Guidelines for an elimination diet to diagnose food allergy in dogs and cats

This is a summary of the article entitled ‘Conducting a successful diet trial for the diagnosis of food allergy in dogs and cats’, the full version of which is available on Wiley Online Library. It provides a detailed overview of the essential clinical steps, challenges and key strategies for accurately diagnosing food allergies in dogs and cats.

Confirmation of a food allergy diagnosis in dogs and cats is based on carrying out a dietary elimination trial followed by a food challenge (reintroduction of the previous diet) that triggers a recurrence of clinical signs. Diet selection is complex and requires consideration of financial factors, the pet’s acceptance of the diet, and the exclusion of proteins that are antigenically related to previous foods.

1. Owner education and adherence to the test

Owner compliance is one of the most significant factors limiting the success of the test. An online study revealed that 75 per cent of owners admitted to dietary lapses (fruit, vegetables, meat, dairy products, bread), 51 per cent provided additional food sources, and 25 per cent reported that their dogs had access to unmonitored food such as rubbish or cat litter.

Most owners prefer to feed their pets a varied diet and do not perceive supplements, dental hygiene products or oral antiparasitic treatments as potential allergens, even though almost all of them contain flavourings. It is essential to obtain a detailed dietary history and to educate the owner on the absolute elimination of any unauthorised items.

Home measures to prevent accidental allergens

• Households with children or elderly people: They may feed the pet accidentally or intentionally, making a strict trial unfeasible.
• Households with multiple pets: All animals must be fed the same test diet, or the patient must be strictly separated during mealtimes.
• Cats: It is recommended to keep them indoors to prevent hunting and the consumption of food from outside, although it must be assessed whether the stress of the lifestyle change outweighs the benefits of the test.
• Treats: A small, measured portion of the same dry food used in the daily diet may be used as a reward.

2. Diet selection: Types, advantages and disadvantages

Historically (data from 1985 to 2015), the most commonly reported allergens in dogs are beef, dairy products, chicken and wheat; whilst in cats they are beef, fish and chicken. However, practices vary geographically and change over time.

The main options for structuring the test are:

1. Single novel protein diet (home-made or commercial): Based on a protein source that the animal has never consumed before.
2. Hydrolysed protein diet: A commercial diet in which the base protein has undergone a hydrolysis process.

Comparison table of test diets

Considerations regarding hydrolysed proteins and cross-reactions

Proteins in hydrolysed diets are usually processed to achieve a molecular weight of less than 5 kD, which theoretically prevents cross-reactivity with immunoglobulin E (IgE). One study showed that dogs allergic to chicken did not react to an extensively hydrolysed diet made from poultry feathers (<1 kD), but 40 per cent of them reacted when the diet was only partially hydrolysed. Furthermore, some hydrolysed e diets contain peptides that activate cell-mediated pathways (T-cell epitopes), meaning they are not 100% effective in all patients.

There is also a risk of serological cross-reactivity (e.g. antibodies against one protein reacting with another that is structurally similar), which has been widely demonstrated in dogs, although its exact clinical relevance remains uncertain. Similarly, a case of Pollen-Food Allergy Syndrome (PFAS) has been documented in a dog with hypersensitivity to Japanese cedar pollen that subsequently developed an allergy to tomatoes.

3. Duration of the test

To achieve an optimal diagnostic success rate, the strict diet must be administered over prolonged periods:

• In dogs: A 5-week trial detects approximately 80 per cent of cases, whilst an 8-week trial increases the effectiveness to 95 per cent. In cases of recurrent otitis externa, the trial period should be extended to exceed the usual interval between flare-ups.
• In cats: An 80 per cent success rate is achieved at 6 weeks and over 90 per cent at 8 weeks.
• If there are concurrent gastrointestinal symptoms, these usually resolve much more quickly than the skin symptoms.

4. Management of concomitant medication

The use of anti-itch and anti-inflammatory treatments at the start of the test is essential for the animal’s wellbeing and improves owner compliance by providing the pet with relief more quickly. This can shorten the duration of the test to between 4 and 6 weeks.

• Pruritus suppression protocol: Prednisolone (0.5 mg/kg daily) or oclacitinib (0.4–0.6 mg/kg daily) is administered for a minimum of 2 weeks until pruritus is reduced to acceptable levels as assessed by the Pruritus Visual Analogue Scale (PVAS). After discontinuing the drug, the diet should remain drug-free for a further 2 weeks to verify stability before performing the food challenge. There are no specific published protocols for cats, although the same practice is followed.
• Secondary infections: Regular topical antimicrobial therapy (for the skin and ears) may be continued as normal. If systemic antibiotics or multi-component ear preparations are used, the diet should be maintained for at least 2 weeks after the antibiotics have been discontinued to assess whether the improvement is sustained on its own.
• Topical corticosteroids: Any topical medication containing steroids must be discontinued completely 2 weeks before the challenge.
• Parasite prophylaxis: Flavoured oral antiparasitic treatments should be replaced with spot-on formulations (pipettes) or unflavoured long-acting oral tablets during the test.

