Another thing that can cause abortion is Rhino/herpes infection. This is the reason people give Pneumabort shots to pregnant mares at 5, 7, and 9 months gestation. We had a mare abort at 8 months and also lost 2 foals at birth that did not get out of the sac, all of which we suspected were due to Rhino, although none were confirmed even when tested at the hospital. But once we started giving the shots, no more losses. We also give the Rhino/flu shots to the show horses and young stock every 2-3 months during the summer as the immunity does not last beyond 3 months. Most people can "get away" with only vaccinating once a year, but not if you have horses coming in and out and pregnant mares.