Your food allergy questions answered

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After the United States Food and Drug Administration’s recent reports on boutique, exotic, and grain-free diets potentially being linked to heart disease,¹ more pet owners are asking veterinary staff what they should or should not feed their pets. Food allergy is a topic that often comes up during these conversations, but before recommending a full workup for this type of adverse food reaction, it’s helpful to determine the answers to 3 broad questions.

1. Is an elimination diet trial (food trial) indicated for the patient?

The patient’s signalment and clinical signs must be taken into consideration to determine how likely a diagnosis of a food allergy is.

Age

The age of onset for clinical signs can vary, but the classic time points are less than 6 months old or greater than 6 years old. Approximately 40% of dogs develop skin signs before they are 1 year old, but cats may be older.²

Breed

Any breed is potentially food allergic, but the suspicion may increase for some, like Labrador retrievers and German shepherds.²

Clinical signs

Clinical signs tend to be nonseasonal. Most food allergic dogs and cats have either noncutaneous or cutaneous signs of food allergies. Only 20% have both.³

• Noncutaneous: Diarrhea and frequent defecations are most common in dogs, whereas vomiting and diarrhea are most common in cats. Other gastrointestinal (GI) signs can include flatulence, borborygmi, tenesmus, and salivating.³
• Cutaneous: Pruritus is the expected sign, often affecting the ears, ventrum, and feet, with or without secondary Staphylococcus or Malassezia infections. Perianal pruritus may not be as common as previously thought. Up to 69% of food allergic dogs may develop otitis, sometimes as the only sign.² Food allergic cats may present more commonly with head and neck pruritus, but they can also have other presentations, such as symmetrical alopecia, miliary dermatitis, and/or eosinophilic granuloma complex.^2,4

2. Is a food trial feasible for the specific client and patient?

Even if food allergies are likely, a food trial may not be a reasonable option.

Owner compliance

Owner compliance is often the biggest obstacle to successfully completing a food trial, contributing to 20% of patients dropping out.^5,6 Sometimes there are temporary reasons to delay performing a food trial, like a vacation or an upcoming holiday with visitors who may be less strict about foods. When owners are willing and able to follow all recommendations, including the amount of food, type of food, duration, follow-ups, and food challenges, then the food trial can be a successful diagnostic tool.⁷

Compliance of patient

Although people can practice self-control, it may be difficult to manage what an animal will or will not eat. Feeding different diets in a multi-pet household can be challenging, although technology with microchip-activated food bowls helps. Outdoor cats need to be kept indoors. Dogs with a passion for scavenging need to wear a basket muzzle. Some pets will only eat dry vs wet food, which can limit diet options. Others require high-reward treats for training or pilling. Finicky appetites are difficult; although a dog’s hunger strike can usually be outsmarted with patience, a cat’s hunger strike may be deadly. If the pet does not eat the food due to palatability, some pet food companies will financially reimburse the client.

Age

Since food allergies can occur at a young age, it is important to consider whether the recommended diet will be appropriate for the pet’s life stage. Currently (as of publication), the only therapeutic food trial diets with the Association of American Feed Control Officials statements of nutritional adequacy for growth are Purina Pro Plan Veterinary Diets HA Hydrolyzed Chicken Flavor Canine Canned Formula and Royal Canin Hydrolyzed Protein HP Dry Dog Food.

Concurrent diseases

It’s important to take a pet’s overall health status into account before recommending a food trial.

• GI problems: For food allergic pets with both GI and skin signs, a single food trial may address both issues. However, if a pet has a history of severe hemorrhagic diarrhea that requires hospitalization after diet changes, then a gradual transition with close monitoring may be required. In this situation, a risk/benefit discussion with the owner should be considered before performing a food challenge if the pet is doing well on a new food.
• Other health problems: Some pets have unrelated concurrent diseases, such as kidney disease, pancreatitis, or urinary stones. Certain diets can help with multiple diseases, such as Purina Pro Plan Veterinary Diets HA Hydrolyzed Canine Vegetarian Formula, which is lower in fat and good for pancreatitis, or Multifunction Renal Support + Hydrolyzed Protein Dry Cat Food, which is lower in protein for chronic kidney disease. Working with a veterinary nutritionist is helpful for pets with multiple diseases, a finicky appetite, or weight management issues, and for owners who want to cook for their pets at home.
• Medications: Many oral medications are flavored. Some can be easily switched, such as changing flavored oral parasiticides to topical versions, but ideally even gelatin capsules should be avoided.⁸ For compounded medications, some pharmacies can select flavorings that should not interfere with a diet trial. Nonessential flavored vitamins and supplements should be discontinued. In other cases, such as behavioral medications for aggression, the risk of changing or stopping the medication is too great for the purposes of a food trial.

Cost

Although all gold standard medical options should be presented to educate and inform owners, real-world financial limitations often must inform medical decisions. If a 2-year-old, adult-onset, seasonally-pruritic golden retriever is likely atopic, then it may be prudent to consider and discuss that the cost of a 2-month therapeutic food trial is equivalent to or even more than the cost of a dermatology consultation, blood allergy testing, or 6 months of immunotherapy, all of which may be more valuable than a food trial. Every situation is different.

