It may be necessary to treat secondary complications of adverse food reaction such as superficial pyoderma, Malassezia dermatitis, and ear infections. Secondary complications do not preclude a dietary trial. In fact, if an animal truly has
a food allergy, switching to a novel protein diet may actually enhance resolution of a pet's condition.
Under these circumstances, however, the animal should be assessed after three or four weeks to ensure antimicrobial therapy
has been adequate. Pruritus typically will have diminished during this time as a result of controlling the secondary infections.
The diet trial should be continued for another two to four weeks to demonstrate improvement is maintained before provocative
challenge is performed.
How much allergen is too much?
The amount of allergen ingested that provokes an allergic reaction varies with allergen type and among individuals. This has
been shown in the case of peanut allergy in people, and there are anecdotal reports of animals that appear to have adverse
reactions to flavored medications. In our practice, we routinely prohibit chewable heartworm prophylactics, flavored antibiotics
and vitamin tablets, and toothpaste during a diet trial. In an open study, the colony of Maltese-beagle dogs at North Carolina
State University was intolerant of flavored milbemycin tablets containing soy and pork. As little as 200 mg/kg of corn (1
g of corn for a 11-lb [5-kg] dog) is sufficient to elicit an allergic reaction in susceptible individuals.7 Clients may better comprehend this small amount if you tell them it is less than the amount of corn in a single corn chip
Figure 7. In some dogs with corn allergies, 1 g of nacho chip pieces is enough to trigger an allergic reaction.
Confirming a food allergy
Adverse food reaction is confirmed only when it has been demonstrated that the same clinical signs recur when previously fed
foodstuffs are reintroduced. I usually recommend reintroducing the previously fed commercial diet first as 10% to 20% of total
food for seven days. If this does not result in a relapse, then commonly fed treats can be reintroduced to determine whether
these induce clinical signs. In my experience, gastrointestinal signs, if present, usually resolve first and may be the first
to reappear after challenge in an animal with adverse food reaction.
How long it takes for dermatologic signs to reappear is a subject of debate. Most authors agree a patient should relapse between
24 hours and two weeks after challenge, although one recent trial reported that two dogs took up to three weeks to show signs.17 Another study reported a significant difference between dairy and cereal products, with patients showing signs 4.1 and 8.3
days after reintroduction, respectively.10 In most reported cases, time to relapse was reported by the owner (often by phone), and the patient was not examined immediately
after the challenge by a veterinarian. Our experience with the Maltese-beagle dogs at North Carolina State University is that
clinical signs of erythema may be seen as early as 15 minutes after challenge. If an animal fails to respond to a correctly
performed diet trial, I often perform a second diet trial with a different diet to definitively rule out food allergy.
Golden rules for successful diet trials
Identifying specific food allergens
Between 35% and 60% of dogs are reported to be intolerant to more than one foodstuff, which underscores the need for challenge
with multiple food sources.10,19,20 Most veterinary professionals agree that beef hypersensitivity has the greatest incidence in dogs in Europe and the United
States. Similar information is not available in cats because owners have typically been reluctant to pursue provocative challenge.