Introduction to the ICADA 2023 canine atopic dermatitis pathogenesis review articles and updated definition

The International Committee on Allergic Diseases of Animals (ICADA) considers recent literature to coordinate reviews of important advances in atopic and allergic diseases over time. These review articles and translations can be found on the website www.icada.org and include the following: feline atopic syndrome—pathogenesis,1 diagnosis2 and treatment3; canine atopic dermatitis—pathogenesis,4-8 diagnosis,9, 10 research guidelines11, 12 and treatments.13, 14 The purpose of the following papers is to include new information since the last ICADA review papers pathogenesis of canine atopic dermatitis were published in 2015. These updated reviews were prepared by searching for articles and meeting abstracts from 2015 to 2022, and the manuscripts were submitted to the ICADA membership for review before submission to this journal. This new definition is useful in that it includes our current broader understanding of the pathogenesis of this condition beyond allergy. This new definition highlights the multiple factors involved in this disease, which supports the recommendation that the management of canine atopic dermatitis must be multimodal (as seen in the former14 and soon-to-be updated Canine Atopic Dermatitis Practice Guidelines). The first paper includes multiple studies showing the importance of environmental factors in the development of atopy, including lifestyle, diet and parasite exposure. Although atopy is unquestionably a hereditary condition and large atopy prevalence studies continue to show strong breed predispositions, unfortunately no consistent genetic markers have yet been found. The next paper reviews skin barrier measurement (which continues to be difficult and controversial), as well as updates on ceramides, filaggrins and other skin proteins, and interesting studies on the cutaneous microbiome and dysbiosis, and cutaneous host defence peptides. Most new knowledge on the pathogenesis of cAD is in the last paper, which examines the role of cytokines and chemokines. However, many studies are conflicting. Several consistently show upregulation of many T-helper (Th) cell cytokines that is not restricted to certain groups (such as only Th2 cytokines, for example). There also is a consistently strong central role for interleukin (IL)-31 in pruritus, in line with successful therapies targeting this cytokine for relief of clinical signs of AD. Certain topics were not included in the updated pathogenesis papers (e.g. the role of eosinophils, basophils, IgE and dendritic cells) because there is a lack of new information in these areas in dogs. In conclusion, these articles summarise the most important studies on cAD since 2015. Our knowledge of the pathogenesis and the definition of cAD will continue to evolve over the years. Hopefully these review papers will be useful to help direct future research endeavours to better understand the multifactorial complexity of cAD. Melissa Eisenschenk on behalf of the group of authors. Self-funded. The International Committee of Allergic Diseases of Animals (ICADA) should be acknowledged as they reviewed all of these review articles including this introduction and coming to a consensus on the new definition of atopic dermatitis. ICADA was also listed as an author, but the system would not accept it, perhaps acknowledgement would be more appropriate. No conflicts of interest have been declared.