SANDRA R. MERCHANT DVM, DIPLOMATE ACVD
DERMATOLOGY CONTINUING EDUCATION SEMINAR
Adverse reactions to food can be manifested by cutaneous, respiratory, gastrointestinal, neurologic or hematologic signs. In the dog and cat, the skin and GI tract seems to be the most common systems affected. Approximately 10-15 % of the food allergic animals with dermatologic signs will have GI disease concurrently. Twenty to 30 % of dogs will have another concurrent allergic dermatitis.
A recent study identified food hypersensitivity in 7.6% of all dogs presented to a referral dermatology practice over a one-year’s period. This represented 32.7% of all dogs presenting with allergic skin disease. Sensitization usually takes 2-3 months or longer to develop. Many studies report the majority of the animals have been exposed to the offending substance for 2 years before symptoms appear. Therefore, it is not usually a "new" foodstuff that is implicated. The reaction is usually specific to a certain protein substance in the diet. Since most commercial pet foods have similar ingredients, merely changing brands of food has little effect on the disease
In dogs, any age can be affected. In one study, Labrador Retrievers, Cocker Spaniels, Golden Retrievers and German Shepherd Dogs were over-represented. Another source noted that the Shar-Pei and Poodle might be at greater risk.
The onset of symptoms is usually sudden and symptoms may be very severe from the outset. Typically the problem is perennial, but can be episodic if the offending food is only fed intermittently or obtained intermittently (predation in cats) or seasonal if the food is a seasonal item (e.g. crawfish.) The most common gastrointestinal change in experimentally induced canine food allergy is an increase in loose stools (to an average of 3 per day).
Food allergy is often associated with other pruritic dermatopathies. Simultaneous occurrence of food allergy with a seasonal pruritic disease may cause a seasonal intensification of the pruritus. The most common complaint is pruritus. However, in a small number of cases, a recurrent staphylococcal folliculitis with no pruritus (or resolution of the pruritus when the infection is resolved) is secondary to a food allergy.
Cutaneous symptoms are nonspecific and can mimic any other allergic dermatosis. Clinical signs can mimic those of canine atopy with facial, ear, extremity and ventral distribution. “Ears and rears” may be a pattern attributed to food allergy or perineal pruritus alone. In a previous study, the ear region was involved in 80% of the cases, feet 61%, inguinal region 53%, and axillary, anterior foreleg and periorbital region 31- 37%. Otitis externa with erythema of the pinnae and vertical canal only (no horizontal canal involvement) may be the only cutaneous manifestation of the food allergy. In the study above, the ear was the only area affected in 24% of the cases. Secondary yeast/bacterial otitis externa is a common sequela. A manifestation comparable to the appearance of canine scabies (generalized papular pruritic disease) has also been associated with food allergy and may be more common in Labrador Retrievers.
Primary lesions are usually absent but papules and less frequently urticaria and angioedema may be seen. In rare cases, primary pustules (non-staphylococcal related) may be noted. Most often the skin lesions are the result of pruritus and/or secondary skin infection.
Symptoms can appear within an hour of eating the offending allergen, but may be delayed with a peak in clinical signs noted to be up to 14 days in one study. Symptoms may become continuous with repeated exposure. Secondary changes resulting from chronic pruritus (seborrhea, malassezia, pyoderma) may continue the clinical signs of pruritus even after the offending substance is avoided. ***Thus it is important to treat these secondary changes prior to evaluation of the patient with a food allergy elimination diet.*** Symptoms may or may not be refractory to corticosteroids.
Other common diseases: scabies, ringworm, demodex and flea allergy need to be ruled out as well as treating any secondary problems(staphylococcal , malassezia infection) before utilizing the diagnostic test of choice which is a food allergy elimination diet. A lymphocyte blastogenic response against food allergens in dogs with food hypersensitivity proved to be a valid diagnostic test in a recent study. However, further work including commercial availability is needed.
