Food allergies

and allergic skin disease

Skin issues are the most common reason why pet owners bring their furry companions to the veterinarian. Dry skin, dull coats, intense scratching, head shaking, and paw licking are just some of the few issues seen with pets who have dermatological problems. It is important to understand what causes these symptoms, how common food allergies actually are, and what to do if you suspect your pet has skin disease.

The immune system

Allergic reactions are mediated, or controlled, by the immune system. The immune system is a complex network of cells and chemical messengers with many roles. One of those roles is to rid the body of foreign material, such as allergens. An allergen is something that causes an abnormal immune response. Allergens in themselves are not inherently dangerous, but the animal’s body perceives that they are, so the immune system initiates a very strong response against them.


There are many different terms that are often used interchangeably when describing unexpected responses to foods. It is important to understand the differences between these terms.

Adverse reaction to food

This is a “catch-all” term to describe any unusual reaction or response to a food or ingredient. It is the most correct term to use when the pathophysiology (mechanism) of the response is unknown. This term encompasses food allergies and all types of intolerances.

Food allergy

A true food allergy is an immune-mediated reaction to a food or ingredient. It can be controlled by different immune cells based on the type of reaction. Immediate hypersensitivity reactions take place within a few hours of eating. They can manifest in the digestive tract, causing inflammation that leads to vomiting and diarrhea; or as skin reactions, causing inflammation that leads to itchiness, redness, scratching, and other signs. (It is important to note that none of these signs inherently indicate a food allergy; there can be many factors at play if a pet has digestive or skin problems.) Intermediate and delayed-type hypersensitivity reactions may occur in dogs and cats, but they are not as well understood as they are in humans.

Allergic reactions require that the animal has come into contact with the specific allergen in the past, usually multiple times. The allergen constantly interacts with immune cells, which may take a long time to “decide” that the allergen is a problem. Therefore, a pet who has eaten the same diet their entire life can become allergic to any component of that diet.

Food intolerance

This is a reaction not controlled by the immune system. Many different types of food intolerances have been described in the scientific literature. They include food poisoning or intoxication (caused by toxins present in the food), food idiosyncrasy (a reaction that mimics a food allergy but does not involve the immune system), and pharmacologic or metabolic reactions to food (relating to how the animal’s metabolism deals with the nutrients found in food). The clinical signs of food intolerance can vary widely. Unlike with allergies, an animal can be intolerant to a food or ingredient it has never contacted before. For example, adult dogs and cats are intolerant to milk because they are unable to break down lactase, the sugar found in milk (a metabolic reaction to the food).

The problem with the above two definitions is that they describe the pathophysiology of the response (how the body reacts), but they do not distinguish the clinical appearance (what we see outwardly). Therefore, it is usually difficult to differentiate food allergies from intolerances.

Food sensitivity

Sensitivity is a looser term that is not defined in the scientific literature. It can be used to describe pets who frequently have digestive signs such as loose stool, flatulence, or vomiting. This digestive upset may be due to ingredients in the diet, the nutrient profile (such as ratios of fat and fiber), or it can be due to other things the pet is eating (like treats or table scraps). This term, unlike the two above, describes the clinical appearance of the problem, not the pathophysiology of the response to the food.

Why dermatitis?

Animals and humans show different symptoms when confronted with the same allergen. In general, dogs and cats often show signs of an allergic reaction in their skin. This is contrasted with human allergies, which typically involve the respiratory system. The reason for the difference in the site of the immune reaction is the distribution of different cells in our bodies. Mast cells are a key part of allergic response: they are responsible for causing itchiness, redness, and swelling. Dogs and cats have a high concentration of mast cells in their skin, while most mast cells in humans are in our respiratory system. This is why many allergic dogs and cats show signs of skin discomfort, while allergic humans frequently cough or sneeze.

Apollo licking his foot
Otitis externa (inflamed/ infected ears)
Inflammation between the toes
Lesions on the mouth from scratching

Common allergens

Allergens can be anything from pollen to fleas to food ingredients. However, despite popular belief, food allergies in pets are pretty uncommon. The most common cause of skin disease in pets is actually fleas. Other common sources of dermatitis include other ectoparasites such as mites, environmental allergens like pollen or dust, contact irritants such as household cleaning products, bacterial or yeast infection, or immune-mediated disease. Atopy, or the genetic tendency towards developing environmental allergies, is another common reason pets have skin issues. One or many of these problems may be present at the same time, contributing to the signs you observe.

