Atopy, in the broad definition used in human medicine includes both food allergy and environmental allergy. Many veterinary dermatologists now also use this definition.
The diagnosis of atopic dermatitis is based on history, clinical examination and ruling out other differential diagnoses! The major causes of pruritus include allergy (flea bite, food, environmental) and ectoparasites. These are frequently complicated by secondary infections which may cause an additional level of pruritus. Some examples of uncommon causes of pruritus include leishmania, skin neoplasias and primary seborrhoea but these are discussed elsewhere.
The diagnosis and management of itchy pets is not easy and a combination of "in house" tests and trial therapy are frequently used in dermatology practice. Which external laboratory tests are available, their merits and my recommendations for their use is the topic of this lecture. However I will also include a brief review of the in house diagnostics or therapeutic trials which I personally use in most cases prior to the use of external laboratory tests
Which Laboratory Tests Can One Use in the Diagnosis of Pruritus Anyway?
A. Tests to Confirm the Presence of Infections or Ectoparasites
In-house tests include cytology and skin scrapes.
External laboratory tests include cytology and skin scrapes, serum flea IgE, serum Sarcoptes, leishmania, a blood count for eosinophilia, bacterial, yeast and dermatophyte culture and skin biopsy.
B. Tests to Identify a Serum IgE Reaction
In house ELISA and in house and external RAST and ELISA methodology measuring IgG and/or IgE are available. The reproducibility of RAST in one study was 79.3%, that of ELISA 93.1%. This means that the same serum sent in twice under different names will show the same results in 80% and 93% of the cases respectively. Thus, ELISA testing seems to be more reliable than RAST testing. These serum tests can not be used to "confirm a diagnosis" of atopic dermatitis. Atopic dermatitis is a diagnosis based on history, clinical examination and exclusion of differential diagnoses. The use of serum tests (even the reportedly most sensitive and best) for the diagnosis and management of adverse food reaction is controversial at best. In contrast, the use of serum testing to select environmental allergens to identify those--if any!--which can be avoided, or to choose those which will be used for hyposensitization is indeed an important part of the management of many atopic animals.
When are These Tests Indicated? How Does One Decide Which Test to Use?
Unfortunately for everybody concerned, (the animal, the veterinarian and the owner) the process of exclusion must be systematically and step by step performed. However, one can take short cuts with time, by running some trial therapies concurrently, not by excluding possible diagnostic tests. Listed below are the major differentials we need to work through and systematically exclude in order to arrive at a diagnosis of atopy.
1. Diagnosing Flea Bite Allergy (or ...how to exclude it as a diagnosis!)
The major clinical clues are pruritus on the caudal half of the animal and seasonal symptoms or worsening of symptoms.
In-house: Intradermal test
Will diagnose approximately 70% of flea allergic patients. Why not 100%? Because only the immediate phase reaction is tested, late phase reactions which may occur within the next 4-8 hours and delayed reactions which occur 24-72 hours later must be either read by the owners at home (easier said than done) or are missed.
My recommendation: I do not use this test except as part of a complete intradermal test, as the rate of the so called "false negatives" works against me in practice, by confirming the owners' opinion that their pet is not flea allergic.
External laboratory test: serum IgE test
This also does not diagnose the basophil and delayed hypersensitivity mediated reactions. Comments are the same as for the flea IDAT. As this is a routine part of most serum tests, I explain to owners that this test is unreliable.
Flea treatment trial:
My personal preference is to use a treatment trial to investigate the degree of pruritus contributed by fleas. Intensive flea control, when used as a diagnostic test should include both an adulticide (fipronil, imidacloprid, moxidectin, selamectin all q 2w initially (twice as frequently as most label recommendations), nitenpyram daily) and environmental treatment. Which exact product or group of products is chosen depends on the situation at home, the number and type of animals, the presence of small children, of carpets and the pocket book of the owner.
Cats in general tolerate the "spot-on" preparations far better than a daily tablet. The need for concurrent treatment of outdoor sleeping areas especially those with sandy soils, or covered and sheltered areas such as "decks" will depend on the presence of other household or neighbourhood animals and wildlife.
In the case of a dog with severe pruritus--with head and ventral involvement where a clinical differential diagnosis of scabies also exists--the treatment of choice would include selamectin and moxidectin, so that a concurrent "miticidal" treatment trial is conducted. Lufenuron takes time to work, but for owners who will not use environmental insect growth regulators, it is better to recommend this than nothing.
The borate salt carpet treatment is one which some owners will gladly undertake in preference to insect growth regulators. But even should the owners decline the use of insect growth regulator products, the modern "spot-ons" have some larvicidal activity. The adulticide should be used on every animal in the house for a 6 week trial period. Should absolutely no improvement be seen within that time, a flea bite hypersensitivity is unlikely.
2. Diagnosing Scabies, Cheyletiellosis & Ear Mites (or ...how to exclude mites as a diagnosis!)
In-house test: Superficial skin scraping, yields a diagnosis in up to a 50%. Ear margins and unexcoriated hocks, axillae and ventral abdomen areas probably give the highest success. Q-tip or cotton bud swabs from ear canals are observed without staining for Otodectes. Sometimes Demodex mites are identified from these ear swabs!