For many owners, the challenges mentioned above are enough for them to decline a food trial, and they’ll often ask about other tests on the market. Unfortunately, blood, saliva, hair, microbiome, and other tests are not diagnostic for food allergies in pets at this time. One study submitted fake fur and water in addition to dog hair and saliva, and the test couldn’t differentiate between allergic, nonallergic, and fake dogs.⁹ The pathogenesis of food allergy likely involves both IgE-mediated (type I hypersensitivity) and non–IgE-mediated (type III and IV hypersensitivity) responses, so testing for IgE alone, as with most blood allergy tests, is insufficient. Although veterinary medicine eagerly awaits a cageside test for food allergies, numerous studies indicate that the current tests are neither specific nor sensitive and should not be used.^10-14

3. How should the food trial be performed?

If a food trial and challenge can and should be performed, then the final question is how best to do so.

Duration

A review of food trial studies found that after 5 weeks in dogs and 6 weeks in cats, 80% of food allergic patients had achieved a remission of clinical signs. Extending the food trial to 8 weeks increases the sensitivity of diagnosis to more than 90% in both dogs and cats. A diet trial of up to 13 weeks was needed for less than 5% of dogs to achieve complete remission of signs.¹⁵

Proteins

The major food allergens in humans are heat-stable, water-soluble glycoproteins between 10 and 70 kDa.¹⁶ The most common food allergens for dogs are beef, dairy, chicken, wheat, and lamb; and the most common food allergens in cats are beef, fish, and chicken.¹⁷ However, even a nonprotein additive could rarely—but theoretically—be an allergen, so some consider home-cooked diets to be the gold standard, but they are often difficult to maintain.

There are 2 approaches to managing dietary protein exposure in food allergic pets.

• Novel proteins: A diet history is important as there are more and more novel proteins available over the counter (OTC). Based on polymerase chain reaction (PCR) and enzyme-linked immunosorbent assay (ELISA) tests, OTC diets are not recommended due to the possibility of contamination with other proteins.^18-22 If a patient has eaten the protein previously—or a potentially related protein (cross-reaction)—then certain diets may need to be avoided. Cow’s milk, beef, and lamb share allergenic molecules.²³ Although cross-reactions within the poultry family may seem obvious,²⁴ there may even be IgE cross-reactions between chicken and fish.²⁵
• Hydrolyzed proteins: Hydrolysis breaks proteins down to less than 10 kDa for higher digestibility and lower allergenicity.²⁶ Thus far, the recognized allergenic proteins for dogs weigh more than 20 kDa.²⁷ Approximately 10% of chicken-allergic dogs may still react to hydrolyzed chicken,²⁸ but they do not seem to react to extensively hydrolyzed feathers.²⁹

Food challenge

Food challenge, typically starting with the pet’s previous diet, completes the food trial process. Without a challenge, clinical improvement falsely attributed to food could instead be due to other variables, such as treatment of infection, administration of anti-inflammatory or antipruritic drugs, or change in season. Relapse of clinical signs may occur within minutes to 14 days after food exposure.³⁰ Single-ingredient challenges can then help with the selection of a long-term diet.

It’s important to remember, and to remind owners, that one of the most common indications for a food trial is to aim for a negative result (ie, not food allergic). In medicine, negative tests are still informative. The prevalence of food allergy in dogs and cats has a wide range depending on the population studied (< 1% for any disease, approximately 5% for skin disease, and 15% to 20% for allergic skin disease),³¹ but the number is likely lower than the 10% to 15% prevalence of atopy in dogs.³² A negative food trial is a necessary step to reach the exclusion diagnosis of atopy.

References

1. Questions & answers: FDA Center for Veterinary Medicine’s investigation into a possible connection between diet and canine heart disease. United States Food and Drug Administration. Updated June 27, 2019. Accessed August 31, 2021. https://www.fda.gov/animal-veterinary/animal-health-literacy/questions-answers-fda-center-veterinary-medicines-investigation-possible-connection-between-diet-and
2. Olivry T, Mueller RS. Critically appraised topic on adverse food reactions of companion animals (7): signalment and cutaneous manifestations of dogs and cats with adverse food reactions. BMC Vet Res. 2019;15(1):140. doi:10.1186/s12917-019-1880-2
3. Mueller RS, Olivry T. Critically appraised topic on adverse food reactions of companion animals (6): prevalence of noncutaneous manifestations of adverse food reactions in dogs and cats. BMC Vet Res. 2018;14(1):341. doi:10.1186/s12917-018-1656-0
4. Hobi S, Linek M, Marignac G, et al. Clinical characteristics and causes of pruritus in cats: a multicentre study on feline hypersensitivity-associated dermatoses. Vet Dermatol. 2011;22(5):406-413. doi:10.1111/j.1365-3164.2011.00962.x
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8. Parr JM, Remillard RL. Common confounders of dietary elimination trials contain the antigens soy, pork, and beef. J Am Anim Hosp Assoc. 2014;50(5):298-304. doi:10.5326/JAAHA-MS-6104
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Editors note: All veterinary technician content for this month is supported by Banfield Pet Hospital.