Diagnosing skin disease

It is very important to not jump to conclusions when you suspect skin disease in your pet. The first thing to do is monitor your pet, record when the problem started, note all the signs they are exhibiting, and even take photos. Call your pet’s usual veterinarian and schedule an appointment. The veterinarian will take a very thorough history, asking you many questions about the problem, your environment, your pet’s lifestyle, and other relevant factors. They will also take a full diet history, where they will evaluate what your pet is eating to see if this could be the source of problems. Aside from food allergies, there are many nutrient deficiencies that can cause skin issues, so the veterinarian will be assessing the diet for this. Then, they will likely perform many diagnostic tests to rule out different types of skin disease. This may include blood work, skin scrapings, or intradermal allergy testing (for environmental allergies only). If they manage to rule out the more common causes of skin disease such as parasites and environmental allergens, they may then turn to trying to diagnose a food allergy.

Diagnosing a food allergy

Many factors may cause suspicion of a food allergy: non-seasonality of symptoms, improving or worsening signs with diet change, or types of skin lesions. However, there is only one way to definitively diagnose a true food allergy: an elimination diet trial. (This is true for pets and humans!) Your pet’s veterinarian will guide you through this process, and if they cannot, they should be able to refer you to a board-certified veterinary nutritionist who can.

The veterinarian will give you options on what to feed your pet as their main diet during the trial period. It must be complete and balanced, as an elimination trial generally takes 8-12 weeks, and can last much longer (many months longer).  Options for the trial diet include:

After a diet is chosen, you will then eliminate all other components of your pet’s diet. This includes treats, table scraps, human food, flavored medications (like flea or heartworm chews), or anything else your pet consumes. These can interfere with the diet trial, so it’s important not to feed anything but the chosen diet.

Once the diet has been fed exclusively for several weeks, you should notice your pet’s allergy symptoms begin to fade. This increases suspicion for a food allergy, and tolerance of the current ingredients being eaten. If the symptoms do not go away, you will need to alert the veterinarian and adjust the main diet.

You will then begin to “challenge,” or test, your pet at your veterinarian’s direction. You will start by giving small amounts of certain ingredients (e.g., chicken, beef, fish), one at a time. Between each challenge of a single ingredient, you will wait several weeks to see if the allergy symptoms return. If they do not, you will test the next ingredient. If they do, your veterinarian will diagnose an allergy to the ingredient most recently challenged. The challenge period can take a very long time, because it takes your pet’s body several weeks to return to normal after a flare-up.

Sometimes, pet parents decide that they do not want to go through with the challenge period. Challenging can be frustrating and time-consuming! If you, your pet, and the veterinarian are happy with the current trial diet, the pet should be able to keep eating it as their sole diet. The only downside to skipping the challenge is that allergies to specific ingredients will go undiagnosed. 

Ineffective methods for diagnosing food allergies

Managing food allergies


The only way to truly control a food allergy is to completely avoid it. Once the allergy to a specific ingredient is officially diagnosed by a veterinarian, you will need to find a diet that does not contain this ingredient. This can be tricky, as most allergic pets react to common ingredients like chicken or beef. Additionally, as exotic proteins (like alligator, kangaroo, etc.) become more widely available, pets have been shown to develop allergies to these as well.

Sometimes, pets are allergic to multiple ingredients, or ingredients so common that it is impossible to find a commercial over-the-counter diet that won’t cause a reaction. In these cases, you may need to consider a therapeutic hydrolyzed protein diet, or a homemade diet formulated by a board-certified veterinary nutritionist. Again, it is important to note that any over-the-counter diet or treat could be contaminated with problem ingredient(s). Therapeutic diets, unlike OTC diets, undergo strict manufacturing quality control protocols to avoid cross-contamination. 

Simply feeding a “hypoallergenic” diet isn’t enough, though. You will need to ensure your pet never eats anything that contains ingredients that cause a reaction. This includes treats, table scraps, people food, and flavored medications. However, you can still reward your pet! You can offer their kibble as treats, use human foods like fresh fruits and veggies, or buy therapeutic hypoallergenic treats with a prescription from your veterinarian.

Parasite control

Ectoparasite control is extremely important for pets with skin disease, no matter the root of the problem. Parasites such as fleas and mites can severely irritate already-sensitive skin and make problems worse. Make sure your pet is on a quality flea/mite/tick preventative year-round.

