Food related allergy treatments in canine and feline patients form part of daily practice for a small animal veterinarian. Dietary elimination trials are often pursued in order to work up an allergic patient exhibiting dermatologic or gastrointestinal disease, and are encouraged as a method to control underlying primary food allergies. Dietary elimination trials also form part of the diagnostic process for allergies unrelated to dietary allergens (atopic dermatitis and flea bite hypersensitivity).
Also referred to as food hypersensitivity, adverse food reaction, or cutaneous adverse food reaction, most dogs and cats with food allergy have an immunologic basis. Type I hypersensitivity reactions are considered to be the most common underlying cause of food allergy, with or without presence of type III and type IV reactions. Individuals with a compromised gastrointestinal barrier function (due to endoparasitism, for example) along with presence of a predisposition to develop IgE hypersensitivity are most likely to be affected by food allergy induced symptoms. In young patients, an immature intestinal mucousal barrier can be inefficient at processing some dietary proteins, typically glycoproteins of 10-70 kD size, that are known to trigger IgE production. These glycoproteins are stable molecules resistant to processing, cooking and digestion; although structural properties of trigger proteins are not well documented.
The majority of food allergies in humans are attributed to a few ingredients such as dairy products, nuts, legumes, eggs and seafood. Any food ingredient can possess allergenic potential in each different individual, and it is incorrect to account for only the commonly known allergic ingredients during work up of a patient. Cross-reactivities occur within and between food groups, thus increasing the potential for a larger range of offending allergens in each individual. Oral allergy syndrome is another well documented cause of allergic flares in humans with pollen allergies that may be exacerbated upon ingestion of a cross-reacting raw fruit, vegetable or nut. In dogs, presence of oral allergy syndrome has also been documented (Fujimura et al 2012) and may be more prevalent that currently described.
Identification of exact food allergens in animals can be challenging & tedious. Commonly fed dietary ingredients are common triggers, with the most commonly reported food allergens in dogs being beef, dairy products, chicken, wheat and lamb (Olivry et al 2016). In cats, the most commonly reported allergens include beef, fish and chicken; while dairy products and lamb have also been reported as food allergens fairly commonly. The only definitive diagnostic method to confirm presence of food allergies in a patient is use of an elimination diet trial followed by provocative exposure testing in order to prove or disprove the presence of food allergies. Where allergic skin disease is suspected in a patient, an elimination diet trial should be initiated based on the patients’ individual dietary history while also treating for secondary infections and pursuing adequate flea prevention. Typically, an 8 week long food trial is considered sufficient in determining the presence of food allergies (Rosser et al 1993), although confirmation relies on provocation testing only. Shorter diet trials, or addition of multiple ingredients (including fruits and vegetables) are considered inadequate for most patients.
A cat or dog affected by food allergy will usually present with non-seasonal pruritus, which can be quite severe and only partially responsive to anti-pruritus treatments such as glucocorticoids and oclacitinib (Apoquel). Any patient presenting with skin or ear disease may be affected by food allergy as there is no known breed, sex or age predilection, with only the Siamese breed known to be predisposed. Dog and cats that are either very young (less than 2 years of age) or past early adulthood, are affected most often. Approximately 7% of dogs were affected by food allergies while about 32%% of dogs exhibiting canine atopic signs were affected by food allergies, in data collected over a duration of 1 year from a referral dermatology practice (Chesney et al 2002). Olivry et al concluded in a literature review that 3 to 6% of cats with dermatological problems and 12 to 21% of cats with pruritus are affected by food allergy. Thus, presence of pruritus, clinical suspicion of allergic skin disease, and the known prevalence of food allergies justify the recommendation for a restricted diet trial followed by provocation trial in dogs and cats (Olivry et al 2016).
Patients affected with food allergy may also be affected by other allergies such as flea bite hypersensitivity or atopic dermatitis. The proportion of patients affected by more than one hypersensitivity disease varies by study, but it is reasonable to expect that 20 to 30% of patients with dermatological disease due to food allergies, may also be affected by additional allergies. Approximately 50% of dogs affected by non-seasonal pruritus possess an underlying food allergy (Loeffler et al 2006). This indicates that not all non-seasonally allergic dogs are food allergic. These findings indicate towards a need for including multiple allergic conditions on the differential diagnoses list at the outset of a diagnostic program, while carefully ruling out individual conditions as work up proceeds. It is also important to address need for treatment of secondary infections and pruritus affecting the patient, in order to achieve a better outcome as well as improving the patient’s quality of life during the work up.
