Update on the horses in this post. Tried to get some photos of the mares with welts but the stud wouldn’t have it. No improvement on the mare with the smaller welt. Unable to see the mare with the larger welt (third photo in original post). Inflammation due to bites or stings should have resolved by now. These photos taken 06/29/18.
The pond seen in the photos from 06/15/18 has lost quite a bit of water, which increases the concentration of dissolved minerals, making it unfit for animal consumption.