# Skin Barrier in Normal and Allergic Horses: What Do We Know?

## TL;DR

This review explores how skin barrier function differs in normal and allergic horses, drawing parallels to human eczema and highlighting the need for further research.

## Contribution

The paper identifies structural and functional similarities between allergic horse skin and human eczema, suggesting a novel perspective on equine allergy mechanisms.

## Key findings

- Allergic horses have disorganized intercellular lipids in the stratum corneum, similar to atopic human and canine skin.
- Lesional allergic horse skin shows downregulation of tight junction and keratin genes, and upregulation of serine proteases and IL-13.
- Transcriptomic differences in allergic horse skin suggest potential molecular pathways for future therapeutic targets.

## Abstract

Skin itself plays an important role in the development of diseases like eczema in both people and dogs. People with eczema have very permeable, sensitive skin that has less protection against the penetration of pollen and dust. The presence of “leaky” skin starts a cascade of events, leading to allergy development. Thus, products to keep the skin moist and “sealed” in eczema patients are very important. Horses suffer from allergies similar to the ones of people, but the role of the skin in the development of equine disease is largely unknown. This review focuses on the current level of knowledge on skin barrier function in normal and allergic horses. Based on the preliminary information, it appears that horses have many similarities to people: the structure of the skin itself in allergic horses is different from the one of normal horses, and it is very similar to the one of people with eczema. It is not known if the skin of allergic horses also absorbs more pollen or what is the best way to correct these abnormalities. More work is needed to explore skin barrier function in allergic horses with the goal of improving the treatment of this disease.

Information on skin barrier in horses is limited. A study on the epidermal ultrastructure of normal and allergic horses documented disorganized amorphous intercellular lipids in the stratum corneum of allergic samples. These findings are similar to atopic canine and human skin. Currently, there is no published study comparing skin barrier function parameters between normal and allergic horses; thus, the functional implications of the ultrastructural changes are unknown. In normal horses, body location, gender, breed, and ambient conditions affect skin barrier parameters, such as Transepidermal Water Loss. Skin microbiome studies on normal horses have highlighted the importance of season and environmental conditions, since horses housed together share similar microbiomes. Skin dysbiosis and predominance of staphylococcus have been described in horses with pastern dermatitis. Transcriptomic studies of the epidermis of normal and allergic horses have found that lesional allergic skin has substantial transcriptomic differences when compared with healthy skin, namely downregulation of genes of tight junctions, keratins, and upregulation of serine proteases and IL-13. Keratinocytes harvested from horses with insect bite hypersensitivity show upregulation of IL-31 gene expression under stimulation. While more research is clearly needed, preliminary results seem to support skin barrier differences between normal and allergic horses.