Dermatology

Early exposure curbs allergies in Icelandic horses

By Krishna Ramanujan

Horses that were exposed at birth to the Culicoides midge, which can cause an extreme allergic reaction, never experienced that reaction, while 62.5% of horses that were first introduced to the midges in adulthood developed an itchy eczema-like reaction, according to a 13-year study of Icelandic horses conducted by researchers at Cornell University in New York.

The results, published in Frontiers in Immunology, were only possible because Culicoides midges are not present in Iceland. This allowed the researchers to test their theories on naïve horses and offspring exposed to the allergen at varying times.

The study focused on hypersensitivity to the bite of Culicoides midges. Female midges take a blood meal as part of their reproductive cycle, injecting anti-coagulant proteins when they bite that can elicit an allergic response in horses. Symptoms manifest as dermatitis, extreme pruritus and hair loss, with severe cases leading to weight loss.

“Training the immune system to develop tolerance against allergens early in life creates a specific ability to adjust to those allergens and recognize them as harmless,” said Bettina Wagner, DVM, PhD, the James Law Professor of Immunology at the Cornell College of Veterinary Medicine, and 1 of the authors of the study. “The earlier it happens, the more successful it is.”

Icelandic horses, often bred in Iceland and exported elsewhere as adults, have been particularly susceptible to the allergic reaction. As a doctoral student, Dr.
Wagner had heard a rumor about the horses. “And the rumor was, if you export an adult horse from Iceland, they more often get a severe allergic reaction to Culicoides,” Dr. Wagner said. “But when the horses are bred in Europe, disease incidence is much, much lower.”

The current study sought to explain why this pattern occurs by asking whether early exposure and maternal immunity might play a role.

The researchers examined 3 cohorts of horses housed at Cornell. Each cohort differed in the timing of their Culicoides exposure and in whether their mares carried and transferred allergy-specific maternal antibodies. Observations and human studies have suggested that immunoglobulin E (IgE) could be transferred from an allergic mother to newborns, leading to higher rates of allergy.

Along with finding high rates of allergy when Culicoides exposure first occurred in adulthood and no hypersensitivity when foals were exposed their entire lives, a moderate allergy rate (21.4%) was found when foals were first introduced to allergens in adolescence. No influence on rates of allergy development was observed for foals born to mares that had been exposed to Culicoides and carried IgE antibodies to the allergens.

The findings support the idea that early exposure to an allergen can prime an individual’s immune system to tolerate it and not become allergic later in life.

Since the equine and human immune systems share many similarities, the study corroborates related findings in humans, including the hygiene hypothesis, which is based on the observation that children who grew up on farms with multiple animal and environmental exposures developed far fewer allergies than children raised in clean, more sterile homes. MeV

Funding for the importation of horses and the maintenance of the Icelandic herd at Cornell was provided by the Harry M. Zweig Memorial Fund for Equine Research at Cornell University.

For more information:
Simonin EM, Torsteinsdottir S, Svansson V, et al. Early allergen introduction overrides allergy predisposition in offspring of horses with Culicoides hypersensitivity. Front Immunol. 2025 Oct 21:16:1654693. doi: 10.3389/fimmu.2025.1654693. eCollection 2025
https://www.frontiersin.org/journals/immunology/articles/10.3389/fimmu.2025.1654693/full

This story originally appeared in the Cornell Chronicle. It has been edited for length and style. https://news.cornell.edu/stories/2025/12/early-exposure-curbed-allergies-icelandic-horses