Unfortunately, there are no clinical signs that are pathognomonic for cutaneous adverse food reaction. A primary eruption may be observed in some patients with presence of erythematous wheals, papules and plaques noted. Secondary lesions due to pruritus and self-trauma develop over time and include excoriations, lichenification, alopecia and hyperpigmentation. Feline patients may exhibit species specific manifestations such as the eosinophilic granuloma complex or miliary dermatitis. Distribution of lesions and pruritus may range from focal pedal or perianal dermatitis to otitis, or there may be a more generalized distribution. Concurrent gastrointestinal symptoms may be present including flatulence, vomiting, diarrhea, increased number of bowel movements, and increased frequency of hair balls (in cats). Vasculitis, urticaria, respiratory signs, and behavioural changes (e.g., lethargy, irritability, aggression) may also be related to underlying food allergies. Most affected dogs and cats have had the same diet for long periods and recent changes aren’t typically the cause for food allergy. Multiple trigger allergens are usually the cause for symptoms, and development of new sensitizations can occur over time.
When considering options for an elimination diet trial for each patient, many options exist including commercial hydrolyzed protein diets, commercial novel single protein home cooked food or commercial novel single protein diet (a.k.a limited ingredient diet). Individualized selection for each patient is encouraged before a diet trial is pursued, and is based on patient age, long term dietary history, body condition, systemic health, as well as client and patient preference to name a few factors. A dietary ingredient that has never been fed to a patient is more likely to be tolerated well compared to a dietary ingredient that the patient has been exposed to regularly. Common food allergens for the species as well as potential for cross reactions should also be considered. It is important to pursue thorough client communication regarding the need and importance of a strict dietary trial in order to achieve desired results. Many pet store diets are labelled as single protein diets but a diet trial using pet store bought diets is not recommended as at least some of these diets have presence of unidentified animal origin sources which would lead to a failed elimination diet trial (Ricci et al 2013). Prescription food based trial or a home-made diet trial should be pursued, when possible. Additional treats, even in small amounts, or rapid changes between diets due to perceived response or lack thereof, should be specifically discouraged. Outdoor cats may need to be kept indoors for the duration of the trial to minimize hunting and access to other sources of food (Mueller and Kirk’s SAD 7th ed.). Need for a provocation challenge, infection control and further work up on additional allergies should also be discussed with clients at the outset. Regular follow ups to confirm and encourage compliance as well as to track patient progress go a long way in obtaining desired results from diet trials.
Apart from dietary elimination followed by re-challenge, other testing options are available but considered to be of inadequate diagnostic benefit in practice. Serologic methods (serum food allergy testing for allergen specific IgE and IgG antibodies) have been evaluated but not currently recommended for diagnosis of suspected food allergy, especially as clinical utility of such testing is questionable, with a lack of correlation in results from different laboratories (Hardy et al 2014). While of immense benefit in helping identify environmental allergens, intra-dermal skin testing has not proven useful in the diagnosis of adverse food reactions in small animals. While easily accessible online for pet owners, results of salivary and hair testing as a resource to help determine food allergies in dogs are largely inaccurate (Coyner et al 2016). These tests should be discouraged until supporting scientific evidence is available for such testing.
Gastroscopic food sensitivity testing has been evaluated in dogs, with reliable results noted based on administration of food extracts dripped on to the dependant aspect of the body of the stomach. Localized swelling of the mucosa was indicative of a food sensitivity (Guilford et al 1994). Colonoscopic allergen provocation testing has been assessed for the diagnosis of IgE-mediated food allergy, and showed higher accuracy than gastroscopic testing (Allenspach et al 2006) but both tests are not routinely used in practice due to their limited clinical application. Patch testing in dogs has been shown to have a strong negative predictive value although a positive reaction on a patient is not diagnostic (Bethlehem et al 2012 ). If performed, negative results should help in choosing ingredients for an elimination diet in a dog with suspected food allergy.
To summarize, food allergies form a fairly common dermatological disease in companion animals. Lack of pathognomonic clinical signs associated with the condition, lack of rapid diagnostic tests and need for reliance on a thorough work up along with need for client compliance make the condition a challenging diagnosis, especially if concomitant skin disease is present. Despite such hurdles, patients diagnosed with food allergy have a good prognosis for life long management of the condition and can have a good quality of life long term.
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Dr. Jangi Bajwa is a Board certified veterinary dermatologist at VetDERM Clinic in Surrey BC. He is also the dermatology feature editor for Canadian Veterinary Journal. Dr. Bajwa’s special interests include otitis and allergic disease in pets; as well as helping improve quality of life of pets and their families.