We had a super intense outbreak of Pigeon Fever here in Central Oregon two summers ago. It is a virus that absesses in the mammory glands and is very contageous and gross. Luckily has a very low mortality rate just a pain to deal with.
Cushings can also have symptoms of abdominal swelling although that picture looks like the swelling is more localized than that.
Your horse could most likely be having a reaction to biting flies/insect. "Swat" is an excellent treatment for this. Heals up the bites and protects it from more irritation, after washing the site of course.
Hope this helps a little...