Diagnosis and management of food allergy in the dog has been made easier in recent years because of new unique protein/limited ingredient diets available in veterinary medicine. The length of the diet trial has been extended from three weeks up to 10-12 weeks. Studies have shown that some dogs may not show significant improvement on their diet trial until 10 - 12 weeks into the food trial. It is doubtful that a dog that is experiencing no change in clinical signs at eight – nine weeks will improve dramatically after that time.
Diets containing fish, lamb, duck, venison, rabbit, kangaroo, modified soy, modified chicken livers etc.. as the protein source are all commercially available. Most of these diets are as limited as possible in the remainder of their ingredients. The concept of protein modification to reduce antigenicity has been utilized in formulation of Hill’s z/d ultra, low allergen and Iam’s H/A. In addition, many diets contain an increased amount of omega three fatty acids or decreased omega 6 to omega 3 fatty acid ratios to decrease the formation of inflammatory mediators in the skin.
Forty percent of dogs in one study reacted to one allergen on dietary provocation, 40% to two allergens, 10 % to three allergens and 10% to five allergens. Cross reactivity between allergens from the same animal species (e.g. beef and cow’s milk) is possible. In a study published in 2004, Bovine IgG was the major allergen in cow’s milk and appeared to be a source of cross-reactivity with beef and probably with lamb because of high homology with ovine immunoglobulins. In addition, there were several other proteins found in bovine and ovine meat to which sera from food allergy dogs showed reactivity. In human medicine there has been a high degree of cross-reactivity with beef, lamb and venison IgG.
The most important concept to remember is an immunologic response can be mounted to any substance (especially protein) that has not been modified to the point of rendering it potentially non-allergenic. Thus, it is very important to obtain a complete dietary history from the client concerning their pet before choosing a food for the food allergy elimination diet trial if choosing a novel protein source diet. None of the substances in the food trial diet should have been in the previous diet(s) of the patient. In addition, the patient should be placed on non-chewable, non-flavored heartworm prevention for the length of the diet trial. The patient should be allowed no other treats, chew toys, raw hides etc. Cheating on the diet by giving treats, animal eating garbage, or scavenging under the high chair can occur.
If the animal is severely pruritic, a short course of oral short acting corticosteroids or other non-steroidal anti-inflammatory medication may be warranted. This therapy must conclude a few weeks before the end of the dietary trial. At the end of the trial, an apparent response to the test diet (no symptoms) should be confirmed with a challenge, using the former diet. Symptoms should reappear on average in a few hours up to 5-7 days. Any secondary skin diseases (seborrhea, Malassezia or staphylococcal folliculitis) should be cleared before the trial or the coexistent problems need to be treated throughout the length of the diet trial and into the re-challenge phase. If the only complaint is recurrent staphylococcal folliculitis or Malassezia dermatitis, a relapse rate must be established prior to institution of the diet trial and the diet must be given for longer than the relapse interval.
After a positive diagnosis has been confirmed by re-challenge, the animal should be placed back on the test diet until asymptomatic again OR if the owners wish to find the problematic ingredient, then suspected ingredients can be added one at a time. Alternatively, the animal can be continued on the test diet. In these instances, only a tentative diagnosis of food allergy can be made.
The only therapy is to avoid the allergen. Many owners will elect to continue on the “food trial” commercial diet. If they do not wish to pinpoint the individual allergen, they should be encouraged to keep a list of the food substances that has been previously fed to their pet. With this information, if they desire to change diets or if the diet being fed becomes unavailable, they may be able to choose another commercially available diet that avoids proteins from the “potentially offending protein” list.
In a study using heat killed Listeria monocytogenes that stimulates interferon gamma production in CD4 T lymphocytes, dogs that were experimentally sensitized to peanuts, wheat, milk and other foods, had mild to no symptoms after oral challenge. This effect was seen for greater than 10 weeks after vaccination.