Should My Vet Test My Dog And Cat For Allergies?
Ron Hines DVM PhD
Skin allergies and itching (canine atopy) are the second most common reason owners bring their dogs to veterinarians (ear infections are the 1st). It’s the 7th most common complaint from cat clients. And if you consider that ear infections and hot spots are often due to underlying allergies, allergic problems are the number one reason dog’s come to visit me.
I live with an allergic dog whom I love dearly. So I am on both the giving and the receiving end of my own advice. When you are dealing with allergic pets, you have a limited number of options: You can treat your pet with soothing lotions and shampoos, give them medications that suppress their immune system (corticosteroids, Atopica®), or you can attempt to have the things your pet is allergic to identified and see if periodic allergy shots might help them (hyposensitization=desensitization injections). We will send you home with other things – antibiotics, fatty acid supplements, antihistamines, tranquilizers, etc. etc. All of those products give less than spectacular results.
Most owners know about the potent side effects of drugs like steroids and Atopica – so allergy testing and desensitization can be quite appealing. I get asked about it all the time. It is certainly a procedure with little downside – other than its expense and time.
Human and veterinary dermatologists, rely on skin testing, a procedure in which a small amount of things (allergens) that pets are commonly allergic to is injected into the pet’s skin. It is an exacting, expensive and time-consuming procedure. Results can also “straddle the fence” and be difficult to interpret. We call this an in vivo test because it duplicates, as best we can, the things that happen in allergic pets – the offending allergen(s) comes in contact with all the resources that your pet’s body will use to destroy it. In the process, redness and swelling at the injection site occurs. Most experts believe that skin test panels such as I described are the best method of detecting true allergies.
Because of the expense and specialized training needed to perform and interpret skin allergy tests, veterinarians have always been on the lookout for a simpler procedure. One that they, themselves, could perform in their offices without the need of a trained specialist. Several times, in the past, blood tests were marketed for that use. All eventually proved to be highly unreliable. They all relied on detecting an antibody called IgE that allergic pets and humans produce against things they become allergic to.
What Blood Sample-based Tests Are Available For My Pet?
Most recently, Heska Corporation, has intensively marketed a blood test to detect allergies in dogs, cats and horses. They call the test, Allercept. You can read about it here. They have spent a great deal of effort and expense in refining the test to make the results reproducible and to purify their antigen extracts to avoid false results. The test measures the amount of a compound one of your pet’s lymphocytes (plasma cells) produce to fight things your pet’s immune system considers a foreign invader. (that is what allergy is) That compound is immune globulin E (IgE). It exists in many forms, each designed to react to a specific allergen. The inflammatory process in your pet’s skin that is begun by IgE is extremely complex and not fully understood.
You can read Heska’s glowing promotional literature for their test here. and the flashy Idexx brochure here . (Idexx is a jobber for Heska) A competitive test, the, Greer Aller-g-complete, is also offered to veterinarians. (ref)
Veterinarian like these tests because they are simple, they can be done at their office, and Heska and Greer maintains a readily available staff to interpret the results. Based on those results, they prepare an extract of the offending allergens for your veterinarian to use in an attempt to desensitize your dog.
Why Are You Hesitant About The Value Of These Blood Tests For My Pet?
I am quite satisfied that the Allercept and Greer tests will accurately measure the levels of IgE in your pets’ blood. But I am less certain that knowing these levels will help your pet. Heska has performed many studies and send their representatives to many veterinary conventions to tout the benefits of the Allercept test. But the study they have not done may tell us most. They have not determined that the level of blood IgE in pets that are healthy – the ones that don’t have itchy skin – is any lower than the level in dogs that do itch. Until they show that blood IgE levels in atopic dogs are higher than IgE levels in healthy dogs, I am going to remain skeptical. We really do not know if high IgE levels to a particular antigen mean anything that is relevant. We do know that many dogs and cats that do not itch also have high IgE levels to many of these antigens.
Proving that a treatment benefits itchy dogs is very difficult. You know that your pet has good days and bad days. You know that the problem can be seasonal and related to boredom and breed. I can tell you that no two dogs react identically to the same medications and treatments for itchy skin. On top of that, veterinary dermatologists will tell you that they strive for modest improvements – not total cures. With such a hazy target, you can see why the benefits of tests like these and treatments they point to can be very hard to judge.
There are veterinary laboratories, other than the two I mentioned, that perform their own in-house blood allergy tests using allergen extracts they do not purchase from Heska. Results obtained from the same dog tend to differ from lab to lab (ref) and test results on multiple samples from the same dog often differ at a single lab. So they are really a waste of time.
At one time, a blood-based test called the RAST test was commonly used. Most human and veterinary dermatologists have given up on the RAST test. It is rarely performed any more because of its unreliability.
Have Any Independent Studies Been Performed To Validate These Tests?
Yes; and in these studies, the value of these blood allergy tests is questionable.
The veterinary school in Queensland, Australia performed a study using the Allercept test on West Highland Whites. They compared the IgE test results of normal Westies to the results of itchy Westies. They found that normal Westies actually had higher levels of IgE than the itchy (Atopic) ones. You can read that study here.
