Vet360 Vet360 Vol 05 Issue 04 - Page 16

DERMATOLOGY • • • • Clinical Diagnosis. Allergy testing. Allergen Specific Immunotherapy. Evidence based treatment guidelines. This highlights the scientific and clinical importance of allergy testing and ASIT in the effective management of canine atopic dermatitis. Allergy Testing Atopic dermatitis is a common diagnosis in veterinary dermatology affecting as much as 10% of the canine population. A key factor in the pathogenesis of the clinical manifestations of atopy is the presence of high levels of allergen specific IgE. However, it should be appreciated that canine atopic dermatitis is a complex and multifactorial disease involving immune dysregulation, allergic sensitization, skin barrier defects, microbial colonization and environmental factors. Only once the clinical diagnosis of canine atopic dermatitis has been made, is it decided whether an allergy test is required or not. Allergy serology should not be used as a front line diagnostic test for atopy, the clinical diagnosis of atopic dermatitis should have been made prior to testing. The following situations would warrant that allergy testing (allergy serology or intradermal skin testing) be performed. • • • • • Severe clinical signs. Prolonged clinical signs that last for more than 3 months of the year. Poor control of atopic skin disease with symptomatic therapy. Side effects to drugs being used in the therapy program. Poor owner compliance. The results of the allergy test are used to identify offending allergens to enable formulation of allergen specific immunotherapy vaccines. Allergy serology has several advantages over intradermal skin testing including no patient risk (as no sedation or general anaesthetic required), it is more convenient (for owner and dog), does not require repeated injections, serology is objective and reproducible and there is lower risk of drug interactions interfering with test results. Evidence based studies have provided no evidence of drug withdrawal prior to allergen-specific IgE serological tests for oral cyclosporine or prednisone / prednisolone. For intra-dermal skin testing on the other hand optimal withdrawal times are antihistamines (7 days), oral glucocorticoids (14 days), topical/ otic glucocorticoids (14 days) and cyclosporine (0) vet360 Issue 04 | SEPTEMBER 2018 | 16 days. It has been shown that the success rate of allergen specific immunotherapy (ASIT) based on allergy serology versus intradermal skin testing is not statistically different. Various allergen specific IgE serology testing assays are available including monoclonal, mixed monoclonal and polyclonal anti-canine IgE assays plus the high affinity IgE receptor alpha subunit assay (Fc-epsilon receptor test/mast cell receptor test). Seum IgE levels are miniscule when compared to IgG, in fact for every IgE antibody there are more than 10 000 IgG antibodies. It should be appreciated that IgG also binds with allergens and therefore, there is huge opportunity for cross reaction. Hence, serological assays based on the use of monoclonal or polyclonal anti-IgE antibodies are complicated by cross reaction with IgG producing false positive results. To specifically identify only IgE in serum, one needs to make use of IgE’s unique affinity for binding to mast cells and basophils, no other class of immunoglobulin is able to perform this. This Fc-epsilon receptor test (mast cell receptor test), shows a strong and highly specific affinity for canine IgE and a complete lack of cross reaction with IgG. This system is specific for the detection of IgE only, it will not identify any other Ig class. Due to this higher sensitivity and specificity plus the absence of IgG cross reaction of the Fc-epsilon receptor test, the use of the monoclonal, mixed monoclonal and polyclonal anti-canine IgE assays have decreased. The complete absence of any IgG cross-reaction in the Fc-epsilon receptor test enables this test to detect IgE in serum down to extremely low concentrations, making it ideally suited for the purpose of allergen selection in an immunotherapy vaccine. Allergy serology only measures circulating allergen specific IgE, it does not measure other allergic pathways and positive reactions are documented in non-allergic dogs. Thus the use of allergy serology should be restricted to only patients with confirmed clinical atopic dermatitis. Quantitative assessment of serum IgE levels can only be achieved with the IgE specific Fc-epsilon receptor test. A test is considered positive if it is above a certain cut-off level. These positive results are then correlated to • • • History of exposure of the patient to the allergen in question. Cross reactivity of allergens within botanical groups of related weed, tree or grass pollens. Level of IgE if more than 12 allergens are positive. There is still a lack of standardisation of the currently