Food and contact allergies
Both these conditions are rare in horses.
Food allergies cause generalised itching and can only be truly diagnosed when all other causes of itching have been ruled out and a response is seen with a food elimination trial. This can take a very long time, however, when the offending food is found the treatment is to simply not allow the horse access to it any more.
Contact allergies usually cause a combination of itching, swelling, inflammation and pain in the area that has come in contact with the allergic substance.
The treatment is to identify and avoid the causal agent.