Although sweet itch can affect horses of any age, the onset of symptoms typically occurs in horses that are between 2-4 years old, although it can develop at any age. There are several factors that contribute to the development of sweet itch in horses, including genetics, environment, and immune system function.


Research has shown that sweet itch has a genetic component, and certain breeds are more predisposed to the condition than others. Breeds such as the Icelandic horse, Shetland pony, and Highland pony are known to have a higher incidence of sweet itch than other breeds.

In addition to breed, certain bloodlines within breeds may also be more susceptible to sweet itch. Horses with a family history of the condition are more likely to develop it themselves, highlighting the importance of considering a horse’s genetic background when assessing the risk of sweet itch.

Environmental Factors

While genetics plays a role in the development of sweet itch, environmental factors also contribute to the condition. The prevalence of biting insects, such as midges, mosquitoes, and black flies, can vary depending on the region and time of year. In areas where biting insects are particularly abundant, horses are more likely to be exposed to the allergens in their saliva, which can trigger an allergic reaction.

In addition to the presence of biting insects, environmental factors such as humidity, temperature, and rainfall can also impact the development of sweet itch. Research has shown that horses living in areas with high humidity and rainfall are more likely to develop the condition, potentially due to the increased number of biting insects that thrive in these conditions.

Immune System Function

The immune system plays a critical role in the development of sweet itch in horses. When a horse is exposed to the allergens in the saliva of biting insects, their immune system overreacts and produces an excessive inflammatory response. This response leads to the release of histamines and other inflammatory molecules, which cause the characteristic itching, hair loss, and skin lesions associated with sweet itch.

Research has shown that horses with weakened immune systems may be more susceptible to developing sweet itch, as their bodies are less able to mount an effective defense against the allergens in biting insect saliva.

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