Nine key allergens in midge saliva linked to insect bite hypersensitivity in horses

Culicoides hypersensitivity on a pony.
Culicoides hypersensitivity on a pony. © Tsaag Valren, CC BY-SA 4.0, via Wikimedia Commons

Horses become sensitized simultaneously to multiple allergens in the saliva of Culicoides biting midges when they develop insect bite hypersensitivity (IBH), according to researchers.

IBH, also known as Culicoides hypersensitivity, is a seasonal allergic dermatitis in horses incited by salivary allergens from Culicoides midges. It is the most common allergic skin disease in horses.

The condition does not occur in Iceland, as the midges are absent. However, a high prevalence is seen in horses exported to Culicoides-rich environments.

Jasmin Birras and her fellow researchers set out to study the natural course of sensitization to Culicoides allergens in horses exported from Iceland, and identify the primary sensitizing allergens.

They tested for 27 Culicoides allergens on serological samples from 110 horses imported to Switzerland from Iceland, 59 of which subsequently developed IBH while 51 remained healthy. The horses were monitored over three summers, which is the high-risk season for IBH.

At the time of the first clinical signs of IBH, affected horses were sensitized to a median of 11 allergens (ranging from 0 to 21), of which nine were major allergens.

This was significantly higher than the summer preceding the first clinical signs, when the median was just three allergens (with a range of 0 to 16), while the healthy unaffected group typically showed sensitivity to one allergen (with a range of 0 to 14).

There was no significant increase in the number of allergens for the summer after the first clinical signs appeared, with sensitivity shown to a median of 12 allergens (with a range of 0 to 23).

The authors also found that IBH-affected horses exported from Iceland had a significantly higher degree of sensitization than those born in Europe, while the duration of IBH did not significantly affect the degree of sensitization.

Interestingly, horses exported from Iceland and exposed to Culicoides midges before seven months of age have the same low risk of developing IBH as locally bred horses, suggesting that early exposure to Culicoides allergens is essential for the development of immune tolerance.

The authors said that significant sensitization is detected in serum only in the year of the first clinical signs of IBH.

“In conclusion,” they said, “this study demonstrates that there is no single primary sensitizing Culicoides recombinant allergen, but that horses become sensitized simultaneously to multiple Culicoides allergens.

The nine major first-sensitizing allergens identified in the study could be used for preventive allergen immunotherapy, they said.

The study team comprised Birras, Samuel White, Sigridur Jonsdottir, Ella Novotny, Anja Ziegler, Douglas Wilson, Rebecka Frey, Sigurbjörg Torsteinsdottir, Marcos Alcocer and Eliane Marti, variously affiliated with the University of Bern in Switzerland, the University of Iceland, the University of Nottingham, and the University of Bristol.

Birras J, White SJ, Jonsdottir S, Novotny EN, Ziegler A, Wilson AD, et al. (2021) First clinical expression of equine insect bite hypersensitivity is associated with co-sensitization to multiple Culicoides allergens. PLoS ONE 16(11): e0257819.

The study, published under a Creative Commons License, can be read here.

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