An allergic reaction to the bite of certain insects (most commonly members of the Culicoides family - biting midges or gnats)

The bite of these insects causes a slight reaction in all horses however horses with a hypersensitivity (allergy) will react particularly strongly to their bites. Specifically, they are reacting to the ‘saliva’ that the insects inject to be able to suck blood effectively. The hypersensitivity reactions occur at the sites where the insects’ prefer to bite, such as crest of the neck, the tail (where the hair of the mane and the tail allow easier access to the skin) or the ventral mid-line (from the udder / prepuce to between the forelimbs), where the hair is sparse.