We went through the same exact thing so I so know how you feel. Its horrible. We had numerous tests done that all came back negative. My vet said even though the fetus was negative for rhino he felt sure that was it. Last year he had us start giving rhino shots every 2 months all year. So they got it before they were bred, and then every two months from then on out. This was every horse on the farm wether they were breeding or not and included the stallion. My vet felt since we attend shows and cant really seperate show horses from breeding horses that the critical thing was to give rhino shots every 2 months. It was a pain giving that many shots that often but I have to think it worked. We have one foal on the ground and the rest of my mares are in their final month. But believe me Im still praying to get the rest here healthy.