Canine atopic dermatitis is a common skin disease characterized by pruritus. One of the most recent advances in understanding this disease has been recognition of the importance of the barrier function of the skin. The current theory is that impaired barrier function may lead to increased allergen penetration and subsequent disease. In humans, a common method of assessing barrier function is measurement of transepidermal water loss (TEWL) or the volume of water that passes from inside the body to the outside through epidermal layers. Although this method was developed for humans, it has been adapted for use in dogs. In this study, TEWL was compared in normal dogs (n = 50), atopic dogs before treatment (n = 50), and atopic dogs in clinical remission (n = 50). In normal dogs, TEWL was 8.81 g/m2 h. In untreated atopic dogs, the TEWL was 22.47 g/m2 h compared with 12.57 g/m2 h in atopic dogs in remission. Atopic dogs had a significantly higher level of TEWL than normal dogs did; however, TEWL was lower in atopic dogs whose clinical signs were in remission.

Commentary
These studies on TEWL are documenting a primary skin barrier defect. Electron microscopy has shown that even the skin of clinically normal atopic dogs has abnormalities in lamellar body secretion and extracellular lamellar lipids when compared with normal skin. In addition, as skin lesions worsen, these changes (eg, widening of intercellular spaces, release of lamellar bodies and abnormal lipid) worsen.1 In this study, the authors were able to document that treatment helps normalize the abnormality (TEWL). We do not know all of the mechanisms involved, but dry skin, as measured via TEWL, is likely a major component. This study helps make the case for treatment (immunotherapy or cyclosporine), including medication and frequent bathing.— Karen A.Moriello, DVM DACVD

Source
Transepidermal water loss in healthy and atopic dogs, treated and untreated: A comparative preliminary study. Cornegliani L, Vercelli A, Sala E,Marsella R. VET DERMATOL 23:41-e10, 2012.

1. Unraveling the skin barrier: A new paradigm for atopic dermatitis and house dust mites. Marsella R, Samuelson D. Vet Dermatol 20:533-540, 2009.

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