4 min read
Insect bite hypersensitivity is one of the most common allergic skin diseases in horses. This frustrating condition results in symptoms similar to human atopic dermatitis, including papules and edema, which progresses to crusts, dandruff, alopecia, excoriations, and lichenification. As expected, insect bite hypersensitivity, or IBH, is uncomfortable for the horse, frustrating for the owner, and difficult to treat as there is no way to remove the cause.
A recent study has discovered a new link between genetic impairment of the epithelial barrier and IBH. Data collected in Investigating the epithelial barrier and immune signatures in the pathogenesis of equine insect bite hypersensitivity, which was published in the peer-reviewed journal PLOS ONE, has suggested that “an impairment of the epithelial barrier in IBH-affected horses that may act as a predisposing factor for IBH development.”
Insect bite hypersensitivity is more than just a bug bite. IBH involves an increase in mononuclear cells and eosinophils in the dermis, elevated counts of mast cells and MHC class II positive cells, and the possibility of secondary bacterial infections in the resulting lesions.
This skin condition is known to be a type I, IgE dependent hypersensitivity, which can be mediated via T helper type 2 cells (Th-2) and their signature cytokines IL-4, IL-5 and IL-13. The cytokines cause B-cell switching, which then creates allergen specific IgE antibodies which bind to mast cells and results in an activation of eosinophils.
New discoveries in human science have, for the first time, demonstrated that an altered epithelial barrier plays a role in the pathogenesis of allergies, alongside Th-2 cell involvement.
The goal of this study was twofold. Researchers hoped to expand on the current understanding of the role of the epithelial barrier and skin immune response in the pathogenesis of this disease in horses, as well as investigate if genetic defects in the epithelial barrier may predispose certain horses to developing insect bite hypersensitivity.
The epithelial barrier of 10 horses with insect bite hypersensitivity was analyzed, as well as skin samples from nine horses without clinical signs or history of IBH. Eight of the horses with insect bite hypersensitivity passed away due to their skin condition prior to the study. Skin samples were collected post mortem.
Horses with insect bite hypersensitivity ranged in age from two to 27 years old and varied in breed, although the most common breed analyzed was the Freiberger. Descriptions of lesions included alopecia, excoriations, and lichenification. Impacted areas included the mane, tail, ears, and abdomen, as is common with IBH.
The control group ranged in age from yearling to 26 years old and also varied in breed, with the most common breed being Haflingers and Freibergers.
Samples from horses with insect bite hypersensitivity were collected from lesional skin at the ventral midline and from non-lesional skin located at the inner thigh. Samples from control horses were only collected at the inner thigh. Each sample underwent extensive testing, including RNA sequencing, differential gene expression analysis, gene ontology analysis, and pathway analysis.
The histopathological evaluation confirmed the diagnosis of insect bite hypersensitivity in all lesional skin samples. Samples from the control group were entirely devoid of inflammation.
Lesional skin samples displayed substantial hyperkeratosis and acanthosis, infiltration with lymphocytes and a strong infiltration with eosinophils in all but one sample, which instead showed strong dermal infiltration with lymphocytes. Non-lesional skin samples from horses with insect bite hypersensitivity also showed infiltration with eosinophils, although to a lower degree than the samples with lesions.
There was a clear separation between lesional skin samples from horses with insect bite hypersensitivity and the control group in terms of the principal component analysis based on expression data of 500 genes. In comparing lesional skin from horses with IBH to the control group, there were 2,228 significantly upregulated and 2,356 significantly downregulated genes.
In short, the lesional skin samples of horses with IBH showed changes in the epithelial barrier and substantial immune signatures. While we won’t dive into the details of which genes were upregulated or downregulated, it’s important to note that a T-cell co-regulator was significantly upregulated.
While it is perhaps only mildly surprising that the lesioned skin of horses with insect bite hypersensitivity differs genetically from healthy horses, it’s doubly surprising that the non-lesional skin samples also differed genetically from those of healthy horses. According to the study, “596 genes were differentially expressed. 461 genes were significantly upregulated… and 135 downregulated.”
Among the top 30 of these differentially expressed genes were those involved in epithelial barrier formation and metabolism of epithelial lipids. Other differentially expressed genes included those for glycerolipid metabolism, pruritus, and immune signatures.
Combined, this data suggests that changes in the epithelial barrier are already present in IBH-affected horses at this non-lesional stage.
Through analysis of differentially expressed genes, this study found “a high number of DEGs (differentially-expressed genes) which indicated that IBH-LE (lesional skin samples) is characterized by changes in the epithelial barrier, substantial changes in immune signature and a strong involvement of the hypoxic pathway.”
Changes in gene expression between the non-lesional skin samples from IBH horses and healthy horses seem to indicate that some horses have genetically different epithelial barriers which may predispose them to developing insect bite hypersensitivity.
Per this study, it appears that horses with insect bite hypersensitivity have a fundamental difference in the building blocks of their epithelial barrier which:
As new data continues to emerge about the root cause and impact of insect bite hypersensitivity, it’s important to look to the future and towards science-backed management options. Thanks to the results of this study, it’s clear that there is a genetic component to insect bite hypersensitivity and that horses who are prone to this condition will require long-term management. Unfortunately, long-term use of the steroidal products that are traditionally used to treat IBH can have significant side effects, including laminitis and muscle wasting.
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