Answer

By Your Horse

Malene Jørgensen says:

These swellings are called urticaria, and they are caused by an allergic reaction.
Preventing the bites is hard, but try keeping him in at dawn or dusk, when the mosquitoes and midges are around.
You could try a Boett blanket, which covers most of the horse, and is made from a fabric that allows his skin to breathe. The rug was originally designed for horses suffering from sweet itch, but it can also be used to help with insect bite allergies. Fly sprays/repellents may help, too.
Conventionally, this condition is treated with antihistamines and/or cortico-steroids, but these will not cure the problem. The most effective preventative is to avoid the insects.
There are two homeopathic remedies that may help – and a simple test to decide which one you need. Soak a cloth or sponge in cold water and bathe the affected area. If your horse seems soothed by the action, he needs Apis mel, if he is aggravated by it, try Urtica urens. Give 200c up to three times daily until you see an improvement.