How Early Insect Exposure Influences Allergies

Research studies and data collected by the KFPS indicate that the occurrence of IBH in Friesian horses is as high as 18.2%.

Insect bite hypersensitivity (IBH), often referred to as sweet itch or summer eczema, is a significant health concern for some Friesian horses. The condition is well known to have a genetic component, and Friesians are among the breeds recognized as being at increased risk. However, genetics alone do not fully explain why some horses develop severe allergic disease while others do not. A newly published long-term study from Cornell University provides important new insights into how early-life environmental exposure, specifically, the timing of first insect exposure, can dramatically influence whether a horse ultimately develops insect bite hypersensitivity.

The study followed a group of closely related Icelandic horses over many years, allowing researchers to isolate the effect of the timing of exposure while minimizing genetic variability. Horses were divided into groups based on when they were first exposed to Culicoides biting midges: as adults, as adolescents, or from birth. Many of the horses had one or both parents affected by insect bite hypersensitivity, meaning they carried a known genetic predisposition to allergy. Despite this inherited risk, outcomes differed substantially by age at first exposure.

• More than 60% of horses first exposed as adults developed clinical allergy.

• Horses first exposed during adolescence showed a much lower disease rate, around 20%.

• Most notably, none of the horses exposed from birth developed chronic insect bite hypersensitivity.

Did you know? Just a small number of Culicoides bites can alter a horse’s immune response for years, making later exposure far more likely to cause chronic allergy.

These findings challenge the long-standing assumption that minimizing early-life insect exposure reduces the risk of allergy later in life. Instead, the results suggest that early, natural exposure may help the developing immune system learn to tolerate insect allergens rather than respond to them in an exaggerated, allergic manner. In practical terms, the immune system appears to be “programmed” early in life, and early exposure may encourage tolerance rather than hypersensitivity.

For Friesian horses bred and raised in the United States, this research raises important considerations. Many breeders and owners understandably work hard to protect foals from insects using stabling, fans, blankets, and aggressive insect control. While these strategies are essential for horses with insect bite hypersensitivity, the study suggests that complete avoidance during early immune development may not always be protective in the long term. This does not imply that foals should be allowed to suffer from insect irritation, but it does suggest that carefully managed, natural exposure may help reduce future allergy risk, particularly in breeding programs where insect hypersensitivity is already present in the pedigree.

The findings are equally relevant for Friesians imported from Europe, including the Netherlands, or relocated across regions within the United States. Horses that grow up in climates with lower midge pressure and are first exposed as adults to high-insect environments may face a substantially higher risk of developing insect bite hypersensitivity. This mirrors what has long been observed in Icelandic horses imported later in life and helps explain why the timing of exposure, not just geographic location, matters so much.

For Friesians bred in one part of the country or world and sold to another, climate differences should also be considered. A horse raised in a cooler or drier region with minimal insect exposure and later moved to a humid, insect-dense area may be at greater risk than a horse exposed earlier in life. Conversely, early-life exposure in a higher-insect environment may provide a degree of immune tolerance that persists even if the horse later moves to a different climate.

One particularly reassuring aspect of this research is its finding regarding maternal allergy. Foals born to mares with insect bite hypersensitivity did not show increased allergy risk if they themselves were exposed to insects from birth. The transfer of maternal antibodies through colostrum did not negate the protective effect of early exposure. This is an important point for Friesian breeders concerned about producing foals from mares with a history of insect hypersensitivity.

FAQs: Early Insect Exposure & Friesian Horses

What is insect bite hypersensitivity, and why is it a concern in Friesian horses?

Insect bite hypersensitivity (IBH), often called sweet itch or summer eczema, is a chronic allergic skin condition caused by an exaggerated immune response to proteins in biting insect saliva. Friesian horses are among the breeds known to have a higher prevalence of allergic and immune-mediated conditions, making IBH a significant welfare and management concern.

Does this mean insects cause allergies?

No. Insects themselves are not the problem. The allergic reaction occurs when the immune system responds abnormally to insect saliva proteins. This research suggests that early exposure may help the immune system learn to tolerate those proteins rather than overreact to them later in life.

Should I stop protecting foals from insects?

No. This study does not suggest allowing foals to suffer or experience severe irritation. It highlights that complete avoidance during early immune development may not always be protective. Any management decisions should balance comfort, welfare, and long-term health.

If a mare has insect bite hypersensitivity, will her foals get it too?

Not necessarily. In this study, foals born to allergic mares did not develop insect bite hypersensitivity if they were exposed to insects from birth. Genetic risk alone did not determine outcome.

Why are imported horses at higher risk?

Horses imported from regions with low insect exposure and first exposed as adults may have a higher likelihood of developing an allergy. Their immune systems encounter these allergens later in life, when tolerance is less likely to develop.

Does this apply only to Icelandic horses?

The study used Icelandic horses because their exposure history could be tightly controlled. However, the immune mechanisms involved are not unique to that breed. The findings are highly relevant to Friesians and other breeds known to be at increased risk of allergy.

Does climate really make that much difference?

Yes. Climate influences insect species, density, and exposure duration. Horses raised in low-insect environments and later moved to high-insect regions may face different immune challenges than horses exposed early in life.

Can this research help prevent insect bite hypersensitivity?

It provides strong evidence that early-life exposure is protective, but it does not yet translate into formal management protocols. This research helps inform risk awareness and future studies rather than offering one-size-fits-all recommendations.

Can this research help prevent insect bite hypersensitivity?

It provides strong evidence that early-life exposure is protective, but it does not yet translate into formal management protocols. This research helps inform risk awareness and future studies rather than offering one-size-fits-all recommendations.

For breeds with higher allergy risk, like Friesians, even limited Culicoides exposure can have long-lasting immune consequences that shape health for years to come.

This research represents an important shift in how we think about the risk of insect bite hypersensitivity in Friesian horses. While genetic predisposition remains relevant, these findings demonstrate that early-life environment, specifically the timing of first insect exposure, can significantly influence whether that genetic risk ever manifests as clinical disease. For Friesian owners, breeders, and buyers, this underscores the importance of considering not only pedigree but also where and how a horse is raised, imported, or relocated.

As with many complex immune-mediated conditions, there is unlikely to be a single universal strategy that applies to every horse. However, studies like this move the conversation beyond genetics alone and toward a more complete understanding of how management, environment, and immune development interact. Continued research will be essential, but this work provides a valuable framework for more informed, thoughtful decision-making around breeding, young horse management, and long-term welfare in the Friesian breed

Reference:

Early allergen introduction overrides allergy predisposition in offspring of horses with Culicoides hypersensitivity. Frontiers in Immunology (2025). Vol 16. 1654693. https://doi.org/10.3389/fimmu.2025.1654693

Disclaimer: This article is intended for educational purposes only and is not a substitute for veterinary advice. Management decisions should always be made in consultation with a licensed veterinarian, taking into account the horse's individual health, local insect pressure, and welfare considerations. Research in this area is ongoing, and recommendations may evolve as additional studies become available.