5. Patient transition and monitoring

The new diet should be introduced gradually over 4 days in 25 per cent increments to prevent gastrointestinal problems.

Complications during the transition

• Food refusal: In dogs, if they refuse to eat, the bowl should be removed and a fresh portion offered at the next meal; their appetite will eventually return. In the event of complete refusal, a novel boiled vegetable (such as sweet potato) may be added, bearing in mind its minimal protein contribution to the final result. In cats, the change may cause stress and worsen pruritus; if the cat is overweight or has other comorbidities, stress-induced anorexia may trigger hepatic lipidosis. Early clinical monitoring is advised in cats.
• Weight loss: This is common due to the removal of treats. The exact daily ration should be weighed, marked on a plastic cup, and the amount of food adjusted in increments of ±10% every two weeks based on regular checks of weight and body condition.
• Initial adverse effects: If persistent vomiting or diarrhoea occurs following the transition, the cat should be temporarily returned to its original diet and an alternative sought. Flare-ups of pruritus may be due to secondary infections or concurrent environmental hypersensitivities; therefore, in patients with a strong seasonal allergic component, it is advisable to carry out the dietary trial during the winter months.

6. Assessment of improvement and clinical monitoring

Regular follow-up, either by telephone or in person, helps prevent problems, provides support and reduces the owner’s frustration. The trial is considered successful if there is a significant reduction in clinical signs (typically a minimum 50 per cent reduction measured using scales such as PVAS, CADESI or FADESI). However, improvement may be influenced by non-dietary factors such as seasonal change, topical therapies or the increased intake of essential fatty acids from the new diet.

In some patients, the diet does not provide an acceptable level of comfort and does not allow anti-pruritic drugs to be withdrawn. This usually indicates a high burden of environmental allergens (atopy) where the dietary component is not clinically significant or is only partial (e.g. paw licking improves with the diet but otitis persists due to environmental factors). In very severely affected cats and dogs, owners sometimes refuse to carry out the challenge test for fear that the animal will suffer again, leaving the diagnosis unconfirmed. Hydrolysed or elemental diets also improve chronic digestive signs as they are highly digestible and modify the faecal microbiome, reducing the inflammatory environment regardless of whether a true allergy was present.

7. The food challenge test

The challenge test is a mandatory step to formally confirm a food allergy and identify the causative allergens. It involves reintroducing the original diet or individual ingredients on a weekly basis. Despite its importance, surveys show that only 33 per cent of vets discuss this step with clients, and only 10 per cent of dogs actually undergo the challenge test due to owners’ fear of a relapse.

Outbreak dynamics following the challenge

• Threshold amount: This is not fully documented in pets, but in humans, as little as 1 to 2 mg of crude protein is sufficient to trigger reactions. A recent study showed that one teaspoon or one tablespoon of the original diet was sufficient to provoke a clinical flare-up on Day 1 and Day 2 of the challenge in several dogs.
• Time to onset: A relapse may occur within hours or days. In a prospective study involving 46 dogs, 23.9 per cent developed pruritus between 3 and 6 hours after the challenge, and 60.9 per cent within 12 hours. A review of multiple studies found that only 9 per cent of dogs and 27 per cent of cats showed worsening symptoms on Day 1, estimating that the time required for 90 per cent of dogs and cats to show a flare-up is 14 and 7 days, respectively. Rapid reactions are IgE-mediated, whilst delayed reactions are due to cellular mechanisms or cumulative dose effects.
• Affected areas: Immediate pruritus following provocation is usually heavily concentrated on the limbs and face. Food-induced anaphylaxis is extremely rare in pets.

Owners should immediately withdraw the challenge diet at the first sign of symptoms to prevent the reaction from escalating. A short course of supportive therapy with oral glucocorticoids or oclacitinib for 3 to 5 days may be administered to manage the discomfort. Sequential challenge testing with individual proteins allows for the design of a clean maintenance diet, although many clients prefer to continue the hydrolysed or novel diet used in the test on a permanent basis and seek out specific ‘treats’ that their pet tolerates well.

8. Long-term management

The cornerstone of treatment is strict avoidance of the identified allergens. Commercial hydrolysed diets are complete and safe for lifelong use. Long-term home-prepared diets must be formulated with the support of a veterinary nutritionist to prevent nutritional deficiencies.

It is important to monitor the introduction of any new or processed food, as industrial processing alters the antigenicity of proteins. Dietary lapses or accidental exposures occurring throughout the pet’s life can be managed with short courses of anti-pruritic therapy. Finally, the vet should bear in mind that some pets develop intolerances or new sensitivities to the established diet after months or years of stability; if this occurs (and other external causes have been ruled out), the entire protocol must be restarted with a new elimination and challenge test.

Would you like me to go into more detail on any of the sections in this summary, such as medication protocols, the management of hydrolysed diets or step-by-step guidelines for carrying out food challenge tests? This will help you to apply these concepts directly in clinical practice.