Endoparasite control is also important for all pets, but particularly those with digestive disease. Endoparasites are things like intestinal worms or other bugs that your pet can pick up anywhere, and they live in your pet’s digestive system. Keeping your pet on a quality heartworm preventative should take care of roundworm issues, and taking your pet to the veterinarian regularly and having their feces tested is a very important part of controlling other types of parasites.

Skin support

Some skin support diets include higher concentrations of vitamins, minerals, and fatty acids that are beneficial for skin health. 

A great way to help your pet out if they have dry skin or a dull hair coat is to use a fatty acid supplement. Omega-3 and Omega-6 fatty acids are nutrients that are essential for skin and coat health. Omega-3s also have great anti-inflammatory properties which can help itchiness and redness. It is important to note that they do not cure skin disease, but they can help the body repair damage to the skin.

If you are going to feed an Omega-3 supplement, make sure you buy from a company that is backed by research and performs good quality control. Supplements are not well-regulated by the FDA or AAFCO, so it’s best to buy from a company that regulates itself well. Ensure the product contains an ingredient list and calorie statement, at the very least, and be sure to take those calories into account with their total daily caloric intake. (To learn more, visit How to calculate your pet's calorie needs and Supplements.)


When a pet has a chronic problem, it is very important to monitor them regularly for any changes or signs they are having a flare-up. Keep a journal of your pet’s skin health, including descriptions of problems, dates they occurred, and how they resolved. Make sure to bring your pet to the veterinarian regularly, to ensure they are staying healthy.

What doesn’t work

Limited-ingredient over-the-counter diets. These diets are a fad and not are based in science. They are centered around the idea that pets who eat a limited number of ingredients are less likely to develop allergies. This is simply not true; allergy development is dependent on exposure, so increased exposure to a single ingredient means there is a greater likelihood a predisposed pet will develop an allergy to the ingredient. Limited-ingredient diets are also often contaminated with other ingredients, due to their being made in facilities that produce other diets, so they are also a poor choice for an elimination trial diet.

Rotational dieting. This trend is based on the idea that constantly changing what a pet eats will lower the likelihood it will develop a food allergy. It contrasts the theory behind limited-ingredient diets, yet the practice is still inappropriate. By constantly exposing a pet to a multitude of different ingredients, the number of “novel” (new, never eaten) ingredients decreases. Because of this, it is extremely difficult to do an elimination diet trial for pets who develop a food allergy after being on a rotational diet. Not to mention that changing a pet’s diet every few weeks or so can upset digestion (causing vomiting or diarrhea), and can impact their body condition (if the foods have different nutrient profiles and calorie contents).

Grain-free/ low-carb diets. These diets are based around the misguided and incorrect theory that dogs and cats cannot digest or utilize nutrients found in grain ingredients, or that all grains are common sources of allergies. You can learn more about these ingredients in this article: Grains as “fillers”.

Gluten-free diets. Gluten is simply the protein part of a grain – the most desirable and expensive nutrient found in the plant. Different types of gluten include wheat gluten (the most often vilified), but also corn gluten, rice gluten, and other types of gluten from many different plants. Gluten-free pet diets are based on a human trend, as well as the incorrect belief that wheat gluten is a common cause of allergies or sensitivities in both humans and pets. There is absolutely no scientific evidence showing avoiding gluten is beneficial for health. Gluten intolerance has only ever been documented in a single inbred family of Irish Setter dogs in Europe, and possibly in Border Terriers. Gluten intolerance has never been documented in other breeds of dogs, nor has it been documented in cats.

Exotic proteins. Diets made with exotic proteins such as alligator, kangaroo, bison, or other uncommon meats have become a fad in recent years. There are two theories behind the development of these diets. The first is that exotic proteins are unusual sources of food allergies when you look at the numbers (most dogs and cats are allergic to chicken and beef). However, chicken and beef are common allergens only because they are very common ingredients. When exotic meats become common ingredients, allergies to them also rise. The second theory behind these diets is that exotic meats are novel ingredients, and therefore should be unlikely to stimulate an allergic reaction. However, as exotic meats make their way into many different diets, they are no longer a novel protein. Additionally, exotic protein diets often contain or are contaminated with common proteins such as chicken and beef, making them useless as an elimination trial diet.

Raw diets. Raw meat-based diets are no more or less allergenic than traditional diets. Raw diets can, however, precipitate food intolerance due to their uncooked nature and ability to harbor pathogenic bacteria. Raw diets are unsafe for pets, their families, and the public.  Read Raw diets to learn more.