Adverse reactions to food in dogs and cats   Adverse reactions to food in dogs and cats (PDF 725 KB)


Several terms, such as food allergy, food hypersensitivity, food intolerance, adverse reaction to food or allergic skin reactions to food ingredients are used to describe a common allergic disease occurring in dogs cats. In dogs adverse reactions to food are the most common form of hypersensitivity after allergy to fleas and atopic dermatitis (allergic reaction to environmental-allergens). In cats the most common cause for allergic dermatitis is a hypersensitivity to fleas followed by adverse reactions to food ingredients.


To better understand the current discussions about adverse food reactions that can be immune- mediated or not immune-mediated, we list a few definitions:

Adverse reaction to food:
a clinically abnormal reaction associated with the ingestion of food without exact knowledge of the causative relationship

Food allergy/food hypersensitivity:
immunemediated reaction associated with the ingestion of food, specific antibodies can be detected

Food intolerance:
non-immune-mediated physiological reaction to food, no formation of antibodies

Food idiosyncrasy:
non-immune-mediated abnormal reaction related to the amount of food intake and not to the common physiological and pharmacological effects. A food idiosyncrasy is similar to an allergic reaction, but does not involve immunological responses (formation of antibodies).

Anaphylactic reaction to food:
formation of allergen specific IgE antibodies causes the release of vascularly active mediators of basophile granulocytes and mast cells.

Pharmacological food reactions:
certain ingredients in food can have a drug-related or pharmacological effect in an individual (e.g. chocolate-intoxication caused by methylxanthine or histamine containing food).

Metabolical food reactions:
adverse reaction as a consequence of the metabolism of the individual (e.g. enzyme-deficiency -> lactose intolerance due to lactase deficiency in small intestine).

Food toxicity and food intoxication:
caused by toxins ingested with food (e.g. alkaloids in onions and garlic) or contamination of food with toxin producing parasites or microorganisms (e.g. aflatoxicosis or botulism).

Although many different substances or conditions can cause food related allergic reactions, the clinical signs are almost indistinguishable, further complicating a correct diagnosis. Once the causative agent is identified, a successful therapy consists of complete avoidance of this agent.

Food allergens – activators of food allergies

Food allergens can be of herbal or animal origin. Any food ingredient is a potential allergen, however usually proteins and their metabolites serve as the main allergens. Water-soluble glycoproteins contained in the most commonly used food, such as beef, milk, soy, lamb, pork, grain, eggs, fish, chicken, and rice have the highest allergic potential. The allergen is generated through the heating and preparation process or simply during regular digestion. Very often cross-reactions occur between allergens, for example an animal that reacts to themain allergen in beef (IgG Bos d7), also shows adverse reactions to meat from lamb, deer and milk.

There is a plethora of potential food allergens for dogs and cats and the long list of ingredients in commercial diets makes it a challenging process to identify the causative agent. Furthermore, cross-reactions between up to 60% of the ingredients are complicating the detective work.

Atopic dermatitis versus food allergy

20-30% of dogs and cats affected by adverse reactions to food also suffer from other allergic diseases. Atopy is defined as the genetic susceptibility to develop hypersensitivities of the immediate type (type 1 allergy) to miscellaneous substances commonly occurring in the environment (pollen, house dust- and storage mites, moulds). This makes it very difficult to distinguish atopic dermatitis and food allergy based on clinical signs. One indication for atopic dermatitis is the seasonality of this condition. Clinical signs of an adverse reaction to food can appear very suddenly after the intake of the causative agent. Atopic dermatitis usually develops gradually. Furthermore, signs of gastrointestinal disease and the response to steroid treatment can give useful hints.

Clinical signs

Most commonly, food allergies affect the skin and/or the gastrointestinal tract. The most obvious clinical sign is an intense non-seasonal pruritus. The onset of puritus can occur at any age and very young and very old dogs can be affected.

Dermatological symptoms
- chronic or recurrent dermatitis, which primary causes pruritus and erythematous skin without specific distributional pattern.

- secondary skin lesions: papules, pustules, erythema, excoriation, epidermal collarettes, hyperpigmentation, lichenification, pododermatitis, seborrhoe as well as alopecia.

- secondary infections caused by bacterial colonization +/- malassezia as consequence of self trauma. In cats food allergy could lead to a milliary dermatitis, self induced alopecia, eosinophilic granuloma complex or more often to self induced ulceration (alopecia, excoriation and crusts) around the head and neck area.

Gastrointestinal symptoms
- not pathognomonic, may include vomitus, diarrhea, abdominal pain, flatulence, borborygmi, alternating fecal consistency, frequent defecation, weight loss and lack of thriving.

- changes in general condition and behavior such as irritability or hyperactivity.

- most symptoms correlate with gastric and or small intestinal dysfunction, but also signs of colitis can occur.

- profuse, aqueous to mucous or haemorrhagic diarrhea, intermittent abdominal pain, tenes mus and changed appetite are noted.


The clinical diagnosis should be based on a thorough medical history, physical examination and the elimination of differential diagnoses. Ideally this includes an elimination diet followed by provocation with the previously fed food. A serological test serves to identify allergens that induce the formation of specific antibodies. Recent scientific research recommends serological tests for the detection of allergen specific immunoglobulin E (IgE) and G (IgG) antibodies to various food components in order to develop an individualized elimination diet. The owner´s compliance to stick to the diet is often increased with a positive test result underlying the dietary recommendations.

