Insect bite hypersensitivity (IBH) is a common seasonal allergic skin condition in horses. It occurs in horses worldwide and is the result of an immediate, type I hypersensitivity reaction to the saliva of biting flies (Simulium and Stomoxys species) and midges (Culicoides spp).

IBH affects between 3 to 60% of horses worldwide, of all age groups. Although the condition has been found to worsen with age. IBH is a multi-factorial disorder influenced by both genetic and environmental factors. Certain breeds of horses have a higher likelihood of developing IBH than others; Icelandic, Shetland ponies, Thoroughbreds, Quarter horses, and German Shires have a genetic predisposition to suffering from IBH.

Horses with IBH develop severe itchiness and discomfort; they are often seen scratching and biting themselves, sometimes using objects they find in the pasture such as shrubs and small trees. The constant scratching often causes skin irritation which leads to hair loss, crusts, papules, and lichenification of the skin. The parts of the horse's body most affected include the mane and tail, ears, dorsal midline and rarely ventral thorax/abdomen.

IBH tends to occur seasonally in horses, with clinical signs coinciding with months where flying insects are most abundant---between April to October. Horses tend to demonstrate increased signs of discomfort during the times of the day that insects are most likely to feed, which is usually the morning and evening hours.