Horse allergy research corroborates findings from human health studies

A 13-year study exploring the development of allergies in horses has corroborated similar findings in humans, its authors say.

The paper, published in Frontiers in Immunology, found horses exposed early in life to an allergen were less likely to react when exposed again later in life.

Human studies have shown early exposure to allergens can help prevent allergies from developing, but the authors – from Cornell University and the University of Iceland in Reykjavik – noted variables in such studies are difficult to control.

Unique opportunity

Scholars sought to address the lack of data in this area in horses, which also provided a unique opportunity for study.

Horses can develop an allergy to a genus of biting midges, called Culicoides hypersensitivity – also known as summer eczema, insect bite hypersensitivity or sweet itch – a seasonal, recurrent skin allergy with clinical signs of pruritus, alopecia and dermatitis that is said to be mechanistically comparable to such allergies in humans.

Beginning in 2012, the study followed 61 Icelandic horses that had not been exposed to the Culicoides allergen, as the midges did not reside in Iceland until 2015. The horses were split into four cohorts – three full-sibling cohorts of 15 horses apiece, and one cohort of their parents (15 unrelated mares and one unrelated stallion).

Allergy status

Each cohort was exposed to Culicoides at different times – the parents as adults, the first sibling cohort at puberty, and the other two cohorts from birth-on – with a follow-up survey in 2021 confirming their allergy status.

In the parent cohort, 62.5% (10 out of 16) quickly developed allergy, which often became severe.

In the puberty cohort, 21.4% (three of 14 surviving horses) developed allergy, but at a delayed onset and with reduced severity.

None of the 30 horses exposed at birth developed the allergy. The authors noted that all of them had one or both allergic parents, suggesting they had increased genetic risk of developing the allergy, but the results suggested “[the] timing of allergen introduction dominated the clinical outcome and that early allergen exposure down-regulated or even eliminated the inherited allergy-associated predisposition”.

‘Disease incidence’

Study co-author Bettina Wagner, James Law professor of immunology at Cornell University’s College of Veterinary Medicine, said that as a doctoral student, there was a rumour that “if you export an adult horse from Iceland, they more often get a severe allergic reaction to Culicoides, but when the horses are bred in Europe, disease incidence is much, much lower”.

She added: “Training of the immune system to develop tolerance against allergens early in life creates a specific ability to adjust to those allergens and recognise them as harmless. The earlier it happens, the more successful it is.”