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Dermatology & Otic Diseases

Food hypersensitivity, commonly called food allergy or food intolerance, is an adverse reaction associated with the ingestion of specific food items. Normally, barriers in the gastrointestinal tract prevent adverse responses to ingested food. These barriers include both physiologic and immunologic protective mechanisms. Abnormalities in the gastrointestinal defense mechanisms, such as mucosal barrier failure with increased antigen absorption or defective immunoregulation, may predispose an animal to the development of food hypersensitivity. Which of these mechanisms is important in the pathogenesis of food hypersensitivity in dogs and cats presently is unknown. Foods contain many proteins that potentially can act as allergens. Although some studies report certain dietary ingredients as common causes of food allergies (e.g. beef), this may be a reflection of greater exposure to certain food ingredients rather than an increased allergenicity of these items. Food hypersensitivity is an uncommon disease of dogs and cats and is usually not associated with a recent change in diet. Actually, up to 70 percent of animals develop allergies to foods that they have been fed for at least two years. Food allergy manifests with pruritus, especially of the face, feet and ears, and with otitis externa and skin disease. Pruritus may be very intense in some animals leading to severe self-mutilation. In some cases animals may also have gastrointestinal signs, although the majority of dogs and cats with food hypersensitivity develop only skin problems. Although food hypersensitivity is associated with the ingestion of food, owners rarely contribute the clinical signs with the diet or a change in diet. This can sometimes cause a problem in owner compliance with diagnostic tests and therapy.

DIAGNOSIS OF DISEASE

ETIOLOGY AND RISK FACTORS

• Causes - The exact mechanism of the reaction is not fully understood, but it is thought to be due to an immunologic reaction. Some reactions are thought to be hypersensitivity type I reactions, which occur within minutes to hours after ingestion of the food. Other reactions are thought to be delayed type IV reactions, which occur hours to days after ingestion. It is usually impossible to distinguish between these two types of reactions, and from a clinical and practical point of view, it is not essential.
  
  The suspected allergens associated with food hypersensitivity are numerous. The foods most commonly incriminated in dogs are beef, dairy products and wheat (accounting for 66 percent of suspected cases of food hypersensitivity in 1 study), whereas chicken, lamb, soy, eggs, pork and preservatives account for only 22 percent of reported cases according to a recent survey of veterinarians in North America. Food allergens incriminated in cats include fish, beef, chicken, dairy products and food preservatives. Adverse reactions to beef, dairy products and fish accounted for more than 80 percent of all reported cases of food hypersensitivity in cats.
  
  Reactions to food additives (colorings, flavorings, preservatives) frequently are suspected, but little objective information supports this perception and their occurrence is likely to be uncommon to rare. Additional studies are needed to confirm the role of food additives in adverse reactions in animals.
• Risk factors
• Age - Any age dog or cat may be affected, although the presence of moderate to severe pruritus in a young dog (< 1 year of age) or in an older dog (> 7 years of age when first affected) should raise the index of suspicion for food allergy.
• Breed/genetics - Terriers and Siamese cats tend to have a higher incidence of food hypersensitivity than other breeds. There is some thought that food hypersensitivity has a genetic predisposition.
• Sex - No known risk
• Geographic/environmental - No known risk
• Other medical disorders - Animals with other allergies may be at increased risk for developing food hypersensitivity. Many dogs with food allergies have concurrent flea allergy dermatitis and/or atopic dermatitis.
• Prevention - There is no known effective way to prevent the occurrence of food hypersensitivity.

HISTORY AND CLINICAL SIGNS

• Species affected - Dog and cat
• Presenting signs and historical problems - Dogs with nonseasonal pruritus, recurrent pyoderma, recurrent yeast dermatitis and/or chronic otitis externa should be suspected to have food allergies. The disease is usually indistinguishable from atopic dermatitis on historical and clinical features alone. Nonseasonal pruritus is the most common clinical sign in dogs. Most often, the feet, ears, face and axillae are the most pruritic. Other locations that are sometimes reported to be pruritic are the perineum and peri-anal area, groin and ventral abdomen, ventral neck and distal limbs. Skin lesions are usually a reflection of the chronic self-trauma associated with pruritus (erythema, alopecia, excoriations, hyperpigmentation, lichenification and seborrhea) or secondary infections (papules, pustules, crusts). Dogs with associated otitis externa may have erythematous ear canals with variable hyperplasia and, if a secondary infection has arisen, a foul odor to the ears, discharge and head shaking. Clinical signs in cats include non-seasonal pruritus, often limited to the head and neck, miliary dermatitis, indolent ulcer, self-induced alopecia, eosinophilic granuloma and eosinophilic plaque.
  
