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My Dog Is Itchy… How Do I Know If It’s A Food Allergy? – Dr. Karen Whala

Dealing with a very itchy pet can be extremely frustrating to you, the pet owner, but there is hope and treatment for your furry friend…but this depends on accurately identifying the cause of your pet’s pruritus (aka itching). There are many causes of itching in dogs of which < 20% iii of dogs that have been properly worked up (sought dermatologist) were diagnosed as purely food allergic. So, before we discuss the possibility for food allergies, let’s identify some of the other common causes of itching: Common causes for itching in dogs can be divided into 2 broad catagories – external parasites (parasites that live on the skin) and allergies (a state of hypersensitivity where the immune system overreacts upon exposure to an otherwise harmless substance – also known as an allergen). In people, this can manifest in respiratory symptoms such as watery eyes, sneezing and runny nose, but in dogs, allergies more often manifest cutaneously with recurrent ear infections, recurrent skin rash, increased skin pigmentation, loss of hair and biting/licking feet. External parasites include mange (sarcoptes, cheyletiella and demodex) and usually require magnification to identify. Fleas can cause itching in 1 of 2 ways – simply by their presence and movement on the pet OR by eliciting an allergic reaction by their saliva. The former is referred to as an "infestation" which is easy to identify and the second as an "allergy" which is more difficult. Other causes of itching include drug reactions, primary or secondary bacterial/yeast infections, hormonal abnormalities and contact allergic reactions (such as to floor detergents). In this article, we will focus on allergies. The 3 most common causes of skin allergies in dogs are flea bite allergy, food allergy and atopy (a hypersensitivity to inhaled allergens such as pollens, dust mites and dander). These 3 types of allergic dermatitis conditions can and do often look identical but as you will see, are treated very differently. Flea bite allergy IS THE MOST common skin disease in dogs (1) and one of the easier allergies to treat. EVEN IF you don’t see any fleas (no infestation), you should still treat regularly with a monthly (or more frequently) preventative because it only takes a few stray fleas and their saliva to cause an itchy breakout in those pets that are allergy-prone. Because of this, 100% flea control is essential for these pets. Also, if your pet has any other types of skin allergies, treating for fleas anyways may reduce symptoms…even if you never see a flea! Remember, in the Central Valley California, fleas don’t take a vacation and are a nuisance all year round!

Atopy (aka environmental allergies) can look IDENTICAL to a pet suffering from food allergies, with the exact same manifestation of symptoms of recurrent ear infections, recurrent skin infections, licking of feet/anal area, hairloss and general itching. Diagnosis can be challenging and is often only arrived at after all other possible causes for itching have been eliminated, making atopy a ‘diagnosis of exclusion’. This condition is lifelong like many allergies and requires a dedicated pet owner to successfully manage. Multiple treatment modalities are often required including antihistamines (Benadryl), frequent bathing (benzoyl benzoate shampoo), immunotherapy (allergy injections), essential fatty acids and often immunosuppressive therapy (steroids, cyclosporine).

Food Allergy – As the name suggests, is an immune-mediated reaction to an allergen in a food. The most common food allergens include beef, chicken, corn, wheat, cow’s milk, soy, eggs and fish, though virtually any food item can elicit an adverse food reaction. An adverse food reaction can occur at any age [and any diet], but itchy dogs less than 1 year and older than 5 can increase index of suspicion. Symptoms of food allergy are nearly identical to those of Atopy – recurrent skin infections, recurrent ear infections, licking of feet/anal areas, hairloss and generalized itching with sometimes minor differences: Food allergies are considered “nonseasonal” whereas atopy can initially start out seasonally.

Food allergy can also present with the moniker “ears and rears” where the combination of perianal itching & ear infections are more common. The ONLY accurate method for diagnosis (once most other causes for itching have been excluded) of canine food allergy is to feed a strict novel protein/carbohydrate or hydrolyzed diet for a minimum of 8-10 weeks(i). This is referred to as a food trial. Other methods for attempting to identify food allergy such as intradermal or serum testing has proven to be inaccurate in dogs(i). A “novel” protein source refers to a protein that the dog’s immune system has never been previously sensitized. Such diets can be homemade or prescription diet. Commercial diets can be a reasonable alternative however it has been demonstrated that many otc (over the counter) diets contain contaminants or trace amounts of potential allergens which are not listed in the ingredient list and can negate all efforts of an effective food trial. During this “food trial”, NOTHING else should pass your pets lips as accidental ingestion of the unknown offending allergen can also negate a food trial. This includes table crumbs, litterbox “candy” and flavored medications.

Once a pets itching has appeared to significantly improve while on a strict food trial, it is important to rechallenge the pet and confirm this diagnosis of food allergy as there may be many reasons why a pet improved including those which may not be related to the diet change at all. Often a rechallenge is done gradually by adding in a single protein or carbohydrate source every 2 weeks to assess for recurrence of symptoms. An apparently ineffective food trial does not necessarily indicate that a pet is not food allergic. There are many reasons for failure to respond.

There are many causes for itching in our beloved companions and as you have read, a definitive diagnosis can be challenging, however, with the proper diagnosis you both have the best chance at effective management and treatment.

For further information on food allergy, check out:

1. Food Allergy Myths: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=468

2. Itching and Allergy in Dogs: http://www.veterinarypartner.com/Content.plx?P=A&A=2604&S=4&SourceID=61

3. Food Allergies: http://www.veterinarypartner.com/Content.plx?P=A&S=0&C=0&A=652

    References

i Robert A. Kennis, Food Allergies: Update of Pathogenesis, Diagnoses, and Management. Vet Clinic Small Anim Practice 36 (2006) 175-184

ii K Hnilica, DVM, MS, DACVD, 12/06 “The Itch Game”

iii C.S. Foil, MS DVM DACVD, 05/2006 “Food Allergy Management: It’s a Piece of Cake!”