Elimination diet

An elimination diet requires a 100% compliance of the pet owner. Usually a food component that is new to the animal will be fed exclusively for at least 8-12 weeks. Initially, elimination diets were followed for only 3 weeks and only 26 % of the allergic animals could be identified within this short time period. An elimination diet usually contains one carbohydrate and one protein source and can be a home made diet or one of the commercially available hypoallergenic or hydrolyzed diets.

Very often some trial and error is involved until the best diet is found for a patient. Some individuals respond best to home made diet, others do better with one of the commercial diets. Hydrolyzed diets, in which the protein is modified to peptides by hydrolysis, are getting more and more popular. Hydrolysis of proteins causes a destruction of antigenic epitopes. The low molecular weight of peptides reduces the antigenic threshold, however the complete absence of allergens in hydrolyzed diets can not be guaranteed.

During an elimination diet, drugs that can contain flavor additives based on meat, should be avoided. It is not necessary to feed supplements to the animal if the diet is discontinued after 8 weeks. If the elimination diet needs to be fed for more than 8 weeks, essential nutrients need to be supplemented because of the unbalanced nature of the diet. Especially in young, still growing animals a commercially available hypoallergenic diet or a calculated diet containing all essential vitamins and minerals should be preferred.

After completing the elimination diet, a series of provocation diets have to be performed to identify the causative food ingredient and to verify the suspected diagnosis of food hypersensitivity. For that purpose, the original diet that caused the symptoms is fed again and/or certain food ingredients are added separately to the diet to determine the causative allergen by provoking symptoms of food allergy again. This provocation is an important part of the golden standard to verify the diagnosis, but many pet owners are reluctant to comply because they are afraid of a relapse. If there is only partial improvement of the clinical symptoms during an elimination diet, it is possible that the animal also suffers from atopic dermatitis, is allergic against an ingredient in the elimination diet or still ingested the original allergen because of bad compliance of the owner. A food allergy, like any other allergy can not be cured, only controlled. The only way to keep the animal healthy and free of symptoms is to avoid the allergen for the rest of its life.

Differential diagnoses

Any disease causing pruritus needs to be considered as a potential differential, such as atopy, flea allergy, sarcoptic mange, pyoderma, malassezia dermatitis and drug eruption.

Clinical Cases

The following cases are meant to give insight into the typical course(s) of disease and possible pitfalls:

German shepherd dog, male, 10 months old

Case history:
Diarrhoe, frequent defecation, borborygmi, flatulence and occasional vomiting
were noticed since the age of 4 months with the dog having an unchanged physical condition. The animal was (de)wormed regularly and the consistency of the faeces was changing. The dog went to dog training and was conditioned using treats. Main food was a commercially available dog food from the supermarket can.

The owners were suspecting a connection between clinical signs of their dog and the food. They asked their veterinarian to perform a serological food test (sensi test) for determination of IgE and IgG antibodies to sixteen conventional food components.

IgE reaction to beef, pork, lamb and IgG to fish. Because of the dogs age and the fact that it was a large breed dog still growing, the diet was changed to a commercial elimination diet according to age and breed and contained only substances that tested negative in the serological allergy test. The owner was instructed to follow the diet strictly and to replace the treats used in dog training with the diet. In less than 1 week the gastrointestinal symptoms disappeared and also the vomiting had stopped. During the time of remission, the conventional treats were fed to provoke the dog one more time, and the same symptoms as before, massive flatulence, change in fecal consistency appeared on the same day, verifying the diagnosis.

British Shorthair Cat, ”Susi”, female neutered and 4 years old

Case history:
the cat showed diarrhea for one year, occasional vomiting and pruritus especially around the head and neck were noticed. The animal was pre-treated with avermectin, antibiotics, antifungal agents and later on with cortisone injections. The pruritus improved somewhat with cortisone, but never disappeared completely.

Clinical signs:
Especially around the head we could see ulcerative skin lesions caused by selfmutilation. Crusty lesions were distributed as a milliary dermatitis over the whole back and flank area which had been unnoticed by the owner. The cat has been fed a commercial food for two years. The previous veterinarian used several hypoallergenic diets that were always discontinued by the owner because - according to him – there was no improvement.

A serological food allergy (sensi test) test was performed, but the results were all negative. The negative results could be explained as follows:

1. The withdrawal time period for corticosteroids was too short (the substance and time/date of last cortisone injection was unknown).

2. Because of the multiple short term diets, the controlled allergen contact was not strong enough to induce an antibody reaction.

3. The allergen responsible for the clinical symptoms was not included in the group of allergens tested.

4. Another possibility is, that in this case we had to deal with a food intolerance which was not immunemediated and not with a food allergy.

Feeding a home-made elimination diet containing only ostrich led to a complete disappearance of the ulcerative lesions around the head and neck (without performing any other therapy). The gastrointestinal symptoms decreased considerably. Some time later provocation diets with single substances were performed and a commercial food could be found that the cat tolerated excellently.

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