Food allergy is defined as an immunologically based reaction to food. In most clinical cases, the exact etiology is not well understood. Immunologic mechanisms of types I-IV have been hypothesized. In contrast, food intolerance is a general term describing any adverse reaction to food that does not have an immunologic basis, including food poisoning (caused by the direct action of a toxin). From a practical basis, the mechanism of action does not impact the clinician faced with a potential case of chronic food-caused cutaneous disease. It is theorized that most food allergens are proteins.
No sex predilection has been reported for food allergy in dogs or cats. In some studies, no breed predilection was noted. In contrast, two studies found that certain dog breeds may have a risk for the development of food allergy:
Soft-Coated Wheaton Terrier, Dalmatian, West-Highland White Terrier, Collie, Chinese Shar Pei, Llasa Apsa, Cocker Spaniel, Springer Spaniel, Miniature Schnauzer, Labrador Retriever Dachshund and the Boxer. Breed data from Colorado State University shows that retrievers may be at greater risk to develop food allergy than other breeds of dogs. While the age at presentation has been reported as variable, several researchers now feel that at least 33% of their cases in dogs are of animals less than one year of age. Clearly, while food allergy may occur at any time in animal’s life, it should always be considered as a differential of pruritus in the young dog
History and Clinical Signs
The most common clinical sign of food allergy is non-seasonal pruritus, which is usually generalized. Pruritus may also be primarily directed at the feet or ears. Very rarely, food allergic dogs with skin lesions but without pruritus have been reported. The most common primary dermatologic lesions are papules and erythema; common secondary lesions are epidermal collarettes (usually indicating a pyoderma) pyotraumatic dermatitis (“hot spots”) hyperpigmentation, and seborrhea. Clinical signs of food allergy have been reported in Cocker Spaniels identical to the idiopathic seborrhea associated with that breed. Food allergy as the underlying cause of idiopathic onychodystrophy (misshapen, splitting claws [nails]) has been reported in two dogs. Food allergy in cats may present as pruritus of the head and face, milliary dermatitis, or one of the manifestations of the eosinophilic granuloma complex.
Reported concurrent gastrointestinal (GI) signs among dogs with cutaneous signs of food allergy are rare; it is unknown if this is due to a true dearth of GI signs or if in fact changes in the stool of these dogs were relatively subtle and/or were not noted or volunteered by the owners while obtaining the history. However, a recent report documented 20 dogs with both pruritus and GI signs typical of colitis: fecal mucus, fecal blood, tenesmus and increased fecal frequency. Both cutaneous and GI signs resolved upon feeding the dogs an elimination diet. Lymphocytic-plasmacytic colitis has been linked to food allergy in cats and cheetahs.
Neurologic signs such as malaise and seizures rarely have been reported. The author feels that malaise may be under-reported, as an increase in energy level (“acting like a puppy, feeling better”) is often noted upon feeding the dog a diet without the offending allergen; this may occur before cessation of pruritus. Respiratory signs, such as asthma, have also been reported, but seem to be quite rare.
Concurrent hypersensitivities have been reported in dogs and include atopy, flea allergy dermatitis, intestinal parasite allergy, and even an allergy to bovine insulin. Concurrent pyoderma and/or Malassezia pachydermatis infection is also common. Dogs may have pyoderma (superficial or deep) as the only clinical sign of food allergy. These dogs are often clinically normal (i.e., non-pruritic) while receiving antibiotics. Therefore, it becomes quite important to diagnose and treat secondary infections, as persistence of pruritus due to these infections may confound the ability of the clinician to diagnose the underlying allergy.
The ideal method of diagnosis is the feeding of an elimination (“hypoallergenic”) diet. The experience of the author and of other researchers has been disappointing in the use of serologic or intradermal skin tests to diagnose food allergy in pets in North America.
The elimination diet generally contains one protein and one starch. These must be based on previous exposure of the dog to various foodstuffs. Important to remember is that dogs who live in households with cats tend to have been exposed to fish, through their consumption of either cat food or cat feces. At UC Davis, we often start dogs with pork and potatoes, although pinto beans and potatoes may also be used. Based on non-exposure, rabbit, duck, and tuna are also options. We have also used ‘exotic’ foods like elk when feasible. Other than fresh water, nothing else should be fed to the dog during the elimination diet trial. This means that vitamins and chewing toys must be eliminated, and that flavored medications (such as certain ecto/endoparasite preventatives) should be replaced by other, equally effective non-flavored preparations. Protein-flavored toothpaste should be replaced by the malt-flavored variety. Because the elimination diet is not a balanced one, owners should be warned that the dog might lose weight, develop a “dull” haircoat or scaling, or be hungrier than usual. In cats, we will use lamb-based baby food for human infants.
Certainly, some owners are unable or unwilling to cook for their pet for the period necessary. In such cases, the dermatology service at UC Davis uses commercially available limited-antigen diets. For dogs these would include Purina LA (salmonid); Iams FP (fish and potato) and KO (kangaroo and oats); IVD duck, venison, whitefish, or rabbit plus potato; Hills D/D (duck or fish and rice); or Waltham fish and rice. For cats, these would include IVD duck, venison, or rabbit plus potato; Hills D/D feline; or Iams lamb and barley. Another option for animals who already have been fed many foods, or whose dietary history is unknown, is the use of hydrolyzed protein diets, in which the protein source is hydrolyzed to small molecular weights, thus avoiding the body’s “immunologic radar.” Such foods include Purina HA (hydrolyzed soy), Hills Z/D, or DVM Exclude. Use of a commercially prepared diet will give an approximately 90% chance of determining a food allergy; however, none of these diets will work for all animals, and failure of an animal to improve on such a diet may warrant trying another one, or a home-cooked diet in another trial.
The length of the elimination diet is somewhat controversial, however, our observations have justified a dietary trial of eight to 12 weeks. Persistence of some pruritus at 12 weeks into the diet trial may indicate the need for continuing the diet, but may also indicate the presence of concurrent hypersensitivities. In cases where antibiotics are given to treat secondary infections, or oral corticosteroids for severe pruritus, the diet must be continued for a minimum of two weeks past discontinuation of these treatments, in order to properly judge its efficacy.
Upon resolution of clinical signs with the feeding of an elimination diet, the animal should be challenged with its regular diet to confirm the diagnosis of a food allergy. Recurrence of clinical signs is usually noted within one week, but may take as long as two weeks. At that point the animal is given its elimination diet again, and the owner may then elect to challenge with suspected allergens, each allergen being given one to two weeks at a time. The most common proven allergens in the dog are beef, chicken, milk, eggs, corn, wheat, and soy; in the cat, fish and milk products. Allergies to more than two allergens are uncommon. Once the offending allergens are identified, commercially prepared dog foods that do not contain them may be fed to the dog. In cases in which the owners refuse to do provocative testing, one of the limited-antigen pet foods may be used as a maintenance diet.
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