Last month, we looked at common culprits as well as the prevalence and clinical signs of food allergies in dogs and cats. But definitively diagnosing a food allergy in dogs and cats remains a challenge. Unfortunately, the only current method to accurately identify patients with food allergies is to perform an elimination diet trial for a sufficient time while controlling all concurrent allergies and secondary infections. This is easier said than done. Both intradermal allergy testing and serologic testing for food allergies remain unreliable, with both false positive and false negative results occurring.1
Three types of diets are available in a veterinary dermatology setting. Novel protein and hydrolyzed protein diets are useful for the diagnosis and long-term management of patients with food allergies. Therapeutic diets are formulated with higher and "balanced" levels of omega 3 and 6 fatty acids and are most useful for patients with atopic dermatitis. They will not necessarily be formulated with novel proteins. Most of the major manufacturers of therapeutic dog foods now provide a line of "hypoallergenic" foods.
Choosing a diet for the trial
There is no foolproof "works every time" test diet. Choosing the best diet to feed a patient with a suspected food allergy requires careful and detailed questioning of the client regarding previous and current diets, treats and flavored medications. Once that information is known, you must choose a diet that 1) consists of proteins to which the patient has not had exposure, 2) has minimal chance of cross reactions with previously fed proteins, 3) will be eaten by the patient and 4) will be readily fed by the client. Because of all these factors, rabbit, kangaroo and, occasionally, fish are the first diets of choice for most patients with suspected food allergies. You should also be confident that the manufacturer of the food has truly kept the food limited to what is stated on the label and not allowed contamination with other feeds or proteins.
In addition to determining which novel protein is appropriate for the test diet, it is also necessary to counsel the owners on what to avoid feeding. We frequently deal with situations in which the owners have fed an appropriate test diet but continued to feed treats and protein-based supplements. Some patients with food allergies will flare or continue to exhibit clinical signs simply from beef- or pork-based additives in chewable medications.
Hydrolyzed diets are also available, with hydrolyzed chicken- and soy-based foods being the most common. Several published studies have reported the majority of patients fed hydrolyzed diets have improvement in clinical signs, even if they are allergic to the parent protein. Yet other studies show up to 50 percent of patients with food allergies flare or fail to improve while eating a hydrolyzed diet.2 In 2010, a report summarized all the various (and sometimes conflicting) articles on the subject and concluded hydrolyzed diets should not be used if a patient could potentially be hypersensitive to the parent (nonhydrolyzed) protein.2 I prefer novel proteins for the test phase. Occasionally, a cat with a food allergy will refuse to eat novel proteins, and hydrolyzed chicken diets are the second choice. Because of limited availability of some of the novel protein diets, once a patient has improved after eating a novel protein diet, I will recommend a challenge with a hydrolyzed diet (soy or chicken) since these diets are always available and not subject to back orders.
Diet trial monitoring
One can find variable recommendations regarding the length of time necessary to see improvement once a patient is placed on the hypoallergenic diet, with some recommending a 12-week diet trial. In my experience, it is rare for a food-allergic patient to not show measurable improvement within four to six weeks, so six weeks is our normal recommended length. Requiring clients and patients to struggle on for 12 weeks without seeing improvement in clinical signs can cause many owners to lose faith with the entire process, leading the owners to abandon the food trial and possibly seek out a different opinion. It may require more than six weeks for the maximum improvement to be seen, but at least the patient is improving during the process, which provides encouragement to continue the trial.
During the food trial, it is important to minimize the other causes of pruritus that will interfere with the ability of the client and veterinarian to determine the success or failure of the food trial. Zealous flea control in flea endemic areas is necessary. Monitoring for and treating secondary infections (pyoderma and Malassezia dermatitis) is also necessary. These infections are often the reason a food trial is being performed in the first place, so it is not uncommon to treat the patient with appropriate antimicrobial therapy potentially for the first half of the food trial. Further counseling is then needed to ensure the medications are not administered in a treat.
In Part 3 of this series, I will discuss common pitfalls to avoid when performing diet trials.
Dr. Lewis sees dermatology patients in California, Arizona, Nebraska, New Mexico and Washington. In 1991, he established Dermatology for Animals, PC.
1. Mueller R, Tsohalis J. Evaluation of serum allergen-specific IgE for the diagnosis of food adverse reactions in the dog. Vet Dermatol 1998;9:167-171.
2. Olivry T, Bizikova P. A systematic review of the evidence of reduced allergenicity and clinical benefit of food hydrolysates in dogs with cutaneous adverse food reactions. Vet Dermatol 2010;21(1):32-41.