  In some animals, vomiting, diarrhea, weight loss, increased number of bowel movements and abdominal pain may be present. In a few animals, seizures, malaise and respiratory distress have been reported.

PHYSICAL EXAMINATION FINDINGS

• General
• Attitude - Mental status is usually normal but some animals may be depressed
• Body condition - Some animals may be thin and in poor body condition
• Vital signs - Usually normal
• Mucous membranes - Usually normal
• Hydration status - Most animals are adequately hydrated
  
  The physical examination usually reveals clinical signs consistent with chronic pruritus (alopecia, erythema, lichenification, variable hyperpigmentation, seborrhea), lesions of secondary bacterial and yeast infections (papules, pustules, crusts) and otitis externa (erythema, hyperplasia, discharge, head shaking, pain on palpation). Seizures have been reported as a rare manifestation of food allergy in dogs. A few dogs may have respiratory distress, similar to an asthma attack.

DIAGNOSTIC STUDIES

• Food elimination trial - A food trial is the only reliable way to diagnose food hypersensitivity. Serum allergy tests and intradermal allergy tests are available but are unreliable and have a high incidence of false positives.
  
  An ideal food trial consists of feeding a small number of highly digestible proteins to which the animal has not been previously exposed. Only a novel source of protein (i.e. never before eaten by that animal) should be used for the food trial, and it is best to use only one source of protein. Cross-reactivity among foods is poorly documented in animals, and it is not known if hypersensitivity to foods closely related to one another (e.g., chicken and turkey) occurs.
  
  Although proteins are the most likely offending allergens, carbohydrates, such as corn and wheat, also contain small amounts of proteins that could serve as allergens. The most effective trial diets are those that contain only one source of protein and one source of carbohydrate with minimal additional ingredients. Finding a novel carbohydrate can be difficult - rice and potato are often given, but most dogs have been exposed to these carbohydrate sources. Allergic reactions to rice and potato have been reported but are thought to be rare.
  
  Some commercial diets advertised for use in the diagnosis of food hypersensitivity are designed on the basis of the molecular size (hydrolyzed diets), so that antigenicity is reduced or eliminated. However, small peptides can aggregate together, potentially causing cross-linking of IgE and an allergic response and depending on the extent of the hydrolyzation, a percentage of proteins will not be reduced in size and remain allergenic. Allergy to hydrolyzed formulas has been reported in children and a similar situation is suspected to occur in dogs. Further, in animals that do not have IgE-mediated food hypersensitivity, the size of the protein may not be so important.
  
  Food trials can be carried out using homemade or commercially prepared diets. No diet is inherently hypoallergenic, and this fact is important to remember when conducting a food trial. Food hypersensitivity may develop with any food if it is fed long enough. Thus each hypoallergenic diet is prescribed for each patient, and what is hypoallergenic for one patient may not be hypoallergenic for another.
  
  A homemade diet is preferred, but commercial diets are commonly used due to the inconvenience of preparing homemade diets for the duration of the food trial. If a homemade food trial is attempted, it is important to feed a nutritionally balanced diet, especially to young dogs and cats. Non-flavored vitamins and supplements should be added.
  
  If commercial diets are chosen, it is important to remember that they often contain a large number of ingredients in addition to those advertised on the label. Chewable medications and flavored toys also should be discontinued during the food trial. Non-chewable and non-flavored tablets should be used for heartworm prevention.
  
  Client education is important, and the success of the food trial depends on the choice of diet and the compliance of the pet owner. One of the major reasons for apparent failure of food trials is non-compliance of owners for the reason that they were not clearly informed of what exactly they are and are not allowed to feed their pet. A handout detailing all the forbidden items can be used for reference as well as providing a source of information for household members not present at the office visit with the pet.
  
  The food trial should be continued for two months minimum. Pruritus and recurrence of skin infections are monitored during the trial. A tentative diagnosis of food hypersensitivity is made if marked improvement in clinical signs occurs during the trial. Challenge, with recurrence of clinical signs, is necessary to confirm the diagnosis. Initially a challenge with all of the "old" dietary ingredients is performed to confirm that the improvement is associated with feeding the hypoallergenic diet, and not as a result of other treatments that may have been given (improved flea control, antimicrobials for bacterial or fungal infections). If the "old" diet induces clinical signs again (usually within days to 2 weeks), then challenge with individual ingredients is performed in order to identify the specific offending food. Worsening of clinical signs may take several hours or up to 2 weeks to occur. The elimination diet is used as the basic diet at this time, and the animal should be challenged with one ingredient at a time for two weeks. If no worsening occurs, challenge with the next ingredient on the list should be done until all ingredients in the original diet have been evaluated. If clinical signs recur, the offending ingredient should be discontinued and the animal fed only the elimination diet until symptoms disappear. When symptoms resolve, challenge with the next ingredient may be carried out.
  
