Call the same day if your cat has open or bleeding sores from scratching, has vomited several times, has diarrhea lasting more than a day, or is eating noticeably less.

Do not switch foods repeatedly to see whether things improve before seeing your vet. Changing diets without a plan rarely gives a clear answer, and the trial only works if it uses ingredients your cat has never eaten, which is easier to work out with your vet's help.

When it's an emergency

Food allergies are rarely an emergency on their own. Go to your vet or the nearest emergency vet immediately, though, if your cat has repeated vomiting and cannot keep water down, has bloody diarrhea, is weak or lethargic, or has a swollen face, trouble breathing or collapses. Those signs need same-day care whatever the cause.

Also call urgently if your cat has refused to eat for more than a day, including during a diet change. Merck warns that cats should never be starved onto a new diet, because a prolonged poor appetite can cause serious liver damage in cats. If your cat will not eat the new food, call your vet rather than waiting it out.

What you can safely do at home

Do not give antihistamines, steroid creams or any human medicine without your vet's advice, and do not put human skin products on your cat. What you can do is note the details your vet will need: everything your cat eats, including treats, table scraps, dental chews, flavored medicines and any food it steals from other pets, plus where it itches and whether the itching changes with the seasons.

Keep up with flea control for every pet in the home, since flea allergy is a common cause of itching that looks similar. Once a diet trial starts, the hardest part is consistency. Feed only what your vet prescribes, keep other pets' food out of reach, ask visitors not to give treats, and give any medicine in an unflavored form your vet approves.

During a food trial, a stainless steel cat bowl or ceramic cat bowl is easy to clean thoroughly between meals.

What the vet will likely check

Your vet will examine the skin and ears, look for fleas, parasites and infection, and may run skin tests for mites or ringworm to rule out other causes. Merck states that blood and skin tests for food allergy are not reliable, so the diagnosis comes from an elimination diet trial. Your vet will choose either a prescription diet made with hydrolyzed proteins or a diet based on a protein your cat has never eaten.

VCA recommends the trial last at least eight weeks, and Merck notes it can take up to three months, as cats respond anywhere from one to nine weeks in. During that time the cat eats nothing but the trial food and water. If the itching settles, your vet will usually reintroduce the old food; if signs return within about one to two weeks, a food allergy is confirmed, and single ingredients can then be tested one at a time to find the trigger. Skin or ear infections are treated alongside.

Living with a food allergy

Food allergies cannot be cured, but they can be controlled by avoiding the trigger ingredients for life, and VCA describes the outlook as generally good. Many cats stay on the prescription trial diet, or your vet can help you find another complete diet that avoids the problem proteins. Treats need to be chosen just as carefully.

VCA notes that retail limited-ingredient foods are not made with the same controls against cross-contamination as veterinary diets, so a sensitive cat may still react to them. If itching returns, check for accidental exposures first, and keep up regular checkups and flea control so other causes do not creep in unnoticed.

This page is general information, not veterinary advice, and it cannot replace an examination by the vet who knows your pet.