From: Equestrian and Horse | Horse Ailments
Insect Bites
Biting insects can cause anything from a small raised lump to a more severe allergic reaction such as sweet itch ( also known as recurrent seasonal pruritus).
Horses at risk
Any horse can get bitten, grey horses are especially susceptible to sweet Itch which is the term used when a horse has an allergic reaction to the saliva produced by midges biting the skin causing the more extreme skin itching to occur.
Symptoms
Treatment
Horses who suffer from reactions to insect bites can be helped by careful management during the spring and summer months when the insects are at their worst.