  Elimination trials are complicated by the fact that many affected animals have other hypersensitivities, in addition to food, e.g. flea allergy dermatitis or atopic dermatitis. Such patients respond only partially to a food trial. For this reason, it is important to control flea exposure, consider the possibility of concurrent atopic dermatitis and challenge patients with isolated food ingredients at the end of the trial.
  
  Commercial diets that have been formulated with a single protein ingredients are manufactured by Innovative Veterinary Diet (Venison and potato, rabbit and potato, duck and potato etc.), IAMS (Eukanuba Response Formula fish and potato or kangaroo and potato), Waltham's (lamb and rice, catfish and rice), Hills d/d (lamb and rice, egg and rice, duck and rice, whitefish and rice) and Nature's Recipe. Available hydrolyzed protein diets are manufactured by Hills (z/d ultra with hydrolyzed chicken and corn starch, z/d low allergen with hydrolyzed chicken and potato), Purina (HA with hydrolyzed soy protein and corn starch).
  
  The size of the molecules used is very small. Purina HA diet is soybean based, and the protein molecules used also are quite small. Eukanuba kangaroo and oat diet contains canola meal and undefined "animal fat" which makes it a less desirable choice. Omega 6 and omega 3 fatty acids have been added to the diet in a ratio of 5:1 to minimize the production of inflammatory precursors. Eukanuba fish and potato contains herring meal, catfish, animal fat and potato as the main ingredients. The presence of animal fat makes this product a less desirable diet for a food trial. Eukanuba lamb and barley could be considered for an elimination trial. Hill's Canine d/d comes in dry and canned formulations. The canine dry d/d may contain duck and rice, salmon and rice or eggs and rice. Canned canine d/d may contain whitefish and rice or lamb and rice. The feline canned d/d contains lamb and rice. Hill's z/d and Hill's z/d ultra are newer commercial diets that contain a unique novel hydrolyzed protein.
• Microbiology - Fungal culture should be performed to rule out dermatophytosis.
• Pathology
• Cytology (fluid or tissue) - Skin scraping should be done to rule out the presence of Demodex mites and skin cytology to check for the presence of yeast or bacterial infections.

DIAGNOSIS AND PROGNOSIS

• Differential diagnosis - Other disorders can mimic food hypersensitivity. These include:
• Atopic dermatitis
• Sarcoptic mange
• Flea allergy dermatitis
• Bacterial folliculitis
• Seborrhea
• Malassezia dermatitis
• Otodectic mange
• Notoedric mange
• Dermatophytosis
• Psychogenic alopecia
• Recommended tests - Skin scraping, skin cytology, fungal culture and food elimination trial
• Summary of diagnostic criteria - Food hypersensitivity can be diagnosed if the animal responds favorably to the food elimination trial.
• Prognosis - Most animals respond favorably if not fed the offending food item. Owner compliance is crucial in preventing relapse. Unfortunately, new food allergies can develop over time, although the incidence of this is not yet known and likely to be low.

TREATMENT OF DISEASE

TREATMENT PRINCIPLES

Treatment for food hypersensitivity is avoidance of the offending food.

LONG-TERM/HOME THERAPY

After the diagnosis has been made, animals are fed a complete well-balanced diet that does not contain the offending food item.

• Up to 80 percent of patients can be managed with commercial diets. A small percentage of animals, however, may require homemade diets, and in those animals it is important to insure that the diet is balanced and nutritionally adequate. When avoidance is not feasible, corticosteroids, such as prednisone 0.5 to 1 mg/kg PO once daily, may be used. Some food hypersensitive animals, however, respond poorly to corticosteroids and, as a general rule, the efficacy of this type of therapy tends to decrease over time.
• Although uncommon, new food allergies can develop over time. These animals require re-evaluation with new elimination diets.
• Response to antihistamines and fatty acid supplements usually is limited in dogs and cats.
• Hyposensitization is not an effective form of therapy for food hypersensitivity.

FOLLOW-UP CARE

Follow-up is usually not needed unless relapse occurs. Most animals do well if the entire family complies with the dietary restrictions.