External laboratory test: A scabies titre is available in many countries and has a reported sensitivity of more than 90%. There are however two potential drawbacks, first a potential cross reactivity with house dust mite is possible and second the test measures antibodies which take time to form after the animal is infected. The test may therefore give false negative results if taken within the first 6 to 8 weeks of the mite infestation. Ear swabs and skin scrapings are frequently sent to the laboratories to look for mites. Demodex will stay on the slide, Otodectes and Sarcoptes mites can move off within a few hours of sampling.
Trial therapy: Is my test method of choice for superficial mites. I need to be 100% sure that I do not have scabies or another mite infection and trial therapy is the best method! Selamectin or moxidectin spot-ons are administered as per the label dose for that animal but at an increased frequency (two weekly intervals) for a total of 3 applications i.e., a total of 6 weeks. All contact animals must be treated. Environmental treatment for the adult mites is not needed with this 6 week regime. An insect growth inhibitor treatment for the eggs is not needed for Sarcoptes scabiei but may be helpful in some rare cases of Cheyletiella and Otodectes. An additional method of treatment for canine scabies, is milbemycin 2mg / kg given twice weekly for 3 weeks, which can be useful for those dogs which are sensitive to the spot on preparations. This has been used with success in those countries which have milbemycin oxime tablets for heartworm control, the formulation which is combined with an anthelmintic is not suitable for this frequent rate and high dose of administration.
3. Diagnosing Secondary Infections (or ...how to exclude secondary infection as a possible contributor to the pruritus!)
In house tests include cytology, with sticky / scotch tape or some form of impression smear. The stain of choice is one such as Diff Quik®. Interpretation, like all tests requires time to learn, but once the eyes are trained the identification of organisms is very simple. Unfortunately, the absence of organisms does not exclude a secondary infection component as we do not sample the entire skin!
The external laboratory tests which are most frequently used to confirm the presence of superficial microbial infections include culture and cytology. Cytology samples from impression smears and ear swabs which have already been stained can be sent to the laboratory for either confirmation of the clinician's suspicions or for the complete diagnosis. Unstained slides will be subject to overgrowth during the transportation and may give a false interpretation.
Trial therapy for bacterial and yeast infections is frequently prescribed in veterinary dermatology. The use of this trial therapy is frequently made even if the cytology is negative. In the case of a diagnosed infection bacteria or yeast are no longer present at a subsequent recheck evaluation. In that case, the success of the therapeutic trial is substantiated with cytology and the degree of improvement in pruritus is evaluated. Even a partial improvement suggests that the infection contributed to the pruritus in that individual animal.
The second outcome is when a clinical suspicion could not be confirmed cytologically. In that case clinical improvement as a result of antimicrobial therapy should support the hypothesis of infection. The problem with this interpretation, where one cannot directly (cytologically) measure a response to therapy, is that the improvement may be truly simply coincidental. In addition, some medications have a concurrent anti-inflammatory or mood-modifying effect. Caution must be exercised when assessing a decrease in pruritus while using systemic medications.
4. Diagnosing Food Allergy
In house tests: There are none which are reliable!
External laboratory tests: The use of serum tests to diagnose food allergy is controversial. The best accuracy level quoted is 60%.
Elimination diet trial: To get an owner to perform an elimination diet is hard work in most cases and to keep the compliance up all the way through the 4-8 weeks needed to make the diagnosis is even more difficult. An elimination diet for canine patients consists of a protein source and a carbohydrate source previously not fed. These are home-cooked and usually fed in a ratio of two to three parts of the carbohydrates and one part of the protein. An elimination diet for cats may be 100% meat as cats are obligate carnivores. Tips and hints to obtain better success in your elimination diets are found in the notes section entitled "Mystery itchy pets".
5. Diagnosing the Type of Allergy Using Serum Tests (remember a positive serum IgE test does not mean that the animal's current symptoms are due to an allergic reaction)
In house tests: Some "in house Elisa tests" are marketed as "screening" tests to determine whether a "full allergy test" is worthwhile. If one has already formed a clinical diagnosis of allergy (based on the exclusion of other possibilities), then such a "screen" is not necessary! Indeed in studies looking at the results of serum allergy tests, normal dogs with negative skin test results as well as dogs with dermatomyositis and scabies (the latter completely responsive to lime sulfur dips or ivermectin) could have been treated with hyposensitization therapy based only on their serum test results.
The external laboratory tests: Once a clinical diagnosis of atopic dermatitis is made and an adverse food reaction is ruled out, the next step is either to treat the animal symptomatically or to perform allergen specific immunotherapy. Avoidance or environmental control measures to reduce allergen exposure are possible in some cases and a test may also be conducted to enable this therapeutic approach. The IgE antibody test has certain advantages compared with the skin test. It is convenient and easily available. We used to think it was less influenced by drugs, but the more developed, sophisticated, specific and reliable blood tests get, the more they seem to be influenced by drugs as well. In humans, serum IgE levels are influenced by parasitic diseases (helminths), infections (viral, mycobacterial, fungal) and others. In the canine, the average IgE concentration in normal dogs is 200 x as high as in atopic humans. The high level of IgE in the canine is believed to be due to their exposure to internal and external parasites and may affect test results leading to false positive reactions.
Intradermal allergy testing is still regarded as the test of choice in canine and feline atopic dermatitis to identify offending allergens.