A 2005 study at the vet school in Urbana, Illinois was equally negative regarding the use of blood allergy tests (although samples were only sent to Greer labs) . “the ability to make clinical determinations of significance was marginal ” You can read that study here.
A 1998 study in Edinburgh, Scotland came to similar conclusions that IgE measurements were valueless – normal, non-itchy dogs has as much circulating IgE as itchy ones. “this study leads one to question the value of either test in the diagnosis of Atopic disease ” You can read that article here.
What About Allergic Cats?
The only study that I know of that compared the test results of allergic cats to test results of non- allergic cats found no difference between the number of cats that had high IgE levels in either group when measured with the Heska Allercept method. (ref ,full text)
Human dermatologists have questions about the accuracy of this test as well. (ref)
Heska claims to have performed an in-house study on beagles. But the purpose was to show that their test was more accurate than two of its competitors – not that it was measuring a clinically significant antibody. If the study was of scientific significance, it would have been journal-published and more details would have had to be given. It was not. They pointedly make no mention of IgE levels in beagles not immunized against allergens. You can read their promotional report here.
Why Are These Blood Tests For Allergies So Inaccurate?
Veterinary dermatologists will be the first to tell you that there is a lot about the allergic process that they do not understand. Obviously, there is more to itchiness than the fact that IgE is present in your pet’s blood. Perhaps one dogs reacts differently to IgE than another due to its genetic or other factors. Perhaps the age at which a pet first encounters these allergens is important is it is in humans. (ref) We also know that in humans, parasites like mange mites (ref) and dog roundworms (ref) influence blood IgE levels. Perhaps factors like that or cross reactivity influence your pet’s susceptibility to atopic skin disease as well.
There is another theory. We know that pets produce at least two kinds (isotypes) of IgE directed against a specific foreign substances (proteins). I do not know if the blood tests currently on the market recognize that. You can read about these two forms IgE+ and IgE- here. (ref , full text)
So What Are These Test Good For?
Blood tests for allergies do seem to have some value in ruling out an allergy to a particular substance. That is, when test results come back saying that your pet is not allergic to something, it probably isn’t allergic to that substance. If it comes back stating that your pet is allergic to something, that may or may not be true. The same conclusions were drawn about using this test in humans. You can read those conclusions here.
If you would rather read the more politically correct answer to that question, go here.
Well, Should I Have This Test Performed Or Not?
Veterinarians are always in a quandary when dealing with allergic pets. The products that actually work all have the potential for serious side effects. When we give them, we give them as sparingly as we can and we feel better when we know that we have exhausted every other option.
That is why I still encourage a few of my clients to have these tests run before resorting to the long term (intermittent) use of powerful medication. Perhaps the tests will benefit their pets, perhaps the articles I linked you to are all wrong. If blood tests and desensitization based on their results don’t work, at least you and I will feel better, knowing that we did everything in our power to avoid powerful drugs. But I want you to understand the major limitations of these tests before you agree to them.
I also have a moderate concern that “desensitizing” your pet to a cocktail of allergen it is not really allergic to might actually make it allergic to those antigens after repeated “desensitizing” injections.
What About Skin Tests Performed By A Veterinary Dermatologist?
I copied a portion of the chapter on skin testing procedures for allergies from my copy of the 2001 edition of Muller and Kirk’s Small Animal Dermatology. It is quite hard to read, but you can view it here.
Skin tests to determine what your pet might be allergic to are considerably more accurate, on the whole, than blood tests. However, they are not 100% accurate either. To have them performed, you will need to locate a board certified veterinary dermatologist. The test requires only mild sedation. A patch of hair will be shaven from your pets rib cage area and a patchwork of injections will deposit small samples of the most common offending allergens in your area into the pet’s skin. When it is performed, your pet will look like this.
Approximately 60% of pets seem to benefit from periodic injections of the extracts prepared according to the skin test results.
What About Blood Tests For Food Allergies?
Most veterinarians believe that the majority of itchy, atopic pets develop their problem due to things they inhale – not things they eat. But we know that some pets (perhaps 10-15%) do become itchy due to allergies to things they eat.
Most of the laboratories that offer blood tests for things like dust mite and flea allergies also offer panels for allergies to food ingredients. Those tests are also highly inaccurate. Just like the blood panels for inhalant allergies, a negative test result might be of some value in planning your pet’s diet. But a positive test is as likely to be a false-positive as a true positive. Studies have found that these tests are really not worth performing. (ref)
You have a much better option: Put your pet on a two-month test diet and see if it improves. Most authorities do not wait that long. They make a judgment after a single month. I would have patience and give the test diet longer.
You have several options, you can purchase a commercial diet that has been processed to make its ingredients non-allergenic (hydrolyzed), diets like Purina’s H/A™ , or you can prepare a diet at home from novel ingredients that your dog has never eaten before. You will find suggestions on preparing your pet’s food at home here. You can read an article on food allergies as they relate to itchy skin disease here.
Remember, dogs on a test diet must consume nothing besides those low-antigen or novel antigen ingredients if the test results are to have meaning. Just like skin testing, the less medications your pet consumes prior to testing, the better.